From 3e8f66f26274fd1a7117b1d423ef8d1768cc8d4a Mon Sep 17 00:00:00 2001
From: Josh de Leeuw <josh.deleeuw@gmail.com>
Date: Wed, 10 May 2023 09:43:37 -0400
Subject: [PATCH 1/2] ignore undefined / null values for `mean()` - #2905

---
 packages/jspsych/src/modules/data/DataColumn.ts | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/packages/jspsych/src/modules/data/DataColumn.ts b/packages/jspsych/src/modules/data/DataColumn.ts
index 6e1153e3..ff29922f 100644
--- a/packages/jspsych/src/modules/data/DataColumn.ts
+++ b/packages/jspsych/src/modules/data/DataColumn.ts
@@ -10,7 +10,18 @@ export class DataColumn {
   }
 
   mean() {
-    return this.sum() / this.count();
+    let sum = 0;
+    let count = 0;
+    for (const value of this.values) {
+      if (typeof value !== "undefined" && value !== null) {
+        sum += value;
+        count++;
+      }
+    }
+    if (count === 0) {
+      return undefined;
+    }
+    return sum / count;
   }
 
   median() {

From f9eb17c376d07003a30cd1aca42ca55e8d2b7488 Mon Sep 17 00:00:00 2001
From: Josh de Leeuw <josh.deleeuw@gmail.com>
Date: Wed, 10 May 2023 09:47:04 -0400
Subject: [PATCH 2/2] add changeset

---
 .changeset/forty-weeks-walk.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/forty-weeks-walk.md

diff --git a/.changeset/forty-weeks-walk.md b/.changeset/forty-weeks-walk.md
new file mode 100644
index 00000000..b0133a1e
--- /dev/null
+++ b/.changeset/forty-weeks-walk.md
@@ -0,0 +1,5 @@
+---
+"jspsych": major
+---
+
+Changed the behavior of `DataColumn.mean()` to exclude `null` and `undefined` values from the calculation, as suggested in #2905