diff --git a/NEWS.md b/NEWS.md
index 8417bae5..e7fc9138 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -20,7 +20,7 @@
 * Default to `quantiles=100` for all dot plots by @behramulukir (#402)
 * Use `"neff_ratio"` consistently in diagnostic color scale helpers to avoid relying on partial matching of `"neff"`.
 * Replace `expand = c(mult, add)` with `ggplot2::expansion()` helper in scale functions for consistency with ggplot2 >= 3.3.0 style.
-* Replace uses of `geom_bar(stat = "identity")` with the more idiomatic ggplot2 form `geom_col()` 
+* Replace uses of `geom_bar(stat = "identity")` with the more idiomatic ggplot2 form `geom_col()`
 * New function `ppc_rootogram_grouped` for grouped rootogram plots by @behramulukir and @jgabry (#419)
 
 # bayesplot 1.15.0
@@ -37,7 +37,7 @@
 
 # bayesplot 1.14.0
 
-* PPC "avg" functions (`ppc_scatter_avg()`, `ppc_error_scatter_avg()`, etc.) gain a `stat` argument 
+* PPC "avg" functions (`ppc_scatter_avg()`, `ppc_error_scatter_avg()`, etc.) gain a `stat` argument
   to set the averaging function. (Suggestion of #348, @kruschke).
 * `ppc_error_scatter_avg_vs_x(x = some_expression)` labels the x axis with `some_expression`.
 * New quantile dot plot functions `ppc_dots()` and `ppd_dots()` by @behramulukir (#357)
@@ -59,7 +59,7 @@
 
 * Expand checking workflows to more platforms by @andrjohns (#324)
 * Skip tests depending on Suggested dependency rstantools if not installed by @MichaelChirico (#325)
-* Skip tests depending on Suggested dependency gridExtra if not installed by @MichaelChirico (#326) 
+* Skip tests depending on Suggested dependency gridExtra if not installed by @MichaelChirico (#326)
 * Fix missing legends for unobserved levels in rhat and neff plots (#328)
 * Document problems with `ppc_stat` with `stat="mean"` (#329)
 * Ensure rank overlay plot starts at 0 even if not all bins present, thanks @sims1253 (#332)
diff --git a/tests/testthat/test-ppc-distributions.R b/tests/testthat/test-ppc-distributions.R
index 93b52e0a..34e1c82f 100644
--- a/tests/testthat/test-ppc-distributions.R
+++ b/tests/testthat/test-ppc-distributions.R
@@ -129,6 +129,97 @@ test_that("ppc_violin_grouped returns a ggplot object", {
   expect_gg(ppc_violin_grouped(y, yrep, group, y_draw = "both", y_jitter = 0.3))
 })
 
+# ppc_data / ppd_data tests -----------------------------------------------
+
+test_that("ppc_data returns the correct structure", {
+  y_small <- c(10, 20)
+  yrep_small <- rbind(c(11, 21), c(12, 22))
+
+  d <- ppc_data(y_small, yrep_small)
+
+  expect_s3_class(d, "data.frame")
+  expect_named(d, c("y_id", "y_name", "rep_id", "rep_label",
+                    "is_y", "is_y_label", "value"))
+  expect_equal(nrow(d), length(y_small) * (nrow(yrep_small) + 1))
+  expect_equal(d$y_id, c(1L, 1L, 2L, 2L, 1L, 2L))
+  expect_equal(as.character(d$y_name), c("1", "1", "2", "2", "1", "2"))
+  expect_equal(d$rep_id, c(1L, 2L, 1L, 2L, NA, NA))
+  expect_equal(d$is_y, c(FALSE, FALSE, FALSE, FALSE, TRUE, TRUE))
+  expect_equal(d$value, c(11, 12, 21, 22, 10, 20))
+  expect_equal(d$value[d$is_y], y_small)
+
+  first_level <- levels(d$rep_label)[1]
+  expect_true(all(as.character(d$rep_label[d$is_y]) == first_level))
+  expect_true(all(as.character(d$rep_label[!d$is_y]) != first_level))
+})
+
+test_that("ppc_data carries group through correctly", {
+  y_small <- c(10, 20)
+  yrep_small <- rbind(c(11, 21), c(12, 22))
+  group_small <- factor(c("a", "b"))
+
+  d <- ppc_data(y_small, yrep_small, group = group_small)
+
+  expect_named(d, c("group", "y_id", "y_name", "rep_id", "rep_label",
+                    "is_y", "is_y_label", "value"))
+  expect_equal(as.character(d$group), c("a", "a", "b", "b", "a", "b"))
+  expect_equal(as.character(d$group[d$is_y]), as.character(group_small))
+})
+
+test_that("ppc_data handles a single replicate matrix", {
+  y_small <- c(10, 20)
+  yrep_small <- matrix(c(11, 21), nrow = 1)
+
+  d <- ppc_data(y_small, yrep_small)
+
+  expect_equal(sum(!d$is_y), length(y_small))
+  expect_equal(d$rep_id[!d$is_y], c(1L, 1L))
+  expect_equal(d$value[!d$is_y], c(11, 21))
+})
+
+test_that("ppd_data returns the correct structure", {
+  yrep_small <- rbind(c(11, 21), c(12, 22))
+
+  d <- ppd_data(yrep_small)
+
+  expect_s3_class(d, "data.frame")
+  expect_named(d, c("y_id", "y_name", "rep_id", "rep_label", "value"))
+  expect_equal(nrow(d), nrow(yrep_small) * ncol(yrep_small))
+  expect_equal(d$y_id, c(1L, 1L, 2L, 2L))
+  expect_equal(as.character(d$y_name), c("1", "1", "2", "2"))
+  expect_equal(d$rep_id, c(1L, 2L, 1L, 2L))
+  expect_equal(d$value, c(11, 12, 21, 22))
+  expect_true(all(grepl("pred", levels(d$rep_label), fixed = TRUE)))
+})
+
+test_that("ppd_data carries group through correctly", {
+  yrep_small <- rbind(c(11, 21), c(12, 22))
+  group_small <- factor(c("a", "b"))
+
+  d <- ppd_data(yrep_small, group = group_small)
+
+  expect_named(d, c("group", "y_id", "y_name", "rep_id", "rep_label", "value"))
+  expect_equal(as.character(d$group), c("a", "a", "b", "b"))
+})
+
+test_that("ppd_data carries observation names through to y_name", {
+  yrep_named <- rbind(c(11, 21), c(12, 22))
+  colnames(yrep_named) <- c("obs_a", "obs_b")
+
+  d <- ppd_data(yrep_named)
+
+  expect_equal(as.character(d$y_name), c("obs_a", "obs_a", "obs_b", "obs_b"))
+})
+
+test_that("ppd_data handles a single replicate matrix", {
+  yrep_small <- matrix(c(11, 21), nrow = 1)
+
+  d <- ppd_data(yrep_small)
+
+  expect_equal(nrow(d), ncol(yrep_small))
+  expect_equal(d$rep_id, c(1L, 1L))
+  expect_equal(d$value, c(11, 21))
+})
 
 
 # Visual tests -----------------------------------------------------------------