Yes - I'd have thought that what is called a "math captcha" is an oxymoron.
The image ones are best, I think - especially if coupled with the option to read it out for visually impaired users. But I hit problems once spammers started paying people in China to answer the CAPTCHAs for them. I suspect mathematic ones would be more of a deterrent, because they are non-standard - but they are more prone to being solved by robots.
So I gave up and went for Mollom - which has (so far) let nothing through it shouldn't and blocked nothing it should have let through.
Yes - I'd have thought that what is called a "math captcha" is an oxymoron.
The image ones are best, I think - especially if coupled with the option to read it out for visually impaired users. But I hit problems once spammers started paying people in China to answer the CAPTCHAs for them. I suspect mathematic ones would be more of a deterrent, because they are non-standard - but they are more prone to being solved by robots.
So I gave up and went for Mollom - which has (so far) let nothing through it shouldn't and blocked nothing it should have let through.