A vector of two characters or numbers indicating the two
phases that should be compared. E.g., phases = c("A","C")
or
phases = c(2,4)
for comparing the second to the fourth phase. Phases
could be combined by providing a list with two elements. E.g., phases
= list(A = c(1,3), B = c(2,4))
will compare phases 1 and 3 (as A) against 2
and 4 (as B). Default is phases = c("A","B")
.