Dr. Jones conducted a study examining the relationship between the amount of sugar in a child's diet and the activity level of the child. A sample of thirty 4-year-old children from a local preschool was used in the study. Sugar consumption was measured by interviewing the parents about each child's diet. Based on the result of the interview, each child was then placed into one of two groups: high sugar consumption and low sugar consumption. Activity level was measured by observing the children during a regular preschool afternoon. Finally, Dr. Jones compared the activity level for the high-sugar group with the activity level for the low-sugar group. Explain why Dr. Jones' study is not an example of the experimental research strategy.