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I am not particularly asking about whether or not Captcha within itself is good UX, but a more generalized question as to when human validation truly makes a difference in terms of a website's integrity, and what "kind" of website those may be?

For example, websites (e.g. social networks) where user accounts are forward facing, and friends/groups are involved and most importantly the integrity of the website is dependent upon user generated content.

VERSUS

An ecommerce site where user accounts are not accessible to the general public and the end goal of the site is to convert via a credit card being used thereby ensuring that the user is a real person (in theory...).

Also are there other instances besides social networks / ecommerce, e.g. Software as a Service, where captcha is essential or not essential.

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If you're not asking whether or not it's good UX, then I'm not sure it belongs on UX. –  DA01 Sep 19 '11 at 1:11
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2 Answers

up vote 2 down vote accepted

You have to look at the positives and negatives of using a captcha. The negative is that it requires more work by the users and it is quite a significant extra piece of cognative work.

The positive is that it can avoid automated bots accessing certain features of a site. If this is more important than the inconvenience, then it is worth using a captcha. In many occasions, there are other ways in which the user is verified - credit card on an e-commerce site, or an identity check on a mobile phone site.

The time I feel they are fully justified is on email forms, where there is a chance of being used for spamming. Also, for fending off automated brute force password attacks. But I don't like them, and I think they should only be used where there is a real benefit ( in particular a benefit for the user ) of having them.

So if you can justify them as providing benefits - ideally to both sides - then use them. Otherwise - and as a default - don't.

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I don't have a solid rule of thumb, but captchas seem most helpful wherever there's an opportunity for automated mis-use of the site. When you look at the feature set for a site, ask whether there is the potential for an automated tool to abuse those features, or otherwise violate the terms, and you'll be able to choose quickly about whether or not a captcha is helpful or unnecessary.

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