Firefox Telemetry records the usage of certain web features via a mechanism called Use Counters. Essentially, for every document that Firefox loads, we record a “false” if the document didn’t use a counted feature, and a “true” if the document did use that counted feature.

(( We technically count it when the documents are destroyed, not loaded, since a document could use a feature at any time during its lifetime. We also count top-level documents (pages) separately from the count of all documents (including iframes), so we can see if it is the pages that users load that are using a feature or if it’s the subdocuments that the page author loads on the user’s behalf that are contributing the counts. ))

To save space, we decided to count the number of documents once, and the number of “true” values in each use counter. This saved users from having to tell us they didn’t use any of Feature 1, Feature 2, Feature 5, Feature 7, … the “no-use” use counters. They could just tell us which features they did see used, and we could work out the rest.

Only, we got it wrong.

The server-side adjustment of the counts took every use counter we were told about, and filled in the “false” values. A simple fix.

But it didn’t add in the “no-use” use counters. Users who didn’t see a feature used at all weren’t having their “false” values counted.

This led us to under-count the number of “false” values (since we only counted “falses” from users who had at least one “true”), which led us to overestimate the usage of features.

Of all the errors to have, this one was probably the more benign. In failing in the “overestimate” direction we didn’t incorrectly remove features that were being used more than measured… but we may have kept some features that we could have removed, costing mozilla time and energy for their maintenance.

Once we detected the fault, we started addressing it. First, we started educating people whenever the topic came up in email and bugzilla. Second, :gfritzsche added a fancy Use Counter Dashboard that did a client-side adjustment using the correct “true” and “false” values for a given population.

Third, and finally, we fixed the server-side aggregator service to serve the correct values for all data, current and historical.

And that brings us to today: Use Counters are fixed! Please use them, they’re kind of cool.

:chutten