diff --git a/packages/apollo-engine-reporting/src/__tests__/contextualizedStats.test.ts b/packages/apollo-engine-reporting/src/__tests__/contextualizedStats.test.ts
index e6f9301dc8d..e82678913fe 100644
--- a/packages/apollo-engine-reporting/src/__tests__/contextualizedStats.test.ts
+++ b/packages/apollo-engine-reporting/src/__tests__/contextualizedStats.test.ts
@@ -1,124 +1,332 @@
-import {
-  Trace,
-} from 'apollo-engine-reporting-protobuf';
-import { dateToProtoTimestamp } from "../treeBuilder";
-import { ContextualizedStats } from "../contextualizedStats";
-import { DurationHistogram } from "../durationHistogram";
+import { Trace } from 'apollo-engine-reporting-protobuf';
+import { dateToProtoTimestamp } from '../treeBuilder';
+import { ContextualizedStats } from '../contextualizedStats';
+import { DurationHistogram } from '../durationHistogram';
 
 describe('Check query latency stats when', () => {
-  const statsContext =  {
-    clientReferenceId: "reference",
-    clientVersion: "version"
-  }
+  const statsContext = {
+    clientReferenceId: 'reference',
+    clientVersion: 'version',
+  };
 
-  let baseDate = new Date();
-  let nonFederatedTrace =  new Trace({
+  const baseDate = new Date();
+  const duration = 30 * 1000;
+  const nonFederatedTrace = new Trace({
     startTime: dateToProtoTimestamp(baseDate),
-    endTime: dateToProtoTimestamp(new Date(baseDate.getTime() + 30*1000)),
-    durationNs: 30*1000,
+    endTime: dateToProtoTimestamp(new Date(baseDate.getTime() + duration)),
+    durationNs: duration,
     root: null,
-    signature: "signature",
+    signature: 'signature',
     details: null,
   });
 
   it('adding a single trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
+    const contextualizedStats = new ContextualizedStats(statsContext);
     contextualizedStats.addTrace(nonFederatedTrace);
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.latencyCount).toStrictEqual(new DurationHistogram().incrementDuration(30*1000));
-    expect(contextualizedStats.queryLatencyStats.requestsWithErrorsCount).toBe(0);
+    expect(contextualizedStats.queryLatencyStats.latencyCount).toStrictEqual(
+      new DurationHistogram().incrementDuration(duration),
+    );
+    expect(contextualizedStats.queryLatencyStats.requestsWithErrorsCount).toBe(
+      0,
+    );
   });
   it('adding a fully cached trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      fullQueryCacheHit: true
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        fullQueryCacheHit: true,
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
     expect(contextualizedStats.queryLatencyStats.cacheHits).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.cacheLatencyCount).toStrictEqual(new DurationHistogram().incrementDuration(30*1000));
+    expect(
+      contextualizedStats.queryLatencyStats.cacheLatencyCount,
+    ).toStrictEqual(new DurationHistogram().incrementDuration(duration));
   });
   it('adding a public cached trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      fullQueryCacheHit: false,
-      cachePolicy: {
-        scope: Trace.CachePolicy.Scope.PRIVATE,
-        maxAgeNs: 1000
-      }
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        fullQueryCacheHit: false,
+        cachePolicy: {
+          scope: Trace.CachePolicy.Scope.PRIVATE,
+          maxAgeNs: 1000,
+        },
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.privateCacheTtlCount).toStrictEqual(new DurationHistogram().incrementDuration(1000));
+    expect(
+      contextualizedStats.queryLatencyStats.privateCacheTtlCount,
+    ).toStrictEqual(new DurationHistogram().incrementDuration(1000));
   });
   it('adding a private cached trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      fullQueryCacheHit: false,
-      cachePolicy: {
-        scope: Trace.CachePolicy.Scope.PUBLIC,
-        maxAgeNs: 1000
-      }
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        fullQueryCacheHit: false,
+        cachePolicy: {
+          scope: Trace.CachePolicy.Scope.PUBLIC,
+          maxAgeNs: 1000,
+        },
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.publicCacheTtlCount).toStrictEqual(new DurationHistogram().incrementDuration(1000));
+    expect(
+      contextualizedStats.queryLatencyStats.publicCacheTtlCount,
+    ).toStrictEqual(new DurationHistogram().incrementDuration(1000));
   });
   it('adding a persisted hit trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      persistedQueryHit: true
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        persistedQueryHit: true,
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
     expect(contextualizedStats.queryLatencyStats.persistedQueryHits).toBe(1);
   });
   it('adding a persisted miss trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      persistedQueryRegister: true
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        persistedQueryRegister: true,
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
     expect(contextualizedStats.queryLatencyStats.persistedQueryMisses).toBe(1);
   });
   it('adding a forbidden trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      forbiddenOperation: true,
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        forbiddenOperation: true,
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.forbiddenOperationCount).toBe(1);
+    expect(contextualizedStats.queryLatencyStats.forbiddenOperationCount).toBe(
+      1,
+    );
   });
   it('adding a registered trace', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      registeredOperation: true,
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        registeredOperation: true,
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.registeredOperationCount).toBe(1);
+    expect(contextualizedStats.queryLatencyStats.registeredOperationCount).toBe(
+      1,
+    );
   });
   it('adding an errored trace ', () => {
-    let contextualizedStats = new ContextualizedStats(statsContext);
-    contextualizedStats.addTrace(new Trace({
-      ...nonFederatedTrace,
-      registeredOperation: true,
-      root: {
-        child: [{
-          responseName: "user",
-          parentType: "Query",
-          type: "User!",
-          error: [{
-            message: "error 1"
-          }]
-        }]
-      }
-    }));
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        registeredOperation: true,
+        root: {
+          child: [
+            {
+              responseName: 'user',
+              parentType: 'Query',
+              type: 'User!',
+              error: [
+                {
+                  message: 'error 1',
+                },
+              ],
+            },
+          ],
+        },
+      }),
+    );
     expect(contextualizedStats.queryLatencyStats.requestCount).toBe(1);
-    console.log(contextualizedStats.queryLatencyStats.rootErrorStats);
-    expect(contextualizedStats.queryLatencyStats.rootErrorStats.children["user"].requestsWithErrorsCount).toBe(1);
-    expect(contextualizedStats.queryLatencyStats.rootErrorStats.children["user"].errorsCount).toBe(1);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .requestsWithErrorsCount,
+    ).toBe(1);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .errorsCount,
+    ).toBe(1);
   });
-})
+  it('merging errored traces', () => {
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        registeredOperation: true,
+        root: {
+          child: [
+            {
+              responseName: 'user',
+              parentType: 'Query',
+              type: 'User!',
+              error: [
+                {
+                  message: 'error 1',
+                },
+              ],
+            },
+          ],
+        },
+      }),
+    );
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        registeredOperation: true,
+        root: {
+          child: [
+            {
+              responseName: 'account',
+              parentType: 'Query',
+              type: 'Account!',
+              child: [
+                {
+                  responseName: 'name',
+                  parentType: 'Account',
+                  type: 'String!',
+                  error: [
+                    {
+                      message: 'has error',
+                    },
+                  ],
+                },
+              ],
+            },
+          ],
+        },
+      }),
+    );
+    for (let _ in [1, 2]) {
+      contextualizedStats.addTrace(
+        new Trace({
+          ...nonFederatedTrace,
+          registeredOperation: true,
+          root: {
+            child: [
+              {
+                responseName: 'user',
+                parentType: 'Query',
+                type: 'User!',
+                child: [
+                  {
+                    responseName: 'email',
+                    parentType: 'User',
+                    type: 'String!',
+                    error: [
+                      {
+                        message: 'has error',
+                      },
+                    ],
+                  },
+                ],
+              },
+            ],
+          },
+        }),
+      );
+    }
+
+    expect(contextualizedStats.queryLatencyStats.requestCount).toBe(4);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .requestsWithErrorsCount,
+    ).toBe(1);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .errorsCount,
+    ).toBe(1);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .children['email'].requestsWithErrorsCount,
+    ).toBe(2);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['user']
+        .children['email'].errorsCount,
+    ).toBe(2);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['account']
+        .requestsWithErrorsCount,
+    ).toBeFalsy();
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['account']
+        .errorsCount,
+    ).toBeFalsy();
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['account']
+        .children['name'].requestsWithErrorsCount,
+    ).toBe(1);
+    expect(
+      contextualizedStats.queryLatencyStats.rootErrorStats.children['account']
+        .children['name'].errorsCount,
+    ).toBe(1);
+  });
+  it('merging non-errored traces', () => {
+    const contextualizedStats = new ContextualizedStats(statsContext);
+    contextualizedStats.addTrace(nonFederatedTrace);
+    contextualizedStats.addTrace(nonFederatedTrace);
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        fullQueryCacheHit: false,
+        cachePolicy: {
+          scope: Trace.CachePolicy.Scope.PRIVATE,
+          maxAgeNs: 1000,
+        },
+      }),
+    );
+    contextualizedStats.addTrace(
+      new Trace({
+        ...nonFederatedTrace,
+        fullQueryCacheHit: false,
+        cachePolicy: {
+          scope: Trace.CachePolicy.Scope.PRIVATE,
+          maxAgeNs: 1000,
+        },
+      }),
+    );
+    for (let _ in [1, 2]) {
+      contextualizedStats.addTrace(
+        new Trace({
+          ...nonFederatedTrace,
+          fullQueryCacheHit: true,
+        }),
+      );
+    }
+    expect(contextualizedStats.queryLatencyStats.requestCount).toBe(6);
+    expect(contextualizedStats.queryLatencyStats.latencyCount).toStrictEqual(
+      new DurationHistogram()
+        .incrementDuration(duration)
+        .incrementDuration(duration)
+        .incrementDuration(duration)
+        .incrementDuration(duration),
+    );
+    expect(contextualizedStats.queryLatencyStats.requestsWithErrorsCount).toBe(
+      0,
+    );
+    expect(
+      contextualizedStats.queryLatencyStats.privateCacheTtlCount,
+    ).toStrictEqual(
+      new DurationHistogram().incrementDuration(1000).incrementDuration(1000),
+    );
+    expect(contextualizedStats.queryLatencyStats.cacheHits).toBe(2);
+    expect(
+      contextualizedStats.queryLatencyStats.cacheLatencyCount,
+    ).toStrictEqual(
+      new DurationHistogram()
+        .incrementDuration(duration)
+        .incrementDuration(duration),
+    );
+  });
+});