It’s happened again. For the fourth time since September and the second time this month, Facebook has disclosed a measurement error.

Facebook had been undercounting iPhone traffic for some publishers using the social network’s proprietary Instant Articles format, the company announced on Friday in an update to a previously published blog post.

Measurement firm ComScore identified the error, which affected publishers’ traffic measurements from September 20 through November 30. Facebook said it has fixed the issue and is working with ComScore to generate corrected traffic estimates for that time period.

The measurement error spawned from a Facebook update that affected publishers who use “our legacy comScore [sic] integration” and whose sites run on the more secure HTTPS protocol, according to the social network.

For most of the publishers exposed to the error, less than 1 percent of their traffic was affected, according to a report by The Wall Street Journal. That reported also said that for some publishers, 10 to 20 percent of traffic was affected. A ComScore spokesperson referred a question asking the company to confirm those figures to Facebook because the measurement firm has not yet produced updated traffic estimates. A spokesperson for Facebook, who earlier confirmed the measurement error, has not responded to a follow-up questions asking the company to confirm the figures cited by The Wall Street Journal.

This isn’t the first Instant Articles-related measurement error Facebook has disclosed. In November, the company said it had been incorrectly calculating the average amount of time people spent reading an Instant Article, leading to the stat being inflated by 7 to 8 percent, on average.

But it is the latest measurement error that Facebook has disclosed (We’ve kept a running tally). The string of inaccuracies has impacted some marketers’ trust in Facebook’s own measurements and created more urgency among marketers for Facebook to allow third-party firms to directly measure the performance of their ads and content, as DigitasLBi exec Jeanne Bright discussed in this week’s episode of the Marketing Land Live podcast.

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