Looking at the four examples they give in the article, I'd likely only get one or two on the first go.

- ntrtyLt (I'm mostly confident)

- tmincrw (I'm really confident)

- rrnmtht (I'm not at all confident of my answer, but I'm really confident that r's, n's, and m's should be prohibited next to each other in captchas like this!)

- MCruncy (I'm really confident of my interpretation, but are capital letters supposed to be reported as capitals on this captcha system, or is this one of the systems that we're only supposed to submit lower case characters even when the character that we're viewing is upper case?)

Probably better? I think my success rate is between 33-50%!

In all seriousness, have there been any good studies of human success rates?