A research team is evaluating the effectiveness of a new breathing technique for reducing anxiety in young adults. A total of $36$ participants aged $18\text{–}19$, all diagnosed with mild anxiety, volunteer for the study. They are randomly assigned to practice either the breathing technique or a control activity (sitting quietly) each day for $4$ weeks. Anxiety levels are measured using two standard psychological scales at the beginning and end of the study. Identify one potential problem in this experimental design and suggest a way to improve it.