core: ensure good and average scores start exactly at control points

lighthouse-core/audits/audit.js

@@ -73,13 +73,28 @@ class Audit {
    * considering a log-normal distribution governed by two control points (the 10th
    * percentile value and the median value) and represents the percentage of sites that are
    * greater than `value`.
+   *
+   * Score characteristics:
+   * - within [0, 1]
+   * - rounded to two digits
+   * - value must meet or beat a controlPoint value to meet or exceed its percentile score:
+   *   - value > median will give a score < 0.5; value ≤ median will give a score ≥ 0.5.
+   *   - value > p10 will give a score < 0.9; value ≤ p10 will give a score ≥ 0.9.
+   * - values < p10 will get a slight boost so a score of 1 is achievable by a
+   *   `value` other than those close to 0. Scores of > ~0.99524 end up rounded to 1.
    * @param {{median: number, p10: number}} controlPoints
    * @param {number} value
    * @return {number}
    */
   static computeLogNormalScore(controlPoints, value) {
-    const percentile = statistics.getLogNormalScore(controlPoints, value);
-    return clampTo2Decimals(percentile);
+    let percentile = statistics.getLogNormalScore(controlPoints, value);
+    // Add a boost to scores of 90+, linearly ramping from 0 at 0.9 to half a
+    // point (0.005) at 1. Expands scores in (0.9, 1] to (0.9, 1.005], so more top
+    // scores will be a perfect 1 after the two-digit `Math.floor()` rounding below.
+    if (percentile > 0.9) { // getLogNormalScore ensures `percentile` can't exceed 1.
+      percentile += 0.05 * (percentile - 0.9);
+    }
+    return Math.floor(percentile * 100) / 100;
   }
 
   /**

lighthouse-core/test/audits/audit-test.js

@@ -280,10 +280,40 @@ describe('Audit', () => {
 
       assert.strictEqual(Audit.computeLogNormalScore(params, 0), 1);
       assert.strictEqual(Audit.computeLogNormalScore(params, 250), 0.99);
-      assert.strictEqual(Audit.computeLogNormalScore(params, 1500), 0.23);
-      assert.strictEqual(Audit.computeLogNormalScore(params, 2500), 0.05);
-      assert.strictEqual(Audit.computeLogNormalScore(params, 4000), 0.01);
-      assert.strictEqual(Audit.computeLogNormalScore(params, 4100), 0);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 1500), 0.22);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 2500), 0.04);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 3500), 0.01);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 3600), 0);
+    });
+
+    it('correctly bins scores relative to control points and allows achievable 100s', () => {
+      const params = {
+        median: 800,
+        p10: 200,
+      };
+
+      // Clamps negative values to a score of 1.
+      assert.strictEqual(Audit.computeLogNormalScore(params, -100), 1);
+
+      // 0 value is always scored with a 1.
+      assert.strictEqual(Audit.computeLogNormalScore(params, 0), 1);
+
+      // A really good value has its score rounded up to 1.
+      assert.strictEqual(Audit.computeLogNormalScore(params, 25), 1);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 50), 0.99);
+
+      // p10 param gets a 0.9.
+      assert.strictEqual(Audit.computeLogNormalScore(params, params.p10), 0.9);
+      // Anything worse than p10 gets < 0.9.
+      assert.strictEqual(Audit.computeLogNormalScore(params, params.p10 + 1), 0.89);
+
+      // Median param gets a 0.5.
+      assert.strictEqual(Audit.computeLogNormalScore(params, params.median), 0.5);
+      // Anything worse than the median gets < 0.5.
+      assert.strictEqual(Audit.computeLogNormalScore(params, params.median + 1), 0.49);
+
+      assert.strictEqual(Audit.computeLogNormalScore(params, 8_000), 0.01);
+      assert.strictEqual(Audit.computeLogNormalScore(params, 10_000), 0);
     });
   });
 });

lighthouse-core/test/audits/mainthread-work-breakdown-test.js

@@ -77,7 +77,7 @@ describe('Performance: page execution timings audit', () => {
 
     assert.equal(Math.round(output.numericValue), 4081);
     assert.equal(output.details.items.length, 7);
-    assert.equal(output.score, 0.49);
+    assert.equal(output.score, 0.48);
   });
 
   it('should compute the correct values for the redirect trace', async () => {

lighthouse-core/test/audits/metrics/first-contentful-paint-test.js

@@ -65,7 +65,7 @@ describe('Performance: first-contentful-paint audit', () => {
 
     const context = getFakeContext({formFactor: 'mobile', throttlingMethod: 'provided'});
     const result = await FcpAudit.audit(artifacts, context);
-    assert.equal(result.score, 0.06);
+    assert.equal(result.score, 0.05);
     assert.equal(result.numericValue, 5668.275);
   });
 });

lighthouse-core/test/results/sample_v2.json

@@ -63,7 +63,7 @@
       "id": "first-contentful-paint",
       "title": "First Contentful Paint",
       "description": "First Contentful Paint marks the time at which the first text or image is painted. [Learn more](https://web.dev/first-contentful-paint/).",
-      "score": 0.02,
+      "score": 0.01,
       "scoreDisplayMode": "numeric",
       "numericValue": 6843.95,
       "numericUnit": "millisecond",
@@ -4191,7 +4191,7 @@
       "id": "uses-long-cache-ttl",
       "title": "Serve static assets with an efficient cache policy",
       "description": "A long cache lifetime can speed up repeat visits to your page. [Learn more](https://web.dev/uses-long-cache-ttl/).",
-      "score": 0.03,
+      "score": 0.02,
       "scoreDisplayMode": "numeric",
       "numericValue": 1258057,
       "numericUnit": "byte",

report/test/generator/report-generator-test.js

@@ -97,7 +97,7 @@ describe('ReportGenerator', () => {
       expect(lines.slice(0, 3).join('\n')).toMatchInlineSnapshot(`
 "requestedUrl,finalUrl,category,name,title,type,score
 \\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"Performance\\",\\"performance-score\\",\\"Overall Performance Category Score\\",\\"numeric\\",\\"0.26\\"
-\\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"Performance\\",\\"first-contentful-paint\\",\\"First Contentful Paint\\",\\"numeric\\",\\"0.02\\"
+\\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"http://localhost:10200/dobetterweb/dbw_tester.html\\",\\"Performance\\",\\"first-contentful-paint\\",\\"First Contentful Paint\\",\\"numeric\\",\\"0.01\\"
 "
 `);