I could really go into detail here (in fact I might tomorrow) but the point of the study is that *switching* to simvastatin is associated with substantially worse outcomes.
Let's examine the potential outcomes when a patient is switched from atorvastatin to simvastatin:
1) lipids go up because it's a less potent statin
2) disruption of treatment results in lack of adherence to simvastatin (switching from one medication to another has been shown to reduce adherence). Lipids go up because you're not taking a statin
Now let's look at possible reasons why they might have been switched:
1) COST (there's an active effort to get patients on simva)
2) Much less prominently, adverse effects with Lipitor. However, few physicians are going to switch to simva from Lipitor if they're seeing AEs. Thus this represents a very small proportion of patients
3) Poor response to Lipitor. Although you advance this as one source of bias, it's highly unlikely that patients would respond poorly to Lipitor and then respond to simva. It's even less likely that a physician would try simva in a poor Lipitor responder.
It's also unlikely that patients would be switched to a less potent statin (simva) if they had a poorer prognosis.
So yes, there is a potential for bias in this study, but it's all against Lipitor.
The point here clearly is that switching from Lipitor to simvastatin has a negative impact on outcomes. It's equally clear to me that this is driven by lipids, whether because of the reduced lipid-lowering efficacy of simvastatin or because patients stop adhering to their medication post-switch. Simple story.
