Facebook Inc. (FB) has come under fire regarding its advertising metrics after a research analyst pointed out a discrepancy between user reach and Census data.

The social network giant claimed to reach a potential audience of 41 million people in the crucial 18 to 24 year old demographic. There is just one problem, however: according to the latest US census data, there are  only 31 million people in the US in that age group.

The discrepancy could be for several reasons, and a Facebook spokesperson was quick to note that the audience reach is “based on a number of factors including Facebook user behaviors, user demographics, location data from devices, and other factors."

Facebook’s reach estimate is designed to educate advertisers on the potential impressions the company could reach when targeting specific demographics.

Warwick Business School professor Zheni Zhang, who specializes in the study of social networks believes that the Census data is not important to potential advertisers however and proposes some reasons why the difference of figures exists.

“The figure is not accurate because its not about legal residents of the US, it is a study about potential viewers or potential visitors of advertisements,” Zhang told Investopedia in an exclusive interview.

“Traditional advertisers may go off census data to estimate their viewer demographics, but Facebook customers who want to advertise care more about effective target viewers," he said. 

Part of the problem is that US census data only includes legal residents, so it would not incorporate illegal aliens and short-term visitors, which are both groups ostensibly counted by Facebook. Facebook may also count younger viewers who lie about their age to view restricted content or multiple account users. 

“It is more of a PR strategy for Facebook, to show advertisers they have more potential viewers, but that does not mean the US has 10 million more people than we are used to seeing,” said Zhang.

In 2016, the company was scrutinized for the way it determined video views, telling ad buying agency Publicis Groupe that it overestimating the average viewing time for video advertising on its platform by between 60 and 80 percent.

"It shows the challenge on how to accurately measure the effectiveness of online advertising, which is less transparent and can be easily manipulated,” concluded Zhang. 

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