What is validity? How does it differ from reliability and what are its types?

Validity: A measurement scale may be considered to be valid if it effectively measures a specific property or characteristic that it intends to measure.

The question of validity does not arise in the case of measurement of physical characteristics such as length, weight and height. This is because the measurement is direct and can be done through standard measuring devices. 

Types of Validity: Following are the main types of Validity:

1. Content Validity: This type of validity may be of two types – a) Face validity and b) Sampling validity. Face validity is determined through a subjective evaluation of a measuring scale. However, the limitation of this type of validity is that it is determined by opinions, rather than through a statistical method. Sampling validity refers to how representative the content of the measuring instrument is. In other words, the measuring instrument’s content must be representative of the content universe of the characteristic being measured.
2. Predictive Validity: This type of validity refers to the extent to which one behavior can be predicted based on another, based on the association between the results yielded by the measuring instrument and the eventual outcome. One limitation of determining predictive validity using this statistical association is that the eventual outcome, in this case, the grade point average of students during the first semester, may be influenced by other “extraneous” variables or factors. In other words, the grade point average may have been influenced by other factors and may not necessarily be linked to the score on the admission test. Therefore, predicting behavior from one situation to another is not always accurate.
3. Construct Validity: A construct is a conceptual equation that is developed by the researcher based on theoretical reasoning. Various kinds of relationships may be perceived by the researcher between a variable under study and other variables. These relationships must be tested in order to determine the construct validity of a measuring instrument. The instrument may be considered to have construct validity only if the expected relationships are found to be true.

Reliability: Reliability refers to the ability of a measuring scale to provide consistent and accurate results.

For example, a weighing machine may be said to be reliable if the same reading is given every time the same object is weighted. 

There are two dimensions of reliability – Stability and Equivalence or Non-Variability.

Stability refers to consistency of results with repeated measurements of the same object, as in the weighing machine example. Equivalence or Non-Variability refers to consistency at a given point of time among different investigators and samples of items.

Reliability can be improved in three ways–

1. By reducing the external sources of variation. This in turn can be achieved by standardizing the conditions under which measurement is carried out.
2. By making the measuring instrument more consistent internally, through an analysis of the different items.
3. By adding more number of items to the measuring instrument, in order to increase the probability of more accurate measurement.