"Wiseguy learned that Ticketmaster's CAPTCHA system had only loaded 30,000 unique images into its database, rather than millions. So Lowson's team downloaded every image they could find as a .jpeg file, stayed up all night typing them out, and taught their bot how to match the images." Wow. I had the impression Ticketmaster had invested a lot of thought into anti bot tech. Apparently not.
What is more interesting about that statement is how did he learn that fact? Possibly some help from the inside? Or some other method...?
Or more concretely, if you take a random sample of 100 images and remember them, then take another sample of 100 images and you recognize half 50 of them. Then you're reasonably confident that 100 is half of the images.
(This does assume that the images presented follow a discrete uniform distribution but it's probably a good assumption given how people write software.)