An aside from your research topic, but to address the frequency: Having done a few rotations on ob/gyn l and d, I'd say ~95% or more of women have at least a grade 1 tear. I've only seen one birth out of ~50 that hasn't required stitches. Now you can get away without stitches, but the stitch hides some of the raw skin and prevents some of the pain during urination. There's a widespread belief that there's a "husband" stitch that occurs. Some background research into the prevalence of this notion would be interesting to look into as well.