A scientist is trying to determine whether the use of cell phone contributes to brain cancer. For his study, he compares the medical records of subjects who have been diagnosed with brain cancer to the records of healthy subjects. What is the most significant weakness in this type of study? The test groups must be unusually large to generate significant data. The relationship between cell phone use and cancer is difficult to determine. This type of study is expensive, and the data are difficult to analyze. This type of study must rely on medical records, which are inaccurate.