[ML] Lowers multi-bucket impact thresholds used for anomaly display (e…

…lastic#24136)

* [ML] Lowers multi-bucket impact thresholds used for anomaly display

* [ML] Adjust thresholds used in multi bucket impact unit tests

x-pack/plugins/ml/common/constants/multi_bucket_impact.js

@@ -7,9 +7,12 @@
 
 
 // Thresholds for indicating the impact of multi-bucket features in an anomaly.
+// As a rule-of-thumb, a threshold value T corresponds to the multi-bucket probability
+// being 1000^(T/5) times smaller than the single bucket probability.
+// So, for example, for HIGH it is 63 times smaller.
 export const MULTI_BUCKET_IMPACT = {
-  HIGH: 4,
-  MEDIUM: 3,
+  HIGH: 3,
+  MEDIUM: 2,
   LOW: 1,
   NONE: -5
 };

x-pack/plugins/ml/common/util/__tests__/anomaly_utils.js

@@ -284,19 +284,19 @@ describe('ML - anomaly utils', () => {
 
   describe('getMultiBucketImpactLabel', () => {
 
-    it('returns high for 4 <= score <= 5', () => {
-      expect(getMultiBucketImpactLabel(4)).to.be('high');
+    it('returns high for 3 <= score <= 5', () => {
+      expect(getMultiBucketImpactLabel(3)).to.be('high');
       expect(getMultiBucketImpactLabel(5)).to.be('high');
     });
 
-    it('returns medium for 3 <= score < 4', () => {
-      expect(getMultiBucketImpactLabel(3)).to.be('medium');
-      expect(getMultiBucketImpactLabel(3.99)).to.be('medium');
+    it('returns medium for 2 <= score < 3', () => {
+      expect(getMultiBucketImpactLabel(2)).to.be('medium');
+      expect(getMultiBucketImpactLabel(2.99)).to.be('medium');
     });
 
-    it('returns low for 1 <= score < 3', () => {
+    it('returns low for 1 <= score < 2', () => {
       expect(getMultiBucketImpactLabel(1)).to.be('low');
-      expect(getMultiBucketImpactLabel(2.99)).to.be('low');
+      expect(getMultiBucketImpactLabel(1.99)).to.be('low');
     });
 
     it('returns none for -5 <= score < 1', () => {