What does imply that a measuring instrument is valid and reliable?
Every scientific measuring instrument has to meet two basic principles: To be Valid and to be Reliable. This is applied to the thermometers, barometers, etc., and certainly to the psychometric test. An instrument is valid for a sphere of behaviors, if it allows to predict performance of the features that it is said it measures. For example, we will say that a rule measures the linear distance between two points, and it will be valid to measure the distance between two linear points, but it is not valid to measure the extension of the line in a circle. Being reliable means that a measurement instrument, always measures the same thing, under the same conditions. Continuing the example of the rule, we will say that it always measures the same distance between two points. The validity and reliability are always limited according to the available evidence, allowing to state that certain instrument is valid and reliable for certain use and under certain conditions. There are many Psychologic
