Guinness is an unfortunate component to this test. Whereas the other beers may have the same color, taste, and mouthfeel, Guinness would be distinguishable from

Guinness is an unfortunate component to this test. Whereas the other beers may have the same color, taste, and mouthfeel, Guinness would be distinguishable from those on any one of those 3 dimensions. Even if you couldn't distinguish based on taste or texture, the color alone would give it away. In other words, you would probably guess that you're drinking a Guinness if you happen to see its color. I think that's a significant confounder here.