Alexandra Bruell and Sahil Patel:

Facebook Inc. is offering millions of dollars in credits to some advertisers after discovering a glitch in a tool that tells advertisers how effective their ads may be in driving results, such as getting consumers to download an app or purchase a product.

Facebook’s “conversion lift” tool overestimated some campaign results for 12 months, the company quietly told its advertisers this month. The glitch skewed data that advertisers use to decide how much money to spend with the company.

It isn’t the first problem Facebook has discovered in its systems to measure advertisers’ campaigns, and it is not likely to dent Facebook’s ad revenue. But some ad buyers said the latest gaffe has hurt confidence in the company’s metrics at a time when many businesses are navigating the pandemic by trying to cut costs and make sure their ad spending performs.