We report on studies showing large differences in student ability to use, and reason with, certain data. Many students incorrectly assume there must be dependence between the axes of any graph irrespective of whether the data suggests a relation. Additionally, students have difficulty with variables and their role in the experiment. A majority of the errors are consistent with failure to control variables or illogical reasoning from the data. We developed and tested two different one-hour group activities to improve student understanding. One activity was laboratory-based and focused on control of variables and experimentation. The other was recitation-based and focused on logical reasoning and data manipulation. Both activities improved student ability to draw conclusions and answer questions about the data in graph or pictorial form. The relative effectiveness of the activities suggests that both logical reasoning and control of variables are important for students working with this data.