Author: Tarn Duong

# Using prim for bump hunting and estimating highest density difference regions

#### Tarn Duong

## Introduction

The Patient Rule Induction Method (PRIM) was introduced by Friedman and Fisher (1999). It is a technique from data mining for finding `interesting’ regions in high-dimensional data. We start with regression-type data (X1, Y1), …, (Xn, Yn) where Xi is d-dimensional and Yi is a scalar response variable. We are interested in the conditional expectation function

m(x) = E (Y | X = x).

## Bump hunting

In the case where we have a single sample then PRIM finds the bumps of m(x).

We use a subset of the `MASS::Boston` data set. It contains housing data measurements for 506 towns in the Boston, USA area. For the explanatory variables, we take the nitrogen oxides concentration in parts per 10 million `nox` and the average number of rooms per dwelling `rm`. The response is the per capita crime rate `crim`. We are interested in characterising those areas with higher crime rates in order to provide better support infrastructure.

    library(prim)
    data(Boston, package="MASS")
    x <- Boston[,5:6]
    y <- Boston[,1]
    boston.prim <- prim.box(x=x, y=y, threshold.type=1)

The default settings for `prim.box` are

- peeling quantile: `peel.alpha=0.05`
- pasting is carried out: `pasting=TRUE`
- pasting quantile: `paste.alpha=0.01`
- minimum box mass (proportion of points inside a box): `mass.min=0.05`
- `threshold` is the overall mean of the response variable `y`
- `threshold.type=0`

We use the default settings except that we wish to only find high crime areas {m(x) \(\ge\) `threshold`} so we set `threshold.type=1`.

We view the output using a `summary` method. This displays three columns: the box mean, the box mass, and the threshold type. Each line is a summary for each box, as well as an overall summary. The box which is asterisked indicates that it is the box which contains the rest of the data not processed by PRIM. There is one box which contains 42.89% of the towns and where the average crime rate is 7.62. This is our HDR estimate. This regions comprises the bulk of the high crime areas, and is described in terms of nitrogen oxides levels in [0.53, 0.74] and average number of rooms in [3.04, 7.07]. The other 57.11% of the towns have an average crime rate of 0.6035.

    summary(boston.prim, print.box=TRUE)
    #>           box-fun  box-mass threshold.type
    #> box1    7.6222290 0.4288538              1
    #> box2*   0.6035267 0.5711462             NA
    #> overall 3.6135236 1.0000000             NA
    #>
    #> Box limits for box1
    #>        nox     rm
    #> min 0.5332 3.0391
    #> max 0.7400 7.0718
    #>
    #> Box limits for box2
    #>        nox     rm
    #> min 0.3364 3.0391
    #> max 0.9196 9.3019

We plot the PRIM boxes, including all those towns whose crime rate exceeds 3.5. Thus verifying that the majority of high crime towns fall inside thebump.

    plot(boston.prim, col="transparent")
    points(x[y>3.5,])

There are many options for the graphical display. See the help guide for more details `?plot.prim`.

The default function applied to the response variable `y` is `mean`. We can input another function, e.g. `median`, using

    boston.prim.med <- prim.box(x=x, y=y, threshold.type=1, y.fun=median)

We compare the results: the box for the mean is in black, for the median in red:

    plot(boston.prim, col="transparent")
    plot(boston.prim.med, col="transparent", border="red", add=TRUE)
    legend("topleft", legend=c("mean", "median"), col=1:2, lty=1, bty="n")

The covariate `x` can also include categorical variables: we replace the average number of rooms per dwelling `rm` with the index of accessibility to radial highways `rad` which takes integral values from 1 to 24 inclusive.

    x2 <- Boston[,c(5,9)]
    y <- Boston[,1]
    boston.cat.prim <- prim.box(x=x2, y=y, threshold.type=1)
    summary(boston.cat.prim, print.box=TRUE)
    #>           box-fun  box-mass threshold.type
    #> box1    7.2629703 0.4822134              1
    #> box2*   0.2148022 0.5177866             NA
    #> overall 3.6135236 1.0000000             NA
    #>
    #> Box limits for box1
    #>        nox  rad
    #> min 0.5380  4.0
    #> max 0.9196 26.3
    #>
    #> Box limits for box2
    #>        nox  rad
    #> min 0.3364 -1.3
    #> max 0.9196 26.3
    plot(boston.cat.prim, col="transparent")
    points(x2[y>3.5,])

## Estimating highest density difference regions

In the case where we have 2 samples, we can label the response as a binary variable with

Yi = 1 if Xi is in sample 1

or

Yi = -1 if Xi is in sample 2.

Then PRIM finds the regions where the samples are most different. Here we have a positive HDR (where sample 1 points dominate) and a negative HDR (where sample 2 points dominate).

We look at a 3-dimensional data set `quasiflow` included in the `prim` library. It is a randomly generated data set from two normal mixture distributions whose structure mimics some light scattering data, taken from a machine known as a flow cytometer.

    library(prim)
    data(quasiflow)
    yflow <- quasiflow[,4]
    xflow <- quasiflow[,1:3]
    xflowp <- quasiflow[yflow==1,1:3]
    xflown <- quasiflow[yflow==-1,1:3]

We can think of `xflowp` as flow cytometric measurements from an HIV+ patient, and `xflown` from an HIV– patient.

    pairs(xflowp, cex=0.5, pch=16, col=grey(0,0.1), xlim=c(0,1), ylim=c(0,1))

    pairs(xflown, cex=0.5, pch=16, col=grey(0,0.1), xlim=c(0,1), ylim=c(0,1))

There are two ways of using `prim.box` to estimate where the two samples are most different (or equivalently to estimate the HDRs of the difference of the density functions). In the first way, we assume that we have suitable values for the thresholds. Then we can use `{r}= qflow.thr <- c(0.38, -0.23) qflow.prim <- prim.box(x=xflow, y=yflow, threshold=qflow.thr, threshold.type=0)`

An alternative is compute PRIM box sequences which cover the range of the response variable `y`, and then use `prim.hdr` to experiment with different threshold values. This two-step process is more efficient and faster than calling `prim.box` for each different threshold. We are happy with the positive HDR threshold so we can compute the positive HDR directly:

    qflow.hdr.pos <- prim.box(x=xflow, y=yflow, threshold=0.38, threshold.type=1)
    summary(qflow.hdr.pos)
    #>              box-fun   box-mass threshold.type
    #> box1     0.551879699 0.05808875              1
    #> box2*   -0.003431327 0.94191125             NA
    #> overall  0.028825996 1.00000000             NA

On the other hand, we are not sure about the negative HDR thresholds. So we try several different values for `threshold`.

    qflow.neg <- prim.box(x=xflow, y=yflow, threshold.type=-1)
    qflow.hdr.neg1 <- prim.hdr(qflow.neg, threshold=-0.23, threshold.type=-1)
    qflow.hdr.neg2 <- prim.hdr(qflow.neg, threshold=-0.43, threshold.type=-1)
    qflow.hdr.neg3 <- prim.hdr(qflow.neg, threshold=-0.63, threshold.type=-1)

After examining the summaries and plots,

    summary(qflow.hdr.neg1)
    #>            box-fun   box-mass threshold.type
    #> box1    -0.6767677 0.05188679             -1
    #> box2    -0.3109244 0.05197414             -1
    #> box3    -0.2778316 0.09023410             -1
    #> box4    -0.2815199 0.05057652             -1
    #> box5*    0.1580895 0.75532844             NA
    #> overall  0.0288260 1.00000000             NA

we choose `qflow.hdr.neg1` to combine with `qflow.hdr.pos`.

    qflow.prim2 <- prim.combine(qflow.hdr.pos, qflow.hdr.neg1)
    summary(qflow.prim2)
    #>            box-fun   box-mass threshold.type
    #> box1     0.5518797 0.05808875              1
    #> box2    -0.6767677 0.05188679             -1
    #> box3    -0.3109244 0.05197414             -1
    #> box4    -0.2778316 0.09023410             -1
    #> box5    -0.2815199 0.05057652             -1
    #> box6*    0.1252819 0.69723969             NA
    #> overall  0.0288260 1.00000000             NA

In the plot below, the positive HDR is coloured orange (where the HIV+ sample is more prevalent), and the negative HDR is coloured blue (where the HIV- sample is more prevalent)

    plot(qflow.prim2, x.pt=xflow, pch=16, cex=0.5, alpha=0.1)

or equivalently as a 3D scatter plot.

    plot(qflow.prim2, x.pt=xflow, pch=16, cex=0.5, alpha=0.1, splom=FALSE, colkey=FALSE, ticktype="detailed")

## References

Friedman, J. H. and Fisher, N. I. (1999) Bump-hunting for high dimensional data. Statistics and Computing 9, 123–143.

– Generated 12 May 2026.