If we were to make an inference about the mean of differences between pre-op and post-op plasma silicone levels in DB5, what would be the df? To make this mean into a t statistic, we subtract what value and then divide by what statistic? To decide if t is significantly larger than 0 or not, we choose a “cut point” greater than which t is significant and smaller than which, not. What do we call this cut point?