You should be aware of varying distribution density. If the metric is taken from a survey, and let's say an avg of 1 in 10 said they experience some form of stutter or stammer, than it's given a claim to affect 1% of the population. If its taken from speech therapists, and 1 in 10 of their patients are for stutters/stammers, than it affects 1% of patients. I'm not certain where you're claims are coming from, and by what metric they're derived. If using personal interaction, you may have less interaction with stutterers than someone whose job is to interact with stutterers. In that regard their claims are as valid to them as yours is to you, based on personal experience. I know I stutter, and if I am in a room with 9 other people, than the 1% holds true.