I was wondering whether someone could help me to solve a statistical issue. I observed that by increasing an independent variable (task complexity), a dependent variable (score) decreases considering all data. Is there a manner to observe whether that reduction differs significantly between two groups, i.e. whether in a group the decrease rate is higher than in the other one? In particular, I am not interested in observing the decrease rate for each group independently, but I want to compare the tendencies of decreasing (how much the score decreases in function of complexity) between groups. Similarly, I am not interested in observing the score for each complexity level.
Thank you in advance!