Reliability and Validity

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Reliability refers to how consistently a test measures a characteristic. For example, if a person is given a test this week and then given the same test next week will that individual get the same test score as last week or will they get a different score? If the test score yields the same results as last week for the same person who sat the test, then a measure of this individual is considered to be reliable. Reliability means the consistency, trustworthiness and the repeatability of a test.
If a person decides to weigh ten pounds of carrots one evening and the scale they sued is reliable, then this same scale should be able to register ten pounds for the carrots when measured two hours later. Unless the carrots were peeled or cooked. Just like in the same way all the instruments and equipment are for tests on classrooms, and public tests should be made reliable – this would mean that there is no difference whether a student sits for the exam in the morning or the evening; today or tomorrow.
Validity simply refers to how good a test measures what it was meant to measure. A test can be reliable and not be valid at the same time (Schutz, 1966). However, for a test to be valid it has to be reliable. We can expect to get an acceptable validity coefficient to be a value of roughly 0.3 because the test is already reliable hence the validity coefficient can be lowered. This is because research may provide an individual with results that are consistent hence making it reliable, however, unless the research is measuring exactly what it is supposed to measure then it is not valid.

Reference
Schutz, W. (1966). Fundamental interpersonal relations orientation—Behavior: Consulting Psychologists press. https://archive.org/details/interpersonalund00schu