Methods for handling treatment switching - MRC Network of Hubs for ...

The problem of unstable weights• Sometimes we get very large weights in IPCW– e.g. if 99% of patients who progressed thenswitched, the poor 1% who didn’t switch get aweight of 100 to “represent” those who did switch• Leads to large standard errors (small effective samplesize)• “Capping” weights avoids large standard errors but reintroducesbias• “Stabilised” weights can help (Robins et al, 2000)• Inherent limitation of the method– e.g. if 100% of patients who progressed thenswitched, IPCW simply fails20