A captcha is often used on the web to ensure that the person browsing the site is human
. It can either be a simple maths sum, such as 4 + 6, an easy to answer question, for example "What colour is the sky?", or more commonly a set of letters which the user must type into a text field.
Captchas were first introduced in 2000 and were invented at Carnegie Mellon University. The main use of a Captcha is to prevent automated programs filling out forms on the internet. For example a website may want to submit hundreds or even thousands of links to a link directory. Adding those links manually would take a long time. With an automated program it would be possible to submit thousands of links quickly and efficiently.
Although some automated programs do fill out the forms correctly there are a lot of spam submissions. For example links to pornography are often submitted by automated programs. Also the same information is often entered thousands of times meaning a lot of hassle for the webmaster to remove the submitted information.
What makes a good captcha?
Firstly it has to be easy to understand and to complete. Having a math sum such as 2x + y = y + 15 or a question such as "How far is the Earth from the Sun?" will confuse people and result in people not being able to submit their information.
The most common type of captcha is shown on the left. It requires the user to enter the string of text displayed. Normally these types of captcha work well; however there are bad examples. One of these bad examples is shown on the left. It is extremely hard to read and would require users to strain their eyes' in order to read it. Users may even give up if they do not get it right after a couple of attempts.
To conclude; make sure your captcha is simple and that anyone would be able to complete it, including children if they're in your target audience.