covariance or correlation

I try to interpret the relationship between two same variables at two
different groups. for this reason, which must I use: covariance or
correlation

=====================
To manage your subscription to SPSSX-L, send a message to
[hidden email] (not to SPSSX-L), with no body text except the
command. To leave the list, send the command
SIGNOFF SPSSX-L
For a list of commands to manage subscriptions, send the command
INFO REFCARD