Psychologists were interested in assessing the effect of commercials on eating behavior in children. They randomly assigned a group of 500 children, 7-11 years old, to two groups. After randomization, each child was asked to watch a cartoon in a private room, containing a large bowl of goldfsh crackers. The cartoon included 2 commercial breaks. The first group watched food commercials (mostly snacks), while the second group watched non-food commercials (games and entertainment products). Once the child fınished watching the cartoon, the psychologists weighed the crackers bowl to measure how many grams of crackers the child ate. Then, they compared the average amount of crackers eaten by each group. What type of statistical study did the psychologists use?