What is the difference between reliability and validity?

These two concepts can be confusing at first, but once you get used to using them they get a lot easier. A golden rule for using these ideas in exam questions is to never use them together or just tack them on to the end of a sentence, as an examiner will think you do not know what you are talking about even if you do. Reliability is the idea that what you are using to measure will measure consistently. So if I have a set of scales and it measures me as 50kg one day, 60kg the next and 45kg the day after, they are unreliable because they are not measuring me to be the same (or nearly the same) weight. Validity is the idea that what you are using to measure will measure correctly. So if I have a set of scales that measures me as 10kg every time I stand on them, they lack validity because I must weigh more than 10kg. The best kind of measure will be both reliable and valid, scales that consistently tell me what I actually weigh. There are different types of reliability and validity, so it will probably help to draw a flow chart that has the definitions of each type written out next to them, then you can test yourself.