Hi there! Thank you for the opportunity to help you with your question!
Psychological tests might produce bias in a few different ways: a study might have expectation bias, where an author/experimenter has a hypothesis and picks up only signals/data that support his hypothesis. This can lead to false conclusions which are not supported by data. In order to avoid this, one should conduct careful experiments.
Also, when picking out the samples for tests, biased in the sample choice can be picked up in the results. If too many people have the same socio-economic status and cultural backgrounds, then the results might be different from that of a study with a well mixed, representative sample. In order to achieve this, one should select a large enough sample, that includes a wide variety of backgrounds to avoid bias.
Please let me know if you need any clarification. Always glad to help!