diff --git a/src/stats/normalizedIntensity.test.ts b/src/stats/normalizedIntensity.test.ts
new file mode 100644
index 0000000..7b99ad3
--- /dev/null
+++ b/src/stats/normalizedIntensity.test.ts
@@ -0,0 +1,75 @@
+import { Interval } from "../ast";
+import { Duration } from "../Duration";
+import { ConstantIntensity, RangeIntensity } from "../Intensity";
+import { normalizedIntensity } from "./normalizedIntensity";
+
+const round = (intensity: ConstantIntensity) => Math.round(intensity.value * 100) / 100;
+
+describe("normalizedIntensity()", () => {
+  it("when no intervals, returns 0 for all zones", () => {
+    expect(round(normalizedIntensity([]))).toEqual(0);
+  });
+
+  it("returns plain average for < 30-seconds workout", () => {
+    expect(
+      round(
+        normalizedIntensity([
+          {
+            type: "Interval",
+            duration: new Duration(10),
+            intensity: new ConstantIntensity(0.5),
+            comments: [],
+          },
+          {
+            type: "Interval",
+            duration: new Duration(10),
+            intensity: new ConstantIntensity(0.7),
+            comments: [],
+          },
+        ]),
+      ),
+    ).toEqual(0.6);
+  });
+
+  it("is the intensity of an interval for single-interval workout", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Interval",
+        duration: new Duration(20 * 60),
+        intensity: new ConstantIntensity(0.75),
+        comments: [],
+      },
+    ];
+    expect(round(normalizedIntensity(intervals))).toEqual(0.75);
+  });
+
+  it("leans towards higher intensity for two-interval workout", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Interval",
+        duration: new Duration(10 * 60),
+        intensity: new ConstantIntensity(0.5),
+        comments: [],
+      },
+      {
+        type: "Interval",
+        duration: new Duration(10 * 60),
+        intensity: new ConstantIntensity(1.0),
+        comments: [],
+      },
+    ];
+    expect(round(normalizedIntensity(intervals))).toEqual(0.85);
+  });
+
+  it("handles range-intensities", () => {
+    const intervals: Interval[] = [
+      {
+        type: "Warmup",
+        duration: new Duration(20 * 60),
+        intensity: new RangeIntensity(0.5, 1.0),
+        comments: [],
+      },
+    ];
+    expect(round(normalizedIntensity(intervals))).toEqual(0.79);
+  });
+});
diff --git a/src/stats/normalizedIntensity.ts b/src/stats/normalizedIntensity.ts
index 656082e..c2346f5 100644
--- a/src/stats/normalizedIntensity.ts
+++ b/src/stats/normalizedIntensity.ts
@@ -7,11 +7,16 @@ import { intervalsToIntensityNumbers } from "./intervalsToIntensityNumbers";
 // Starting at the beginning of the data, calculate 30-second rolling average
 const windowSize = 30; // equals to nr of seconds, but also to nr of entries in intensities array
 const rollingAverages = (intensities: number[]): number[] => {
-  if (intensities.length < windowSize) {
-    throw new Error(`Workout must be at least ${windowSize} seconds long`);
-  }
-  const averages: number[] = [];
   let rollingSum: number = sum(intensities.slice(0, windowSize));
+
+  if (intensities.length === 0) {
+    return [0];
+  }
+  if (intensities.length < windowSize) {
+    return [rollingSum / intensities.length];
+  }
+
+  const averages: number[] = [];
   averages.push(rollingSum / windowSize);
   for (let i = 0; i < intensities.length - windowSize; i++) {
     rollingSum -= intensities[i];