All I'm saying is don't disregard findings on the basis of it involving self-report. Self-report methods have been proven to be a viable measure of attitudes and personal experiences and are used in studies published in very well established journals. I think it's unfair to disregard something on the basis of it being self-report.
This for example suggests that a certain self-report assessment of discrimination is very reliable and valid-
http://www.sciencedirect.com/science/article/pii/S0277953605000973A quick search shows they can even be useful in diagnosing depression
http://apm.sagepub.com/content/1/3/385.short It is ridiculous to completely disregard something on the basis of it being self-report, especially with something concerning personal everyday experiences such as sexism. I can't really think of a way of measuring everyday sexism without using a degree of reflection and self-report.
Self-reports can be a really good measure of sexism, but obviously they can also be ****e, dependent on the questions used, obviously.
They may have asked the girls to provide frequencies of specific experiences or they may have been more general (not good). Unfortunately the website doesn't seem to provide us with the questions, but if you look at the statistics concerning harassment-
Six in ten girls have had comments shouted at them about their appearance at school (60%)
62% have been shouted or whistled at in the street about their appearance (rising to 76% of 16- to
21-year-olds).
Seven out of ten girls aged 13 and over report further experiences of sexual harassment at school or college. These include sexual jokes or taunts (51%), seeing images of girls or women that made them uncomfortable (39%), seeing rude or obscene graffiti about girls and women (33%), and unwanted sexual attention (28%) and touching (28%). Those figures are really specific, which suggests that the questions must have been. It seems the questionnaire asked for specific experiences, rather than general notions such as 'Have you experienced verbal- harassment within the school environment?'
Obviously we can't really make a judgement, because we can't see the questionnaire, but I think it's wrong to jump to conclusions and so quickly and willingly disregard something which such strong figures on the basis of it being self-report. To me it feels like a lot of people don't want to believe it so interpret it to suit their beliefs, rather than contemplate that maybe that amount of girls do actually experience sexism.