Since 1991, the year the Gulf War ended, veterans and their families voiced concern about subsequent health problems. In 1999, an assessment of the first 1,000 veterans involved in the Ministry of Defence's Gulf War medical assessment programme found "no evidence" of a single illness, physical or mental, that would explain the pattern of symptoms observed in the group. Nevertheless, in 1999, MEDACT petitioned for the WHO to conduct an investigation into illnesses in veterans and Iraqi civilians. A major 2006 review of peer-reviewed literature by a US Institute of Medicine committee concluded that, "[because the symptoms vary greatly among individuals", they do not point to a syndrome unique to Gulf War veterans, though their report conceded that the lack of objective pre-deployment health data meant definitive conclusions were effectively impossible. Simon Wessely praised the IOM's review, and noted that, despite its central conclusion that no novel syndrome existed, its other findings made it "equally clear that service in the Gulf war did aversely affect health in some personnel". Aside from the lack of baseline data to guide analysis of the veterans' postwar health, because no detailed health screening was carried out when the veterans entered service, another major stumbling block with some studies, like the thousand-veteran one, is that the subjects are self-selected, rather than a random sample, making general conclusions impossible.