Weaknesses of birth certificate follow-back surveys include that they are resource intensive, particularly given the high level of effort to track children which is particularly difficult with highly mobile populations. These studies do not include children who immigrate into the study area and often exclude children who migrate out of the study area due to resource limitations and it may not be possible to find these children. Additionally, completion rates are often low (80% or less) which has the potential to introduce bias if coverage differs among children for whom data are collected compared with children who can not be found or data are not collected. As with any measure of immunization coverage, the validity of data may not be good if the immunization histories are not verified or validated.