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4: Types of bias generate, create none none on none projectsprint data matrix Once one moves from the abs none none tract to the paper, one begins to see some questions rise up. As with all RCTs ( 8), the rst question is whether the results being reported were the primary outcome of the clinical trial; in other words, was the study designed to answer this question (and hence adequately powered and using p-values appropriately) Was this study designed to show that if you took antidepressants for a few months after stroke, you would be more likely to be alive a decade later Clearly not. The study was designed to show that antidepressants improved depression 3 months after stroke.

This paper, published in AJP in 2003, does not even report the original ndings of the study (not that it matters); the point is that one gets the impression that this study (of 9-year mortality outcomes) stands on its own, as if it had been planned all along, whereas the more clear way of reporting the study would have been to say that after a 3 month RCT, the researchers decided to check on their patients a decade later to examine mortality as a post-hoc outcome (an outcome they decided to examine long after the study was over). Next one sees that the researchers had reported only the completer results in the abstracts (i.e.

, those who had completed the whole 12-week initial RCT), which, as is usually the case, are more favorable to the drugs than the intent-to-treat (ITT) analysis (see 5 for discussion of why ITT is more valid). The ITT analysis still showed bene t but less robustly (59% with antidepressants vs. 36% with placebo, p = 0.

03). We can focus on this result as the main nding, and the question is whether it is valid. We need to ask the confounding question: were the two groups equal in all factors when followed up to 9-year outcome The authors compared patients who died in follow-up (n = 50) versus those who lived (n = 54) and indeed they found di erences (using a magnitude of di erence of 10% between groups, see 5) in hypertension, obesity, diabetes, atrial brillation, and lung disease.

The researchers only conducted statistical analyses correcting for diabetes, but not all the other medical di erences, which could have produced the outcome (death) completely unrelated to antidepressant use. Thus many unanalyzed potential confounding factors exist here. The authors only examined diabetes due to a mistaken use of p-values to assess confounding and this mistake was pointed out in a letter to the editor (Sonis, 2004).

In the authors reply we see their lack of awareness of the major risk of confounding bias in such post-hoc analyses, even in RCTs: This was not an epidemiological study; our patients were randomly assigned into antidepressant and placebo groups. The logic of inference di ers greatly between a correlation (epidemiological) study and an experimental study such as ours. Unfortunately not.

Assuming that randomization e ectively removes most confounding bias (see 5), the logic of inference only di ers between the primary outcome of a properly conducted and analyzed RCT and observational research (like epidemiological studies); but the logic of inference is the same for secondary outcomes and post-hoc analyses of RCTs as it is for observational studies. What is that logic The logic of the need for constantly being aware of, and seeking to correct for, confounding bias. One should be careful here not to be left with the impression that the key di erence is between primary and secondary outcomes; the key issue is that with any outcome, but especially secondary ones, one should pay attention to whether confounding bias has been adequately addressed.

. VS 2010 Clinical example 3 Negative confounding: substance abuse and antidepressant-associated mania The possibility of negative confounding bias is often underappreciated. If one only looks at each variable in a study, one by one (univariate), compared to an outcome, each one of them might be unassociated; but, if one puts them all into a regression model, so that confounding 17.
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