This could get long and complicated! Put simply, psychometrics is a set of techniques used to ensure, among other things, that:

• you are actually testing what you think you are testing. A written test of mathematics should be testing maths, not writing for instance.
• your test gives the same results if its given to the same person twice or administered by different people
• it’s fair to everyone• you know how accurate the measure is and how far you can depend on it. No measure – whether of your height or your profit is 100% accurate (just ask an accountant about the latter). Sometimes this can be significant (in the latter case ask the taxman!).

Psychometrics allow you to weigh up the accuracy of your decision.