The authors note that it is a contributing factor, which means it is not the only factor.
I'm not really sure why people referred for semen analysis would be a biasing factor in this when it is present in both test groups. It seems like it would be the only reasonable way to get statistical power in a study like this since if the effect is small and most people don't suffer from it there would be a much larger sample size required.
> I'm not really sure why people referred for semen analysis would be a biasing factor in this when it is present in both test groups. It seems like it would be the only reasonable way to get statistical power in a study like this since if the effect is small and most people don't suffer from it there would be a much larger sample size required.
You're right, everything else being equal, that probably wouldn't be the biggest issue... Although in this specific case, it seems like a way to amplify the effect - if this study would be done on the general population, where (1) almost everybody uses mobile phones extensively, and (2) presumably, men are less likely to have poor sperm than in this focus group, the resulting effect would be diluted (as in most cases, mobile phone usage would not result in poorer sperm quality).
I'm not really sure why people referred for semen analysis would be a biasing factor in this when it is present in both test groups. It seems like it would be the only reasonable way to get statistical power in a study like this since if the effect is small and most people don't suffer from it there would be a much larger sample size required.