<s>[INST] This question requires the application of scientific evidence evaluation skills, specifically in the context of medical research. 

Dr. Smith has conducted a study on the effects of a new medication on a specific medical condition. The study was published in a reputable medical journal. 

The study involved 100 patients with the condition, who were randomly assigned to either a treatment group (receiving the new medication) or a control group (receiving a placebo). The study found that 80% of patients in the treatment group experienced significant improvement in their symptoms, compared to 20% of patients in the control group.

However, upon closer examination of the study, you notice that the patients in the control group were significantly older and had more severe symptoms than the patients in the treatment group. Additionally, the study did not control for any other variables that could affect the outcome, such as diet, exercise, or other health conditions.

What is the main limitation of this study, and how would you recommend addressing it?[/INST]