Hopefully this isn't considered a spoiler...
So at one of the two tapings today, I went into the studio, watched all of the audience members as they came in, and quickly marked down whether they were wearing 1) a custom-made-for-TPiR shirt, 2) a shirt with a university name, military uniform, or similar clothing that isn't TPiR-related, but still kind of personalized, or 3) normal clothes that one would wear in any public place (including generic location shirts such as "OKLAHOMA" or "I heart NY").
At this particular taping, roughly 167 people were wearing normal, everyday clothes, 141 were wearing custom shirts mentioning the show or Bob, and 25 were wearing college/military clothes (allow a small margin of error as I tried to decipher shirts while people were quickly being herded into the studio).
Of the contestants that were called to come on down, four of them were wearing normal, everyday clothes, four of them were wearing custom shirts, and one was wearing a college shirt. To be honest, I was amazed at how closely it reflected the audience as a whole (I expected a slightly higher percentage of custom shirts being called, since they would be the people more excited about the show in general).
Granted, it's only a sample of one episode, but I think it does speak to the fact that the production staff isn't lying when they say that they don't pick based on the shirts.
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Scott Robinson