SAN FRANCISCO: Facebook is apologising to advertisers for what it calls an error that overstated the average length of time users watched videos on the site.

The measurement didn't affect how much Facebook charges to run video spots, but analysts say ad agencies may have used the Facebook estimates as a key metric when they plan campaigns and decide how much advertising to place on Facebook or competing sites.

Facebook executive David Fischer said his company recently discovered its method of calculating the average viewing time didn't include times when people watched a video for less than three seconds. That had the effect of making average times seem longer.

Fischer said Facebook has corrected the error, but analysts say it underscores the need for independent verification of such advertising metrics.