Figure 6.

Immortal time bias as a form of selection bias. Immortal time bias is introduced as a form of selection bias in cohort studies when a period of ‘immortal time’ is excluded from the analysis. This exclusion occurs because the start of follow-up for the group receiving treatment (a biologic DMARD in this example) is defined by the start of treatment and is, by design (or by practice pattern), later than that for the comparison group (receiving a conventional DMARD). a | A depiction of the comparison group's follow-up starting at the time of RA diagnosis. b | A depiction of the comparison group's follow-up starting sometime after RA diagnosis (matched on certain time factors other than RA duration), but before biologic DMARD use. In both cases, unless the excluded period of nonbiologic agent use before biologic agent use (i.e. the unexposed immortal time) is appropriately assigned to the nonbiologic group in a time-varying manner,⁶⁴ the immortal time-induced selection bias could lead to a major survival advantage for biologic agent users. Abbreviation: RA, rheumatoid arthritis.