A researcher is looking at the foot-in-the-door phenomenon (in which asking somebody to a small task increases their willingness to a larger task later) and door-in-the-face phenomenon (where asking for something big first results in people turning it down; they are then more likely to agree to something smaller out of guilt). He sets aside a week to collect data at FIU. From 9:00 am until 11:00 am each day, he carries out the foot-in-the-door session, where he asks people if they would be willing to donate a dollar to help fund an on-campus charity that helps underprivileged students with book fees. When they donate, he asks if they would be willing to attend an on-campus event the next week that will last about two hours. From 11:00 am until 1:00 pm each day. He carries out the control condition where he only asks them if they would be willing to attend the on-campus event. Finally, from 1:00 until 3:00 he first asks if they would be willing to donate $20. When they turn him down, he then asks if they would be willing to attend the event (door-in-the-face condition). He finds that those in both the foot-in-the-door and door-in-the-face conditions are more willing to go to the two-hour event than those in the control condition.
How could the confound be fixed?