Animal Welfare Charity Evaluation: Update	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.

Facebook

The survey says 7% of page visitors clicked 'like'. To get these users to fill out a survey they placed ads set to display only to these users who clicked 'like'. The author tells me these were a "picture of a movie ticket, did not mention group or veg stuff, said something to the effect of 'Want free movie tickets? Fill out this one minute survey and you have a one in ten chance of winning free movie tickets.'" They "did not have to mention the site at all because it was only shown to those who had "liked" the site." This sounds good: the people brought in are biased in that they have free time and want movie tickets, but I don't think that's a big problem. One thing that does worry me is that Facebook may have added text to the ads about them showing up "because you like hidden face of food" or something similar. Does anyone know whether ads shown because you 'like' something do that?

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.

Email

As I wrote last time, 1.5% of the visitors to the site ordered a VSP. The author tells me "roughly 1/3rd of those who ordered a VSK left their email, we emailed all who ordered (within a selected time period), and about 10% replied." [1] The people who left their email were doing that in a field labeled "To receive our E-newsletter, enter your email (optional)", so it's likely that these were the more enthusiastic ones. I don't know what these emails looked like or what fraction failed to complete the survey after clicking through to the survey page, but if these are like the Facebook ads then they're probably not an issue.

Overall

For 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.

[1] 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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