Facebook acknowledged the plight of advertisers dealing with changes to its ad platform, mostly coming from Apple’s restrictions on sharing data from devices, and the company said that many brands are now getting imprecise metrics.

On Wednesday, Graham Mudd, Facebook’s VP of product marketing, issued a blog post outlining some of the challenges facing more than 10 million advertisers on its platform. In the past year, Apple has updated software on its ubiquitous iPhones, most recently this week with iOS 15, which has put a lock on data that marketers use to target ads and measure their effectiveness. Many marketers have reported lower conversion rates, a key metric that shows when an ad led to an actual sale or other action.

“Our estimate is that in aggregate we are underreporting iOS web conversions by approximately 15%,” Mudd said in the blog post on Wednesday. “However, there is a broad range for individual advertisers. We believe that real world conversions, like sales and app installs, are higher than what is being reported for many advertisers.”

Facebook issued a similar notice earlier this month about a bug it found in its integration with Apple’s SKAdNetwork, which is one of the new ways that apps are able to report some stripped-down data back to marketers to fill in the picture about how ads are doing. Facebook said that it found a glitch that undercounted conversions by an average of 10%. That meant that marketers that sell products and generate app downloads were getting a fuzzy picture of how their media was doing on Facebook. In this case, the error would have made Facebook ads appear less successful than they were since it was underreporting the number of successful ad transactions.

Facebook, like much of the mobile and digital ad marketplace, is trying to adjust to the new privacy edicts coming from platforms like Apple. There is an increased focus on making sure that consumers can choose when their data is shared, data such as device identifiers that could track their online whereabouts. Apple has implemented App Tracking Transparency, which has forced all apps to obtain permission from users before they can use the Apple Identifier for Advertisers. The ID is helpful for advertisers to detect when a consumer saw one of their ads on Facebook and then track that consumer to a sale on the brand’s app or website.

Facebook’s guidance on Wednesday suggests that marketers could assume that their ads are leading to 15% higher conversion rates. However, as all these changes take hold marketers have been concerned about the lack of hard data that they can rely on from Facebook. There have been worries about how Facebook could rely more on estimates rather than exact data.

Facebook has warned about the fallout from all the online data changes, suggesting they could impact millions of small businesses that have relied on sophisticated ad targeting to power their sales and growth. Facebook’s blog post mentioned the upcoming holiday season and said it was trying to adjust its ad platform in time for that critical sales period.

Facebook issued some tips to marketers, some of which it has been promoting for months to get ahead of Apple’s changes, like encouraging marketers to adopt its Conversions API, a system that helps brands track ad performance. Facebook also suggested that marketers wait three days to make some adjustments to ad campaigns, because there is a lag time before its models can offer better guidance to inform their marketing strategies.

“We expected increased headwinds from platform changes, notably the recent iOS updates, to have a greater impact in the third quarter compared to the second quarter,” Mudd said. “We know many of you are experiencing this greater impact as we are.”