Are psychological tests reliable?

If you want to measure your length you can use a tape measure or a carpenter’s rule. These kind of measurement tools are very reliable. If you measure your length today, and tomorrow, and the day after tomorrow, and after a month there is a certain probability that you will reach almost exactly the same results. This means that the tools have a very high reliability, indeed.
But are they really measuring length? Are you sure?
Yes, you could rest assured. They do measure length. The tools have a very high validity.
But let us look at a common psychological test instrument that is said measuring intellectual capacity. You try it today, and then tomorrow, and the the day after tomorrow and then after a month.
Will you get exactly the same results?
Probably not. Not exactly. It may differ. Perhaps you will get a higher result after two days because you have learned how to solve the questions. Or, you will get lower results because after two days you are quite tired, or you are occupied by something else that is very important to you.
The reliability is in fact not so very high. If a perfect match between test day 1 and test day 2, or 3 or 4 exists the reliability is said to reach 1.0.
But most tests have a reliability around 0.6-0.8 at the most,
And is the test really measuring intellectual capacity?
Well, that depends on how to define intellectual capacity. And furthermore, the intellectual capacity could of course, as I guess that you know by experience, vary quite a lot from day to day. The human being is a very complex creature. We are all affected by so many emotional and other conditions so you will never know how your brain works tomorrow.
A lot of the most common psychological tests have a validity of around 0.3 – 0.5.
There are very large areas of uncertainties around the results of psychological tests.
It is possible to reduce the uncertainties, but then you have to meet and talk to and discuss a lot of issues with the candidate.
Tests that are used by themselves only are not particularly reliable.
That goes especially for so called personality inventories or personality tests used in selection programs.
Most psychological tests are reliable in the same way that you want to measure the length of a person by using a bathroom scale, primarily pointing out your weight.