Contrastable Overview

This vignette describes some of the contrast coding helpers this package provides. It should be noted that this package does not seek to provide an interface for creating hypothesis matrices and their respective contrast matrices. For that functionality you might look into the multcomp or hypr packages. The contrast functionality of this package is focused on the following:

This vignette will not go into detail how different contrast schemes are derived or interpreted, but will describe some of the high level differences. I recommend reading the following:

I also have a few blog posts about contrast coding; if you need further explanation of contrasts and more working examples beyond this vignette, read the first one below:

Introduction to contrasts

Statistical models such as linear regression need numbers to operate with, but often we have discrete categories that we want to model such as ethnicity, country, treatment group, etc. Typically, we have some comparisons in mind before we do our statistics: compare the group receiving treatment to a reference level who did not, or something like compare group A to group B. Contrast coding is the process of assigning numbers that encode the comparisons we want to make prior to fitting a statistical model. What this allows us to do is strategically set up our model to be interpretable in a way that’s meaningful to us even though the fit of the statistical model is unchanged. So, contrast coding is important for inferential statistics and reporting coefficients correctly, but is not that important for group-level predictions.

To start with an example, consider an experiment where people are tasked with keeping track of the location of different shapes under different levels of cognitive load, such as needing to remember different strings of numbers. Basically, a change detection task with a digit span cognitive load manipulation. We’ll call the levels of this cognitive load factor none, low, and high. A priori, the speed with which people detect a shape that changes location should vary depending on the cognitive load. Specifically, reaction time (RT) should be slower when cognitive load is higher.

What kinds of comparisons could we make, and how do we encode these in our statistical model? One comparison might be to compare low to none, then high to none. Another could be to compare low to none, then high to low. Yet another could be to compare low to none, then high to low and none together. Or, we could compare each level to the average across the conditions.¹ Specifying these different comparisons, which will really just amount to differences in group means, is our goal here.

In our example, what if we went found that our high cognitive load condition wasn’t high enough? What if we needed to add an extra-high level? We could add a comparison of extra-high to none, or compare extra-high to high. Or, we could compare extra-high to none plus low plus high together. Ideally, if we’re going for pairwise comparisons from a reference level, successive differences between levels, or some kind of nested set of comparisons, we would want to be able to specify this consistently without needing to revisit every instance of specifying our comparisons just to accommodate the new level.

Throughout this vignette and package, I refer to different contrast schemes, which I define as an encoding of a principled set of comparisons for an arbitrary number of factor levels. The important bit here is the arbitrary number part such that if the goal is to take pairwise comparisons from a reference level, then adding a new level should only amount to encoding an additional pairwise comparison to the same reference level. I’ll contrast this approach to a more manual approach to contrast coding, where you would explicitly write out, in code, every single comparison for every single factor you want to make. Consider if we added a new three-level predictor crossed with our four level example from before (we’ll call the new predictor group with levels A, B, C). If we also just wanted to do pairwise comparisons, a scheme approach would encode pairwise comparisons for cognitive load and pairwise comparisons for the new factor. The manual approach would say to encode low-none, high-none, and extra high-none; then, encode B-A, and C-A. If we introduce a new group D later, we have to remember to go in and specify D-A too.

So, while the manual approach is maximally explicit about exactly what we want to compare, it’s also more things to write out and more chances that we make a mistake while implementing it. Given that contrasts are specified using matrices in R, the chances of a mistake are high. Accordingly, R provides a few functions that implement common contrast schemes such as contr.treatment and contr.sum, which return matrices given a number of levels. However, there’s some trickiness with them. Below is a quick list of some issues this package attempts to circumvent; if you’re familiar with contrast coding from your own work, some of this may sound familiar and some of it may surprise you.

When setting contrasts to a factor that differ from the R session’s default contrasts, the labels for the comparisons in the output of the statistical model are reset. This requires the researcher to keep track of which numeric indices (1, 2, etc.) correspond to which comparisons—which also requires keeping track of the alphabetical order of the levels. Alternatively, the researcher can set the comparison labels using dim(), dimnames(), or colnames(). In my experience, I rarely see people in my field do this, and so I believe it would be naive to expect people to suddenly (learn how to and) start doing so.
The default contrast scheme, contr.treatment, places the reference level as the first level alphabetically while contr.sum places the reference level as the LAST level alphabetically. To verify for yourself, try running contr.treatment(5) and contr.sum(5). respectively, the reference levels are identifiable by the rows containing either all 0s or all -1s.
Related to the above, when people want to use \(\pm.5\) for their contrasts for a two-level factor, it is not uncommon to see something like -contr.sum(2)/2. However, this does not generalize when the number of levels is greater than 2, i.e,. -contr.sum(3)/2 will not give you what you expect.
contr.helmert returns unscaled matrices, giving effect estimates that need to be manually scaled. Tests of statistical significance are still meaningful, but interpreting the magnitude of effects using matrices from this function are likely to be incorrect if not scaled manually.
Quite a few contrasts simplify to \(\pm.5\) when there are two levels. For example, helmert coding and backward difference coding both give +.5 and -.5. But, if we were to add a third level to these, the values for the newly added comparison are NOT the same. Accordingly, if we’re not explicit about what the goal of the comparisons are (nested comparisons vs successive comparisons) then future researchers following up on our work might attempt to use the same contrast scheme but not understand what the new comparison is.

Motivation

I’m going to use the mtcars dataset just to have something to work with. We’ll convert a bunch of the columns to factors up front.

mdl_data <- 
  mtcars |> 
  as_tibble() |> 
  mutate(cyl = factor(cyl), 
         twolevel = factor(rep(c("a", "b"), times = nrow(mtcars) / 2)),
         gear = factor(gear),
         carb = factor(carb))

We can inspect the contrasts for the factors using contrasts(), which will use the default contrasts for unordered factors

options("contrasts") # Show defaults for unordered and ordered
#> $contrasts
#>         unordered           ordered 
#> "contr.treatment"      "contr.poly"
contrasts(mdl_data$twolevel)
#>   b
#> a 0
#> b 1
contrasts(mdl_data$carb) # Note the reference level
#>   2 3 4 6 8
#> 1 0 0 0 0 0
#> 2 1 0 0 0 0
#> 3 0 1 0 0 0
#> 4 0 0 1 0 0
#> 6 0 0 0 1 0
#> 8 0 0 0 0 1

We can see that the levels for carb are reflected in the coefficient labels:

summary(lm(mpg ~ carb, data = mdl_data))
#> 
#> Call:
#> lm(formula = mpg ~ carb, data = mdl_data)
#> 
#> Residuals:
#>    Min     1Q Median     3Q    Max 
#> -7.243 -3.325  0.000  2.360  8.557 
#> 
#> Coefficients:
#>             Estimate Std. Error t value Pr(>|t|)    
#> (Intercept)   25.343      1.854  13.670 2.21e-13 ***
#> carb2         -2.943      2.417  -1.218  0.23435    
#> carb3         -9.043      3.385  -2.672  0.01285 *  
#> carb4         -9.553      2.417  -3.952  0.00053 ***
#> carb6         -5.643      5.243  -1.076  0.29174    
#> carb8        -10.343      5.243  -1.973  0.05927 .  
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> 
#> Residual standard error: 4.905 on 26 degrees of freedom
#> Multiple R-squared:  0.4445, Adjusted R-squared:  0.3377 
#> F-statistic: 4.161 on 5 and 26 DF,  p-value: 0.006546

But if we changed the contrasts ourselves those helpful labels go away and are replaced with numeric indices. Note that the level names were originally also numbers, so it would be easy to get mixed up at this point.

mdl_data2 <- mdl_data
contrasts(mdl_data2$carb) <- contr.sum(6)

contrasts(mdl_data2$carb) # Note the reference level
#>   [,1] [,2] [,3] [,4] [,5]
#> 1    1    0    0    0    0
#> 2    0    1    0    0    0
#> 3    0    0    1    0    0
#> 4    0    0    0    1    0
#> 6    0    0    0    0    1
#> 8   -1   -1   -1   -1   -1
summary(lm(mpg ~ carb, data = mdl_data2))
#> 
#> Call:
#> lm(formula = mpg ~ carb, data = mdl_data2)
#> 
#> Residuals:
#>    Min     1Q Median     3Q    Max 
#> -7.243 -3.325  0.000  2.360  8.557 
#> 
#> Coefficients:
#>             Estimate Std. Error t value Pr(>|t|)    
#> (Intercept)  19.0888     1.3373  14.274 8.18e-14 ***
#> carb1         6.2540     2.0198   3.096  0.00465 ** 
#> carb2         3.3112     1.8418   1.798  0.08383 .  
#> carb3        -2.7888     2.6710  -1.044  0.30605    
#> carb4        -3.2988     1.8418  -1.791  0.08493 .  
#> carb5         0.6112     4.2221   0.145  0.88602    
#> ---
#> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#> 
#> Residual standard error: 4.905 on 26 degrees of freedom
#> Multiple R-squared:  0.4445, Adjusted R-squared:  0.3377 
#> F-statistic: 4.161 on 5 and 26 DF,  p-value: 0.006546

Usage

Now I’ll introduce the main workhorse of this package: the set_contrasts function. This function features a special syntax to quickly set contrast schemes. I’ll introduce features one at a time.

First, we’ll set carb to use sum coding as we did before. We do this using two-sided formulas, where the left hand side of the formula is a column in our dataset and the right hand side includes information about the contrasts we want to set. For example, this takes the form varname ~ code_by. Below, we specify that we want carb to use contrasts generated by the sum_code function. Two things to note: set_contrasts will automatically coerce specified columns into factors, so you don’t need to worry about calling factor() on columns first. Note that the function will also give messages about other factors that you haven’t set explicitly.

mdl_data3 <- set_contrasts(mdl_data, carb ~ sum_code)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel

contrasts(mdl_data2$carb) # matrix from before
#>   [,1] [,2] [,3] [,4] [,5]
#> 1    1    0    0    0    0
#> 2    0    1    0    0    0
#> 3    0    0    1    0    0
#> 4    0    0    0    1    0
#> 6    0    0    0    0    1
#> 8   -1   -1   -1   -1   -1
contrasts(mdl_data3$carb) # new matrix, note the column names
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1

Note that, unlike last time, the reference level is now the first level alphabetically and the labels are retained (compare the contrast matrices) For schemes that have a singular reference level, setting contrasts with set_contrasts will always set the reference level to the first level alphabetically. Currently, the reference level for carb is 1. If we want to change the reference level, we use the + operator in our formula. The change to the formula looks like varname ~ code_by + reference. Let’s change it to 3:

mdl_data4 <- set_contrasts(mdl_data, carb ~ sum_code + "3")  
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel

contrasts(mdl_data4$carb)
#>    1  2  4  6  8
#> 1  1  0  0  0  0
#> 2  0  1  0  0  0
#> 3 -1 -1 -1 -1 -1
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1

Note that when using sum coding, the intercept will be the mean of the group means:

# mean of group means:
group_means <- summarize(mdl_data4, grp_mean = mean(mpg), .by = "carb")
group_means
#> # A tibble: 6 × 2
#>   carb  grp_mean
#>   <fct>    <dbl>
#> 1 4         15.8
#> 2 1         25.3
#> 3 2         22.4
#> 4 3         16.3
#> 5 6         19.7
#> 6 8         15
mean(group_means$grp_mean)
#> [1] 19.08881

# model coefficients
coef(lm(mpg ~ carb, data = mdl_data4))
#> (Intercept)       carb1       carb2       carb4       carb6       carb8 
#>  19.0888095   6.2540476   3.3111905  -3.2988095   0.6111905  -4.0888095

Let’s say, hypothetically, we wanted to get differences of each comparison level to the grand mean, but we also wanted the intercept to be not the grand mean but the mean of our reference level? We can set this using the * operator: varname ~ code_by + reference * intercept.

mdl_data5 <- set_contrasts(mdl_data, carb ~ sum_code + 3 * 3)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel

contrasts(mdl_data5$carb)
#>   1 2 4 6 8
#> 1 2 1 1 1 1
#> 2 1 2 1 1 1
#> 3 0 0 0 0 0
#> 4 1 1 2 1 1
#> 6 1 1 1 2 1
#> 8 1 1 1 1 2
coef(lm(mpg ~ carb, data = mdl_data5))
#> (Intercept)       carb1       carb2       carb4       carb6       carb8 
#>  16.3000000   6.2540476   3.3111905  -3.2988095   0.6111905  -4.0888095

Finally, let’s say we wanted labels that were a bit more clear about what the comparisons are here. We can set the label names ourselves using the | operator: varname ~ code_by + reference * intercept | labels

mdl_data6 <- set_contrasts(mdl_data, 
                           carb ~ sum_code + 3 * 3 | c("1-3",
                                                       "2-3",
                                                       "4-3",
                                                       "6-3",
                                                       "8-3"))
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel

contrasts(mdl_data5$carb)
#>   1 2 4 6 8
#> 1 2 1 1 1 1
#> 2 1 2 1 1 1
#> 3 0 0 0 0 0
#> 4 1 1 2 1 1
#> 6 1 1 1 2 1
#> 8 1 1 1 1 2
coef(lm(mpg ~ carb, data = mdl_data6))
#> (Intercept)     carb1-3     carb2-3     carb4-3     carb6-3     carb8-3 
#>  16.3000000   6.2540476   3.3111905  -3.2988095   0.6111905  -4.0888095

Here’s what it would look like if you tried to manually do this yourself. It’s highly unlikely you would correctly specify every single value and every single label correctly on the first go (I actually messed up myself while writing out the matrix).

mdl_data7 <- mdl_data

my_contrasts <- matrix(c(2, 1, 0, 1, 1, 1, 
                         1, 2, 0, 1, 1, 1, 
                         1, 1, 0, 2, 1, 1, 
                         1, 1, 0, 1, 2, 1,
                         1, 1, 0, 1, 1, 2),
                       nrow = 6)
dimnames(my_contrasts) <- 
  list(
    c("1", "2", "3", "4", "6", "8"), 
    c("1-3", "2-3", "4-3", "6-3", "8-3")
  )


contrasts(mdl_data7$carb) <- my_contrasts
contrasts(mdl_data7$carb)
#>   1-3 2-3 4-3 6-3 8-3
#> 1   2   1   1   1   1
#> 2   1   2   1   1   1
#> 3   0   0   0   0   0
#> 4   1   1   2   1   1
#> 6   1   1   1   2   1
#> 8   1   1   1   1   2

To see how this can get even more error prone, consider a contrast matrix like the one below. Personally I would not trust myself to get the matrix correct, and if I ever had to add a level to the design it would be obscene to try and edit it.

mdl_data8 <- set_contrasts(mdl_data, carb ~ helmert_code * 4)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel

MASS::fractions(contrasts(mdl_data8$carb))
#>   <2   <3   <4   <6   <8  
#> 1 -1/2 -1/3   -1    0    0
#> 2  1/2 -1/3   -1    0    0
#> 3    0  2/3   -1    0    0
#> 4    0    0    0    0    0
#> 6    0    0 -3/4    1    0
#> 8    0    0 -3/4  1/5    1

The work would compound when we have multiple factors, but with the set_contrasts function we can specify what our desired scheme is easily:

mdl_data9 <- set_contrasts(mdl_data,
                           carb ~ helmert_code * 4,
                           cyl ~ scaled_sum_code + 4,
                           twolevel ~ scaled_sum_code + "a",
                           gear ~ helmert_code)

MASS::fractions(contrasts(mdl_data9$carb))
#>   <2   <3   <4   <6   <8  
#> 1 -1/2 -1/3   -1    0    0
#> 2  1/2 -1/3   -1    0    0
#> 3    0  2/3   -1    0    0
#> 4    0    0    0    0    0
#> 6    0    0 -3/4    1    0
#> 8    0    0 -3/4  1/5    1
MASS::fractions(contrasts(mdl_data9$cyl))
#>   6    8   
#> 4 -1/3 -1/3
#> 6  2/3 -1/3
#> 8 -1/3  2/3
MASS::fractions(contrasts(mdl_data9$twolevel))
#>   b   
#> a -1/2
#> b  1/2
MASS::fractions(contrasts(mdl_data9$gear))
#>   <4   <5  
#> 3 -1/2 -1/3
#> 4  1/2 -1/3
#> 5    0  2/3

Practically speaking, the most common things you’ll be doing is setting a contrast scheme and maybe changing the reference level, so the + operator is very important.

enlist_contrasts

A related function is the enlist_contrasts function, which follows the same exact rules as set_contrasts but will return a list of contrasts instead of setting the contrasts to the data itself. Henceforth, when I’m not going to use the dataframe in an example, I’ll just use this to show the contrast matrices.

enlist_contrasts(mdl_data,
                 carb ~ helmert_code * 4,
                 cyl ~ scaled_sum_code + 4)
#> Expect contr.treatment or contr.poly for unset factors: gear twolevel
#> $carb
#>     <2         <3    <4  <6 <8
#> 1 -0.5 -0.3333333 -1.00 0.0  0
#> 2  0.5 -0.3333333 -1.00 0.0  0
#> 3  0.0  0.6666667 -1.00 0.0  0
#> 4  0.0  0.0000000  0.00 0.0  0
#> 6  0.0  0.0000000 -0.75 1.0  0
#> 8  0.0  0.0000000 -0.75 0.2  1
#> 
#> $cyl
#>            6          8
#> 4 -0.3333333 -0.3333333
#> 6  0.6666667 -0.3333333
#> 8 -0.3333333  0.6666667

Contrast functions

I showed a few functions that generate contrasts like sum_code and scaled_sum_code and helmert_code. Below is a list of functions provided by this package:

treatment_code: Pairwise comparisons to a reference level, intercept equals the reference level (equivalent to contr.treatment)
sum_code: Comparisons to the grand mean, intercept is the grand mean (equivalent to contr.sum, but reference level is the first level)
scaled_sum_code: Pairwise comparisons to a reference level, intercept equals the grand mean
helmert_code: Nested comparisons starting from the first level. Note this is NOT equivalent to contr.helmert, as the latter gives an unscaled matrix.
reverse_helmert_code: Nested comparisons starting from the last level.
backward_difference_code: Successive differences. For levels A, B, C, gives the differences B-A and C-B
forward_difference_code: Successive differences. For levels A, B, C, gives the differences A-B and B-C.
cumulative_split_code: Dichotomous differences. For levels A, B, C, D, gives the differences A-(B+C+D), (A+B)-(C+D), (A+B+C)-D.
orth_polynomial_code: Orthogonal polynomial coding, equivalent to contr.poly.
raw_polynomial_code: Raw polynomial coding, I do not recommend using this unless you know what you’re getting into.
polynomial_code: An alias for orthogonal_polynomial_code

You can also use contrast functions from any other package so long as the first argument is the number of levels used to generate the matrix. However, again, if there’s a singular level used for the reference level, this will always be set to the first level regardless of what the original matrix was.

# all equivalent to carb ~ sum_code
foo <- sum_code
enlist_contrasts(mdl_data, carb ~ contr.sum)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1
enlist_contrasts(mdl_data, carb ~ sum_code)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1
enlist_contrasts(mdl_data, carb ~ foo)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1

If you want to suppress this behavior, you can use the wrapper I from base R. Note this will also result in not setting the labels for you: it will use the generated matrix as-is. This is useful if you explicitly create a matrix and want to make sure the reference level isn’t shifted around.

enlist_contrasts(mdl_data, carb ~ I(contr.sum))
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> Warning in .postprocess_matrix(new_contrasts, code_by, reference_level, : No
#> comparison labels set and as_is=TRUE, contrast labels will be column indices.
#> $carb
#>    1  2  3  4  5
#> 1  1  0  0  0  0
#> 2  0  1  0  0  0
#> 3  0  0  1  0  0
#> 4  0  0  0  1  0
#> 5  0  0  0  0  1
#> 6 -1 -1 -1 -1 -1

One last quick note is that you can keep the parentheses with the function, so if your tab-autocomplete puts parentheses you don’t have to worry about it.

enlist_contrasts(mdl_data, carb ~ contr.sum())
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1

However, the function does need to exist; the following won’t run:

foo <- contr.sum(6) # foo is a matrix
enlist_contrasts(mdl_data, carb ~ foo()) # foo is not a function

tidyselect functions

If you know you’re going to use the same scheme for multiple variables, but don’t need to set the reference level or labels for any one in particular, you can use tidyselect functionality to set multiple contrasts. I’ll show a few examples of how to do this.

First, you can specify multiple variables on the left hand side using the + operator.

# equivalent to: enlist_contrasts(mdl_data, cyl ~ sum_code, gear ~ sum_code)
enlist_contrasts(mdl_data, cyl + gear ~ sum_code)
#> Expect contr.treatment or contr.poly for unset factors: carb twolevel
#> $cyl
#>    6  8
#> 4 -1 -1
#> 6  1  0
#> 8  0  1
#> 
#> $gear
#>    4  5
#> 3 -1 -1
#> 4  1  0
#> 5  0  1

You can also use selecting functions like the below:

enlist_contrasts(mdl_data, where(is.factor) ~ sum_code)
#> $cyl
#>    6  8
#> 4 -1 -1
#> 6  1  0
#> 8  0  1
#> 
#> $gear
#>    4  5
#> 3 -1 -1
#> 4  1  0
#> 5  0  1
#> 
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1
#> 
#> $twolevel
#>    b
#> a -1
#> b  1
# see also enlist_contrasts(mdl_data, where(is.unordered) ~ sum_code)
# see also enlist_contrasts(mdl_data, where(is.numeric) ~ sum_code)

This also lets you programmatically set contrasts like so:

these_vars <- c("cyl", "gear")
enlist_contrasts(mdl_data, all_of(these_vars) ~ sum_code)
#> Expect contr.treatment or contr.poly for unset factors: carb twolevel
#> $cyl
#>    6  8
#> 4 -1 -1
#> 6  1  0
#> 8  0  1
#> 
#> $gear
#>    4  5
#> 3 -1 -1
#> 4  1  0
#> 5  0  1

However, you have to ensure that no variables are duplicated. The following are not allowed regardless of whether the right right hand sides evaluate to the same contrast schemes.

enlist_contrasts(mdl_data, 
                 cyl ~ sum_code, 
                 where(is.factor) ~ sum_code) # cyl is a factor for mdl_data

enlist_contrasts(mdl_data, 
                 cyl ~ sum_code, 
                 cyl + gear ~ sum_code) # cyl can't be specified twice

You can check out the tidyselect package for more information. is.unordered is provided in this package as an analogue to is.ordered.

Other ways to set contrasts

You can also set contrasts using things other than a function. Specifically, you can pass the following classes to code_by in the two-sided formulas:

array
matrix
hypr
function (must return a contrast matrix, cannot be an anonymous function)
symbol (must evaluate to a contrast matrix though)

If you’re using a custom matrix for a tricky analysis, then you can specify the matrix yourself and then pass it to set_contrasts:

my_matrix <- contr.sum(6) / 2
enlist_contrasts(mdl_data, carb ~ my_matrix)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>      2    3    4    6    8
#> 1 -0.5 -0.5 -0.5 -0.5 -0.5
#> 2  0.5  0.0  0.0  0.0  0.0
#> 3  0.0  0.5  0.0  0.0  0.0
#> 4  0.0  0.0  0.5  0.0  0.0
#> 6  0.0  0.0  0.0  0.5  0.0
#> 8  0.0  0.0  0.0  0.0  0.5

You can also specify a list of formulas and pass that to different functions.

my_contrasts <- list(carb ~ contr.sum, gear ~ scaled_sum_code)

enlist_contrasts(mdl_data, my_contrasts)
#> Expect contr.treatment or contr.poly for unset factors: cyl twolevel
#> $carb
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1
#> 
#> $gear
#>            4          5
#> 3 -0.3333333 -0.3333333
#> 4  0.6666667 -0.3333333
#> 5 -0.3333333  0.6666667
mdl_data12 <- set_contrasts(mdl_data, my_contrasts)
#> Expect contr.treatment or contr.poly for unset factors: cyl twolevel

This functionality is very important for the next function I’ll discuss.

glimpse_contrasts

It can be very useful to get a summary of the different factors and their contrasts in your dataset. The glimpse_contrasts function does this for you by passing it your dataset and your contrasts.

my_contrasts <- list(carb ~ contr.sum,
                     gear ~ treatment_code + 4,
                     twolevel ~ scaled_sum_code * "b",
                     cyl ~ helmert_code)
mdl_data$twolevel
#>  [1] a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b a b
#> Levels: a b

enlist_contrasts(mdl_data, cyl ~ scaled_sum_code + 6 * 6)
#> Expect contr.treatment or contr.poly for unset factors: gear carb twolevel
#> $cyl
#>   4 8
#> 4 1 0
#> 6 0 0
#> 8 0 1

mdl_data13 <- set_contrasts(mdl_data, my_contrasts)

glimpse_contrasts(mdl_data13, my_contrasts)
#>     factor n  level_names          scheme reference  intercept
#> 1     carb 6 1, 2, 3,....       contr.sum      <NA> grand mean
#> 2     gear 3      3, 4, 5  treatment_code         4    mean(4)
#> 3 twolevel 2         a, b scaled_sum_code      <NA>    mean(b)
#> 4      cyl 3      4, 6, 8    helmert_code      <NA> grand mean

Note that if you haven’t actually set your contrasts to the dataset, you’ll receive a series of warnings:

glimpse_contrasts(mdl_data, my_contrasts)
#> Warning: Contrasts for these factors in `mdl_data` don't match formulas:
#>  - carb
#>  - gear
#>  - twolevel
#>  - cyl
#> To fix, be sure to run:
#> mdl_data <- set_contrasts(mdl_data, my_contrasts)
#>     factor n  level_names          scheme reference  intercept
#> 1     carb 6 1, 2, 3,....       contr.sum      <NA> grand mean
#> 2     gear 3      3, 4, 5  treatment_code         4    mean(4)
#> 3 twolevel 2         a, b scaled_sum_code      <NA>    mean(b)
#> 4      cyl 3      4, 6, 8    helmert_code      <NA> grand mean

You can also specify the formulas directly, like with set_contrasts and enlist_contrasts— but usually you’ll want to have a list of formulas and pass this to the different functions. Note how we retyped everything we typed above just to get the summary table.

glimpse_contrasts(mdl_data13, 
                  carb ~ contr.sum,
                  gear ~ treatment_code + 4,
                  twolevel ~ scaled_sum_code * "b",
                  cyl ~ helmert_code)
#>     factor n  level_names          scheme reference  intercept
#> 1     carb 6 1, 2, 3,....       contr.sum      <NA> grand mean
#> 2     gear 3      3, 4, 5  treatment_code         4    mean(4)
#> 3 twolevel 2         a, b scaled_sum_code      <NA>    mean(b)
#> 4      cyl 3      4, 6, 8    helmert_code      <NA> grand mean

If you don’t provide any contrast information, glimpse_contrasts will assume that factors should be using their default contrasts. If the contrasts are not the same as the defaults, you’ll receive warnings:

glimpse_contrasts(mdl_data)  # no warnings
#>            factor n  level_names          scheme reference intercept orthogonal
#> cyl           cyl 3      4, 6, 8 contr.treatment         4   mean(4)         NA
#> gear         gear 3      3, 4, 5 contr.treatment         3   mean(3)         NA
#> carb         carb 6 1, 2, 3,.... contr.treatment         1   mean(1)         NA
#> twolevel twolevel 2         a, b contr.treatment         a   mean(a)         NA
#>          centered dropped_trends explicitly_set
#> cyl            NA             NA          FALSE
#> gear           NA             NA          FALSE
#> carb           NA             NA          FALSE
#> twolevel       NA             NA          FALSE
glimpse_contrasts(mdl_data13) # warnings
#> Warning in .warn_if_nondefault(default_contrasts, unset_factors, factor_sizes, : Unset factors do not use default contr.treatment or contr.poly. Glimpse table may be unreliable.
#>  - cyl
#>  - gear
#>  - carb
#>  - twolevel
#>            factor n  level_names scheme reference  intercept orthogonal
#> cyl           cyl 3      4, 6, 8    ???      <NA> grand mean         NA
#> gear         gear 3      3, 4, 5    ???         4    mean(4)         NA
#> carb         carb 6 1, 2, 3,....    ???         1 grand mean         NA
#> twolevel twolevel 2         a, b    ???         a    mean(b)         NA
#>          centered dropped_trends explicitly_set
#> cyl            NA             NA          FALSE
#> gear           NA             NA          FALSE
#> carb           NA             NA          FALSE
#> twolevel       NA             NA          FALSE

If there are mismatches between the data and the contrasts specified by the formulas, you’ll also get warnings:

glimpse_contrasts(mdl_data, 
                  carb ~ contr.sum, 
                  gear ~ treatment_code * 4,
                  cyl ~ contr.treatment | c("diff1", "diff2"))
#> Warning: Contrasts for these factors in `mdl_data` don't match formulas:
#>  - carb
#>  - gear
#> Comparison labels for contrasts in `mdl_data` don't match:
#>  - cyl   (expected `diff1, diff2` but found `6, 8`)
#> To fix, be sure to run:
#> mdl_data <- set_contrasts(mdl_data, 
#>                           carb ~ contr.sum,
#>                           gear ~ treatment_code * 4,
#>                           cyl ~ contr.treatment | c("diff1", "diff2"))
#>     factor n  level_names          scheme reference  intercept
#> 1     carb 6 1, 2, 3,....       contr.sum      <NA> grand mean
#> 2     gear 3      3, 4, 5  treatment_code      <NA>    mean(4)
#> 3      cyl 3      4, 6, 8 contr.treatment      <NA>    mean(4)
#> 4 twolevel 2         a, b contr.treatment         a    mean(a)

Remember that set_contrasts will automatically coerce dataframe columns into factors when specifying a contrast, but glimpse_contrasts does not because it does not modify the provided dataframe.

Note on polynomial contrasts

There is an additional operator for the formulas that only applies to polynomial contrasts. For background, polynomial contrasts encode polynomial “trends” across the levels of a factor. So, testing for a linear-like trend, a quadratic-like trend, etc. One thing you can do, though I don’t necessarily recommend it is removing higher-order trends. For instance, if you have 8 levels but don’t want to bother with polynomials over degree 3 (even though you’ll get up to degree 7 for “free”). In this situation you can use the - operator: varname ~ code_by + reference * intercept - low:high | labels

enlist_contrasts(mdl_data, carb ~ contr.poly)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>           .L         .Q         .C         ^4          ^5
#> 1 -0.5976143  0.5455447 -0.3726780  0.1889822 -0.06299408
#> 2 -0.3585686 -0.1091089  0.5217492 -0.5669467  0.31497039
#> 3 -0.1195229 -0.4364358  0.2981424  0.3779645 -0.62994079
#> 4  0.1195229 -0.4364358 -0.2981424  0.3779645  0.62994079
#> 6  0.3585686 -0.1091089 -0.5217492 -0.5669467 -0.31497039
#> 8  0.5976143  0.5455447  0.3726780  0.1889822  0.06299408
enlist_contrasts(mdl_data, carb ~ contr.poly - 3:5)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> $carb
#>           .L         .Q
#> 1 -0.5976143  0.5455447
#> 2 -0.3585686 -0.1091089
#> 3 -0.1195229 -0.4364358
#> 4  0.1195229 -0.4364358
#> 6  0.3585686 -0.1091089
#> 8  0.5976143  0.5455447

Note that you can ONLY use this with enlist_contrasts and glimpse_contrasts. This is because if you remove columns from a contrast matrix, R will fill them back with something else.

mdl_data14 <- set_contrasts(mdl_data, carb ~ contr.sum)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
contrasts(mdl_data14$carb)
#>    2  3  4  6  8
#> 1 -1 -1 -1 -1 -1
#> 2  1  0  0  0  0
#> 3  0  1  0  0  0
#> 4  0  0  1  0  0
#> 6  0  0  0  1  0
#> 8  0  0  0  0  1
contrasts(mdl_data14$carb) <- contrasts(mdl_data14$carb)[, 1:2]
contrasts(mdl_data14$carb)
#>    2  3                                    
#> 1 -1 -1 -0.23570226 -0.23570226 -0.23570226
#> 2  1  0 -0.23570226 -0.23570226 -0.23570226
#> 3  0  1 -0.23570226 -0.23570226 -0.23570226
#> 4  0  0  0.90236893 -0.09763107 -0.09763107
#> 6  0  0 -0.09763107  0.90236893 -0.09763107
#> 8  0  0 -0.09763107 -0.09763107  0.90236893

What do these values correspond to? If we check out the hypothesis matrix, we can see that it seems like it’s trying to encode 0 but with some floating point error:

MASS::fractions(
  contrastable:::.convert_matrix(
    contrasts(mdl_data14$carb)
  )
)
#>      [,1]              [,2]              [,3]              [,4]             
#> [1,]               1/6              -1/3              -1/3  -1293633/5488420
#> [2,]               1/6               2/3              -1/3  -1293633/5488420
#> [3,]               1/6              -1/3               2/3  -1293633/5488420
#> [4,]               1/6                 0                 0 11956585/13250218
#> [5,]               1/6                 0                 0 -1293633/13250218
#> [6,]               1/6                 0                 0 -1293633/13250218
#>      [,5]              [,6]             
#> [1,]  -1293633/5488420  -1293633/5488420
#> [2,]  -1293633/5488420  -1293633/5488420
#> [3,]  -1293633/5488420  -1293633/5488420
#> [4,] -1293633/13250218 -1293633/13250218
#> [5,] 11956585/13250218 -1293633/13250218
#> [6,] -1293633/13250218 11956585/13250218

This is why I don’t recommend actually using the - operator. If you use something like lm to fit functions, you can use the output from enlist_contrasts as the argument to contrasts. Although, I’m pretty sure the model matrix will just add in these values anyways. If you try to use the - operator with set_contrasts you’ll get a warning, and the - operator will be ignored:

mdl_data15 <- set_contrasts(mdl_data, carb ~ polynomial_code - 3:5)
#> Expect contr.treatment or contr.poly for unset factors: cyl gear twolevel
#> Warning: Cannot use `-` with set_contrasts, ignoring in carb ~ polynomial_code - 3:5.
#>   Use enlist_contrasts instead.

Technically, due to degrees of freedom, we’d compare only 2 of the 3 levels to the average.↩︎

Contrastable Overview

Introduction to contrasts

Motivation

Usage

enlist_contrasts

Contrast functions

tidyselect functions

Other ways to set contrasts

glimpse_contrasts

Note on polynomial contrasts

decompose_contrasts

Typical patterns

Pattern 1: No glimpsing

Pattern 2: With information about contrasts

Conclusion

Citation examples