HW 2 Q10 Answer Key

HW 2 Q10 Answer Key

by Jean Digitale -

The answer key states:

"These two features combined may lead to an under-representation of participants who have aggressive cancer and are not on anticoagulation, which is a bias that would bias the results away from the null (i.e. towards the conclusion that anticoagulant use is protective against mortality associated with prostate cancer)."


This example states that the E-D+ cell is underrepresented. This means that those anticoagulants are relatively more likely to die, making anticoagulant use look more harmful.  Is this correct? This is the contrary to the key.

Re: HW 2 Q10 Answer Key

by Monica Ospina Romero -

Thank you for catching this error in the answer key. Yes, in our example of selection bias the E-D+ group is underrepresented which makes the risk in the nonAD group underestimated. Since the risk ratio =  Risk in AC/Risk in nonAC, this bias will over-estimate the numeric risk ratio. Under the null, this bias would make AC look harmful (RR>1). If the AC were truly protective for PCSM (non-null), this type of bias is towards the null, making AC group less effective on preventing PCSM.

Grading for question 10 was not based on the directionality of the bias but on your ability to explain a scenario where sampling was differently in exposed/unexposed and high/low risk for the outcome.  We made corrections in the answer key and we are happy to discuss any further question.