That’s a fairly typical ad, which is now running on TV, and which is also on Glad’s web site. Looks like a clear majority would rather buy Glad’s fine trash bag than some other, lesser, bag. Right?

Not exactly.

So what is the probability that a “consumer” would prefer a Glad bag? You’ll be forgiven if you said 70%. That is exactly what the advertiser wants you to think. But it is wrong, wrong, wrong. Why? Let’s parse the ad used and see how you can learn to cheat from it.

The first notable comment is “over the other leading brand.” This heavily implies, but of course does not absolutely prove, that Glad commissioned a market research firm to survey “consumers” about what trash bag they preferred. The best way to do this is to ask people, “What trash bag do you prefer?”

But evidently, this is not what happened here. Here, the “consumer” was given a dichotomy, “Would you rather have Glad? Or this other particular brand?” Here, we have no idea what that other brand was, nor what was meant by “leading brand.” Do you suppose it’s possible that the advertiser gave in to temptation and chose, for his comparison bag, a truly crappy one? One that, in his opinion, is obviously inferior to Glad (but maybe cheaper)? It certainly is possible.

So we already suspect that the 70% guess is off. But we’re not finished yet.

Because in tiny type at the bottom of the screen (following the asterisk), we find these words: “Versus the other leading brand’s Tall Kitchen Drawstring trash bag” and, here’s the kicker, “Among those with a preference.” So now we know that the “other leading brand” was not just some other bag, but a very specifically chosen one. Just as we suspected.

But how about that other bit? The phrase “Among those with a preference” should have your suspicions announcing Red Alert! Because it tells us that there were some people who just didn’t give a damn about trash bags, or, at least, the two trash bags presented to them. How many people? We have no idea. But we might suspect it’s a lot. Which means that the original guess of 70% for the implied, but false, question “What proportion of people prefer Glad”, is way off, and certainly far too large.

Suppose, for example, only 50% of the people surveyed expressed a “preference”. Then, only 35% actually selected Glad!
We also know the original 70% is wrong because we can infer that if the advertiser did have better evidence in his favor, he certainly would have used it.

This ad, then, earns a 4 on the Briggs Statistical Deception Scale.? The rating is low because this kind of statistical manipulation is common; plus, I actually like the Glad bags!