|March 18th, 2012|
|vegetarianism, giving, charity_evaluation|
I wrote a before about a survey that tries to estimate effect on people's eating habits of seeing Facebook ads for the Hidden Face of Food. I'm following up to look more at sample bias: how the people taking the survey might be different from the overall group we're trying to learn about.
Ideally you could sample from the whole population of people seeing the ads, but instead you can only email people who left their address in ordering Vegetarian Starter Packs (VSPs) and advertise to people that 'like'd the page. Depending on how these emails and ads were set up, you could have quite a bit of sample bias: imagine if the ads said "Given up eating meat? Let us know why!" To find out how these requests to take surveys were set up, I wrote to the author.
I'm going to go through this separately for the people who they contacted through email and the people who they brought back in with Facebook ads.
Some people may have, after seeing the survey, been embarrassed not to have changed their behavior and not filled it out. How many? Asking, the author wrote "aside from the 44 that filled out the survey, only 1 did not complete it". This sounds good, but is suspiciously high. A one in ten chance of tickets is pretty good, though, so this could well be that people just wanted their tickets.
OverallFor the most part, sample bias appears not to be a problem beyond the low rates for liking and VSP-requesting that Alan and I already talked about. The VSP numbers I trust less, because I know less about them and they were selected only from people who left their email address, but the Facebook numbers are the larger fraction and seem quite good.
 This is consistent with what I calculated last time: a "10% response rate indicates they sent surveys to 39% of their VSPers".
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