characterisitics of good measuring instrument

Characteristics of good measuring instrument:     

Reliability is the consistency of your measurement, or the degree to which an instrument measures the same way each time it is used under the same condition with the same subjects. In short, it is the repeatability of your measurement. A measure is considered reliable if a person's score on the same test given twice is similar. It is important to remember that reliability is not measured, it is estimated. A good instrument will produce consistent scores. An instrument’s reliability is estimated using a correlation coefficient of one type or another.

Validity  Validity is the extent to which a test measures what it claims to measure. It is vital for a test to be valid in order for the results to be accurately applied and interpreted. Validity isn’t determined by a single statistic, but by a body of research that demonstrates the relationship between the test and the behavior it is intended to measure. There are three types of validity: It is the strength of our conclusions, inferences or propositions. More formally, Cook and Campbell (1979) define it as the "best available approximation to the truth or falsity of a given inference, proposition or conclusion." In short, were we right? Let's look at a simple example. Say we are studying the effect of strict attendance policies on class participation. In our case, we saw that class participation did increase after the policy was established. Each type of validity would highlight a different aspect of the relationship between our treatment (strict attendance policy) and our observed outcome (increased class participation).

Practicability: It should be feasible & usable. Quality of being usable in context to the objective to be achieved. USABILITY(practicality) ease in administration, scoring, interpretation and application, low cost, proper mechanical make – up

Measurability: It should measure the objective to be achieved.