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?

It is very hard to determine the link between cellphone use and brain cancer

If a scientist is trying to determine whether the use of cell phone contributes to brain cancer, then the most significant weakness in this type of study is the relationship between the cell phone and brain cancer itself. It is hard to identify its connections and consequences with each other.