We conducted a case-control study to investigate a possible positive association between drinking diet soda and kidney disease. We selected our cases from patients hospitalized for kidney disease. We selected our controls from hospitalized individuals who were diagnosed with obesity-related conditions. We were apparently unaware that obesity-related conditions have positively associated with drinking diet soda.

By selecting these controls, will we likely overestimate or underestimate the true association between drinking diet soda and kidney disease? Or will this control selection have no impact? Justify your answer.

Hint: Draw the 2x2 table for your exposure and outcome, then think about how each cell might be affected by the selection of these controls and what that would do to your calculation of excess risk.