The internal validity of the study results is doubtful, given the study designs and the quality of information used, in particular, to estimate the number of eligible women in the area (based on data from the 1991 census). As the authors recognized, the lack of adequate control of the underlying population in the study could lead to biases. The authors pointed out that the impossibility of determining the precise reason for Pap tests or whether the tests were carried out on women who were due for screening, overdue for screening or presenting for screening earlier than required, was likely to result in the effectiveness of the intervention being understated. They further pointed out that the study results could be biased by external factors. As an example they citedtwo legal cases involving negative test results in women who later developed cervical cancer, both of which were widely reported by the media during the course of the study - the publicity given to these cases resulted in a significant upsurge in pap testing, predominantly amongst women who were already adequately screened.