What would you do with a bathroom scale that gave you a different weight every time you stood on it? You'd throw it out. In the same way, if you are going to have any faith in a psychological measure (such as intelligence or extroversion), then you at least have to get the same score (or at least something close) each time you give the test to someone. That's reliability. You gotta have it or no one will believe that you know what you are talking about.

What about all the items on a standardized test that are supposed to measure some construct such as mathematical ability? You would think that all the items would tend to agree with each other. One subset of items shouldn't say you stink in math while another subset says that you are great. This is another form of reliability. Ever take a survey over the phone and they seem to ask you the same question twice? They are checking to see if you are reliable.

Based on the examples, how would you define reliability?