The practicality attribute of a measuring instrument can be estimated regarding its economy, convenience and interpretability. From the operational point of view, the measuring instrument needs to be practical. In other words, it should be economical, convenient and interpreted.
Economy consideration suggests that some mutual benefit is required between the ideal research project and that which the budget can afford. The length of measuring instrument is an important area where economic pressures are swiftly felt. Even though more items give better reliability, in the interest of limiting the interview or observation time, we have to take only few items for the study purpose. Similarly, the data-collection methods, which are to be used, occasionally depend upon economic factors.
Convenience test suggests that the measuring instrument should be easily manageable. For this purpose, one should pay proper attention to the layout of the measuring instrument. For example, a questionnaire with clear instructions and illustrated examples is comparatively more effective and easier to complete than the questionnaire that lacks these features. Interpretability consideration is especially important when persons other than the designers of the test are to interpret the results. In order to be interpretable, the measuring instrument must be supplemented by the following:
detailed instructions for administering the test,
evidence about the reliability, and
guides for using the test and interpreting results.
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.