A researcher was interested in teaching couples to communicate more effectively. She had 20 volunteer couples, 10 of which were randomly assigned to receive the training program and 10 of which were not. After they had been trained (or not), she presented each of the 20 couples with a hypothetical problem situation and asked them to resolve it while she tape-recorded their conversation. She was blind as to which 10 couples had taken the training program until after she had analyzed the results.