Most effective human verification questions?

[wisebread]

Yesterday I turned on the Q&A verification for the registration process. The question is: What are the first 4 letters of our website's name? (the answer is "wise"). This question didn't really slow down the spammers (we still had 5 new spammers in 12 hours, which is our usual rate).

Did I just pick a bad question? What are the characteristics of a good question? What questions do you guys find effective?

p.s. We tried recaptcha and that didn't slow down the spammers either.

[Xtrigit]

I have a simple but clear anti-spam policy notice on my site. I really hate the captcha images so I decided to implement the human verification questions instead. The question is: How many hours are there in a week? quite simple. 168.

So far, 0 spammers, registration rates are good.

I hate it when I visit a site and they ask hard/uncommon questions, just makes me leave the site and never go back.

[IsMaR]

Nothing is absolutely 100% effective because not all spammers are automated. Alot of spammers outsource this kind of thing to humans.

[LorinPotter]

We use "What is five plus three, answer with a single number", this seems to work for us. But we are a niche group of motorcyclists, and probably don't get as much attention as more active forums.