What does it mean when people say that a t-test performed on ranked data is equivalent to a Mann-Whitney U-test? Does that mean they just test the same hypothesis/are useful in the same situations or are they are supposed to give the exact same p-values? The reason I ask is I tried both in
R and compared two groups with very small sample sizes (3 and 4). I got completely different answers: one significant and one not.
The two groups are A=(1,2,3) and B=(4,5,6,7).
t-test: p = 0.01
Mann-Whitney U-test: p = 0.06