Author: Dr. Simon Müller
Date: 2026-02-26

# Get Started

#### Dr. Simon Müller

#### 2026-02-26

# 1 Motivation

Case-Based Reasoning (CBR) solves new problems by finding similar past cases. This package uses regression models—Cox Proportional Hazards (CPH), linear, and logistic—to define a principled distance between cases based on model coefficients. The workflow is: prepare data, fit a model, then query for similar cases.

# 2 Cox Proportional Hazard Model

We demonstrate the CPH model using the `ovarian` dataset from the survival package.

    ovarian$resid.ds <- factor(ovarian$resid.ds)
    ovarian$rx <- factor(ovarian$rx)
    ovarian$ecog.ps <- factor(ovarian$ecog.ps)

    # initialize R6 object
    cph_model <- CoxModel$new(Surv(futime, fustat) ~ age + resid.ds + rx + ecog.ps, ovarian)

During initialization, cases with missing values are removed via `na.omit` and character variables are converted to factors.

# 3 Available Models

The package provides four model classes for estimating case similarity:

## 3.1 Linear Regression

- Simple, fast, and interpretable via coefficients.
- Limited to continuous dependent variables.

## 3.2 Logistic Regression

- Suited for binary outcomes (e.g., success/failure).
- Assumes a linear relationship on the logit scale.

## 3.3 Cox Proportional Hazards Regression

- Designed for time-to-event (survival) data with right-censoring.
- Assumes constant hazard ratios over time.

## 3.4 Random Forests

- Captures non-linear relationships and feature interactions.
- More computationally expensive and less interpretable than regression models.

# 4 Case Based Reasoning

## 4.1 Search for Similar Cases

We split the data into training and query sets, then retrieve the most similar training cases for each query case.

    set.seed(42)
    n <- nrow(ovarian)
    trainID <- sample(1:n, floor(0.8 * n), FALSE)
    testID <- (1:n)[-trainID]

    cph_model <- CoxModel$new(Surv(futime, fustat) ~ age + resid.ds + rx + ecog.ps, ovarian[trainID, ])

    # fit model
    cph_model$fit()

    # get similar cases
    matched_data_tbl <- cph_model$get_similar_cases(query = ovarian[testID, ], k = 3)
    knitr::kable(head(matched_data_tbl))

| futime | fustat | age | resid.ds | rx | ecog.ps | scDist | caseId
--- | --- | --- | --- | --- | --- | --- | --- | ---
10 | 563 | 1 | 55.1781 | 1 | 2 | 2 | 0.7533753 | 1
7 | 464 | 1 | 56.9370 | 2 | 2 | 2 | 1.1760552 | 2
24 | 353 | 1 | 63.2192 | 1 | 2 | 2 | 1.4624169 | 3
71 | 464 | 1 | 56.9370 | 2 | 2 | 2 | 0.3736327 | 1
241 | 353 | 1 | 63.2192 | 1 | 2 | 2 | 0.9489132 | 2
14 | 770 | 0 | 57.0521 | 2 | 2 | 1 | 1.0646258 | 3

After identifying the similar cases, you can extract them along with the verum data and compile them together. However, keep in mind the following notes:

Note 1: During the initialization step, we removed all cases with missing values in the data and endPoint variables. Therefore, it is crucial to perform a missing value analysis before proceeding.

Note 2: The data.frame returned from `cph_model$get_similar_cases` includes four additional columns:

1. `caseId`: This column allows you to map the similar cases to cases in the data. For example, if you had chosen k=3, the first three elements in the caseId column will be 1 (followed by three 2’s, and so on). These three cases are the three most similar cases to case 0 in the verum data.
2. `scDist`: The calculated distance between the cases.
3. `scCaseId`: Grouping number of the query case with its matched data.
4. `group`: Grouping indicator for matched or query data.

These additional columns aid in organizing and interpreting the results, ensuring a clear understanding of the most similar cases and their corresponding query cases.

## 4.2 Check Proportional Hazard Assumption

Verify that the proportional hazards assumption holds for the fitted model:

    cph_model$check_ph()

# 5 Distance Matrix Calculation

You can also compute and visualize the full distance matrix:

    distance_matrix <- cph_model$calc_distance_matrix()
    heatmap(distance_matrix)

`cph_model$calc_distance_matrix()` computes the distance matrix between the train and test data. If test data is omitted, it calculates distances within the training data. Rows correspond to training observations and columns to test observations. The result is also stored internally as `cph_model$dist_matrix`.