Though several other studies have yielded similar findings, they have all been observational, showing only a correlation, not cause and effect.
Because all the studies done so far, including the latest one, have been observational rather than clinical, their findings do not provide evidence that hair dye causes lymphoma, said Gerald McEwen, vice president for science at the Cosmetic, Toiletry and Fragrance Association, a trade group in Washington.
Most studies have been observational, unreplicated, and lack the experimental controls needed to eliminate or account for confounding sources of variation.
Nonetheless, most clinical studies in this area have been observational in nature, retrospective in design, small in size, and subject to bias, leaving this issue unresolved for more than 20 years.
The risk of individual bias within studies was assessed, focusing on blindness of the investigators, power analysis, intra-/inter-rater reliability of measurements and on selective/incomplete data presentation, since most studies would have been observational and bias generation from intervention-related factors (i.e., random allocation and concealment, percentage of lost-to-follow up, etc).
However, much of the data regarding hyperandrogenic insulin resistance in humans has been observational [5], [6], [22], [23].
