diff --git a/R/utils.R b/R/utils.R
index 34e48b5a..aaa82e46 100644
--- a/R/utils.R
+++ b/R/utils.R
@@ -270,6 +270,11 @@ perm2 <- function(r, v) {
 #'
 #' @keywords internal
 .create_hypotheses_mat <- function(histories, reference, comparison) {
+  # Create a matrix with length(histories) rows.
+  # Each column sets the value -1 for the reference group row
+  # and the value 1 for the comparison group row. 
+  # When multiplied by the predicted values it does 
+  # R' b = b_comparison - b_reference
   sub_mats <- lapply(reference, function(ref) {
     n_cols <- length(comparison)
     cus_comps <- matrix(0, ncol = n_cols, nrow = length(histories))
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
index 4bf4e8e4..44990c04 100644
--- a/tests/testthat/test-utils.R
+++ b/tests/testthat/test-utils.R
@@ -12,3 +12,16 @@ test_that(".extract_time_pts_from_vars", {
   ) 
   expect_equal(unname(x2), c(1, 2, NA))
 })
+
+
+test_that(".create_hypotheses_mat", {
+  mat = .create_hypotheses_mat(
+    histories = c("a", "b", "c"), 
+    reference = "c", 
+    comparison = c("a", "b")
+  )
+
+  expect_equal(mat[, 1], c(1, 0, -1))
+  expect_equal(mat[, 2], c(0, 1, -1))
+  expect_equal(colnames(mat), c("(a) - (c)", "(b) - (c)"))
+})