David Conducted an Experiment Where He Assigned One Group of Couples

David conducted an experiment where he assigned one group of couples to go on weekly dates and to do something that neither spouse had ever done before or had not done very often (e.g. ,rollerblading or attending the opera or cooking classes);he called this the "novel" group.He assigned a second group of couples to go on a date every Saturday but to do the same activity every week;David called this the "routine" group.The couples were asked to go on their date nights every Saturday for 6 weeks.The couples completed relationship satisfaction questionnaires prior to starting the date nights and again at the end of the 6 weeks.The answers allowed David to see whether the couples in the novel activity group were happier in their relationships at the end of the study compared to the couples in the routine group.When couples volunteered to be in the study,David asked them which group they would prefer to be in because he thought if couples could choose their own group they would be more likely to complete the 6-week study,and thus David could avoid attrition bias.What is the problem with David's study,and why is it a concern?