Dplyr - Mean for multiple columns

You don't need to group, just select() and then mutate()

library(dplyr)
mutate(df, IVMean = rowMeans(select(df, starts_with("IV")), na.rm = TRUE))

Use . in dplyr.

library(dplyr)
mutate(df, IVMean = rowMeans(select(., starts_with("IV")), na.rm = TRUE))