Can someone explain the meaning of "shared frailty" in survival analysis Can someone explain the meaning of "shared frailty" in survival analysis in layman english ? a concrete example of where one would use this would also be very helpfull. thx
kindly Stig
 A: This is an interesting question. 
In a survival setting, where we are typically concerned with studying the time to "failure", there are two types of frailties: 1. Individual frailties and 2. Shared frailties. ("Failure" could be death, transplant failure, etc.) 
Individual frailties are specific to each subject in the target population being studied, whereas shared frailties are common to all subjects belonging to the same subset of that population. 
Individual frailties aim to capture the fact that some subjects in the target population are more failure-prone than others (hence, more "frail"). Shared frailties aim to capture the fact that some subsets of the population (as a whole) are more failure-prone than others. The factors responsible for this proneness to failure are either (i) unknown or (ii) known but unmeasured/unmeasurable. In this sense, frailties are umbrella terms for all of these (risk) factors. 
This heterogeneity of individual subjects or entire population subsets with respect to proneness to failure needs to be taken into account at the modelling stage so that it doesn't lead to misleading results and conclusions. 
Please refer to this document for an excellent explanation of the concept of frailty: http://psfaculty.ucdavis.edu/bsjjones/frailtyslides.pdf. 
