“…7 A control group of "normal" subjects was included in 11 studies (Table 2), with an important division occurring in these studies. In 7 of them, the control group consisted of hospital employees 13,47,50,64 or volunteers from the community, 49,59,61 a fact that can be considered another important selection bias. In 4 of these studies, a higher prevalence of MVP was detected in the case group.…”