However, there is another, more important bias, which may help at least in part to explain the results presented by Sobolev and colleagues: waiting lists are not managed as perfect queues. In theory, patients receive treatment in the order in which they were placed on the waiting list, but in practice, treatment may be provided in a nonchronological order. This may lead to underestimation of real queuing times measured prospectively because waiting time in the queue-jumping subpopulation lowers mean foreseeable waiting times. This is possible in practice because those providing the services tend to keep some spots open, i.e., programmed productivity is slightly less than maximal service availability (and slightly less than actual service productivity).