The key to thinking about what the t-shirt test means is thinking beyond the numbers. This is hard for us as designers, because we want to break everything down into probabilities to know how likely an action is to succeed at any point. But that’s not what players are thinking when they grab their dice. They’re thinking “Can I hit this dragon?” “Can I dodge this punch?” “Can I convince the starship captain to help us?” The d20’s easy probabilities make it easy for a player to judge the likelihood of success, but nobody is doing the math when death is on the line.