使用dcast时如何获取每个ID的value.var？

I try to rephrase my question. I have the following data frame, bb1, and I'm decasting it using dcast from dply. In this example I want to calculate how much obersvations I have for in the "rt" column for each "subject" by "condition" (subject ~ condition), but I want only observations that have a "z.score" that meets a cretain condition. In the bellow example I used abs(z.score) > 1.5, but sometimes it will be 1.5, sometimes 1 and sometimes 2. The 1.5 is just an example. Also in the example bellow I calculate the length, but I would like also to be able to calculate the mean (e.g., the mean for "rt" column for each "subject" by "condition" only for observations that have "z.score" > 1.5, so the length is just an example here).I try to rephrase my question. I have the follo