While evaluating a measurement tool, three major considerations must be taken into account: validity, reliability and practicality. A sound measurement should fulfill all of these tests.
Test of Validity
It is the most important criterion. It indicates the degree to which an instrument measures what it is supposed to measure. There are three types of validity: Content validity, Criterion-related validity, and Construct validity.
Content validity refers to the extent to which a measuring instrument adequately covers the topic under study. Its determination is mainly judgmental and intuitive. It cannot be expressed in numerical terms. It can also be determined by a panel of persons who judge the extent of the measuring instruments standards.
Criterion-related validity refers to our ability to predict or estimate the existence of a current condition. It reflects the success of measures used for empirical estimating purposes. Criterion-related validity is expressed as the coefficient of correlation between the test scores. Here, the concerned criterion must possess the following characteristics:
- Relevance: When a criterion is defined in terms judged to be the proper measures, it is known to be relevant.
- Unbiased: When the criterion provides each subject an equal opportunity to score, it is unbiased.
- Reliability: When a criterion is stable or reproducible, it is considered as reliable.
- Availability: The information specified by the criterion should be easily available.
Construct validity is most complex and abstract. It is the extent up to which the scores can be accounted for by the explanatory constructs of a sound theory. Its determination requires association of a set of other propositions with the results received from using the measurement instrument. If the measurements correlate with the other propositions as per our predictions, it can be concluded that there is some degree of construct validity.
If the above criteria are met, we may conclude that our measuring instrument is valid and provides correct measurement; if not, we may have to look for more information and/or depend on judgment.