Calculate change since base year?
问题 I have a dataset that looks something like this: df1 <- data.frame(id = c(rep("A1",4), rep("A2",4)), time = rep(c(0,2:4), 2), y1 = rnorm(8), y2 = rnorm(8)) For each of the y variables, I want to calculate their change since time==0 . Basically, I want to do this: calc_chage <- function(id, data){ #y1 y1_0 <- data$y1[which(data$time==0 & data$id==id)] D2y1 <- data$y1[which(data$time==2 & data$id==id)] - y1_0 D3y1 <- data$y1[which(data$time==3 & data$id==id)] - y1_0 D4y1 <- data$y1[which(data