diff --git a/packages/utils/src/assertions.js b/packages/utils/src/assertions.js
index 26fe0bb13..e8bc70e3f 100644
--- a/packages/utils/src/assertions.js
+++ b/packages/utils/src/assertions.js
@@ -287,7 +287,7 @@ function resolveAssertionOptionsAndLhrs(baseOptions, unfilteredLhrs) {
     optionsToUse = _.merge(_.cloneDeep(presetData), optionsToUse);
   }
 
-  const {assertions = {}, matchingUrlPattern: urlPattern} = optionsToUse;
+  const {assertions = {}, matchingUrlPattern: urlPattern, aggregationMethod} = optionsToUse;
   const lhrs = urlPattern
     ? unfilteredLhrs.filter(lhr => doesLHRMatchPattern(urlPattern, lhr))
     : unfilteredLhrs;
@@ -312,6 +312,7 @@ function resolveAssertionOptionsAndLhrs(baseOptions, unfilteredLhrs) {
     assertions,
     auditsToAssert,
     medianLhrs,
+    aggregationMethod,
     lhrs: lhrs,
     url: (lhrs[0] && lhrs[0].finalUrl) || '',
   };
@@ -323,10 +324,14 @@ function resolveAssertionOptionsAndLhrs(baseOptions, unfilteredLhrs) {
  * @return {AssertionResult[]}
  */
 function getAllFilteredAssertionResults(baseOptions, unfilteredLhrs) {
-  const {assertions, auditsToAssert, medianLhrs, lhrs, url} = resolveAssertionOptionsAndLhrs(
-    baseOptions,
-    unfilteredLhrs
-  );
+  const {
+    assertions,
+    auditsToAssert,
+    medianLhrs,
+    lhrs,
+    url,
+    aggregationMethod,
+  } = resolveAssertionOptionsAndLhrs(baseOptions, unfilteredLhrs);
 
   // If we don't have any data, just return early.
   if (!lhrs.length) return [];
@@ -338,14 +343,14 @@ function getAllFilteredAssertionResults(baseOptions, unfilteredLhrs) {
     const [level, assertionOptions] = normalizeAssertion(assertions[assertionKey]);
     if (level === 'off') continue;
 
-    const lhrsToUseForAudit =
-      assertionOptions.aggregationMethod === 'median-run' ? medianLhrs : lhrs;
+    const options = {aggregationMethod, ...assertionOptions};
+    const lhrsToUseForAudit = options.aggregationMethod === 'median-run' ? medianLhrs : lhrs;
     const auditResults = lhrsToUseForAudit.map(lhr => lhr.audits[auditId]);
     const assertionResults = getAssertionResultsForAudit(
       auditId,
       auditProperty,
       auditResults,
-      assertionOptions
+      options
     );
 
     for (const result of assertionResults) {
diff --git a/packages/utils/test/assertions.test.js b/packages/utils/test/assertions.test.js
index f6ca43d80..e3635ff7e 100644
--- a/packages/utils/test/assertions.test.js
+++ b/packages/utils/test/assertions.test.js
@@ -164,6 +164,16 @@ describe('getAllAssertionResults', () => {
   });
 
   describe('aggregationMethod', () => {
+    it('should default to aggregationMethod optimistic', () => {
+      const assertions = {
+        'first-contentful-paint': ['warn', {minScore: 1}],
+        'network-requests': ['warn', {maxLength: 1}],
+      };
+
+      const results = getAllAssertionResults({assertions}, lhrs);
+      expect(results).toMatchObject([{actual: 0.8}, {actual: 2}]);
+    });
+
     it('should use aggregationMethod optimistic', () => {
       const assertions = {
         'first-contentful-paint': ['warn', {aggregationMethod: 'optimistic', minScore: 1}],
@@ -243,6 +253,26 @@ describe('getAllAssertionResults', () => {
       ]);
     });
 
+    it('should use file-wide default when set', () => {
+      const assertions = {
+        'first-contentful-paint': ['warn', {minScore: 1}],
+        'network-requests': ['warn', {maxLength: 1}],
+      };
+
+      const results = getAllAssertionResults({assertions, aggregationMethod: 'pessimistic'}, lhrs);
+      expect(results).toMatchObject([{actual: 0.6}, {actual: 4}]);
+    });
+
+    it('should override file-wide default when set', () => {
+      const assertions = {
+        'first-contentful-paint': ['warn', {minScore: 1, aggregationMethod: 'pessimistic'}],
+        'network-requests': ['warn', {maxLength: 1}],
+      };
+
+      const results = getAllAssertionResults({assertions, aggregationMethod: 'median'}, lhrs);
+      expect(results).toMatchObject([{actual: 0.6}, {actual: 3}]);
+    });
+
     it('should handle partial failure with mode optimistic', () => {
       const assertions = {
         'first-contentful-paint': ['warn', {aggregationMethod: 'optimistic'}],
diff --git a/types/assert.d.ts b/types/assert.d.ts
index b2b5ea877..ec043ae35 100644
--- a/types/assert.d.ts
+++ b/types/assert.d.ts
@@ -28,6 +28,7 @@ declare global {
 
       export interface BaseOptions {
         matchingUrlPattern?: string;
+        aggregationMethod?: AssertionAggregationMethod;
         preset?: 'lighthouse:all' | 'lighthouse:recommended';
         assertions?: Assertions;
       }