Well, short of surveying all women, everywhere, we can't really 100% prove that his behavior doesn't extend beyond a single one-off event because, to your point, 95% support would still suggest a 5% group who may or may not have experienced some kind of abuse. However, the context the letter provides can be taken for its absolute value. The absolute value of the letter is that where someone might have assumed these women experienced some kind of similar abuse at the hands of Franken, this letter makes clear that such an assumption might be false. The value gleaned from such support (a tiny, tiny, tiny, tiny, tiny subset of population of the population is saying he had similar opportunities and didn't act that way with them) isn't exoneration of his behavior, but it gives some motivation to the possibility that his behavior was an outlier or a standard deviation away from the median of his identity, not actually the median of his identity. And, we as a society seem far more concerned with people who oscillate around harassing behavior over people who might have gone too far outside of their normally respectful behavior. That's why people spend so much time trying to push someone into the latter category over the former category.