Epi Methods II forum

"Anytime you conduct disease-outcome focused research, i.e., you are studying only a group of people already diagnosed with a condition, there is the potential for collider bias. This potential only becomes a a reality if the exposure you are interested in influences the risk of disease incidence and also some other factor that influences disease incidence also influences the outcome you are interested in. Even then, it may not be a big problem (we have lots of biases we live with because they are not big enough to meaningfully change interpretation of the results) unless the exposure and that other factor interact when determining disease incidence.  By "interact" here we mean specifically interact on a  multiplicative scale.  We can walk through the math of that, but it's a bit of a detail.  The big picture is - yes, always be worried about the potential for collider bias in disease prognosis research if your exposure influences disease incidence; - consider whether there are any other major factors with a strong influence on disease incidence that also influence your outcome; - if there are such factors, ask whether the influence of those factors on disease incidence might be really different for people with vs without the exposure.  You usually won't be sure about the last two parts of this, so you may have to use a sensitivity analysis."