I’m not an expert—I’ve just talked about stuttering with some speech pathologists as a person who stutters. Stuttering severity is generally determined by percentage of syllables stuttered on, 3-4% being the bottom threshold to be considered stuttering (mild stuttering). Severe stutters can get up to 75% of syllables stuttered. Again these might not be accurate numbers, I’m just repeating what I heard. Percentage of stuttered syllables is not a very reliable statistic, since people can hide their stutter with techniques or change their responses to avoid stuttering whatsoever (word replacement, one word phrases) Mild/moderate stutters are associated with developmental causes, like being born with a stutter (disconnect between speech and motor aspects of the brain). Severe stutters are more commonly associated with life events such as PTSD or physical trauma, and there is a physically observable cause in the brain that can be determined to be causing the stutter. These are generalities and there can be exceptions. For those of you who have studied this and know better—let me know if any of this is wrong or misleading and I’d be happy to modify. Sincerely, 22F who has stuttered since birth (and talked with many other people who stutter/studying stuttering)