Dr. Thompson researches gang-related crime in a large city. She notices more graffiti appears when there are more ice cream trucks in town. She does a study that measures the frequency of new graffiti being reported and ice cream truck sales for each week over a year. She finds a positive association between ice cream truck sales and graffiti and concludes the ice cream trucks are encouraging graffiti. Her colleague, Dr. Richardson, points out that both ice cream sales and graffiti might be influenced by rising temperatures during the summer. Which kind of problem has Dr. Richardson identified?