Reliability of Research instruments-internal consistency procedures


Instrument reliability is a way of ensuring that any instrument used for measuring experimental variables gives the same results every time.

 In the physical sciences, the term is self-explanatory, and it is a matter of making sure that every piece of hardware, from a mass spectrometer to a set of weighing scales, is properly calibrated.

Instruments in Research

As an example, a researcher will always test the instrument reliability of weighing scales with a set of calibration weights, ensuring that the results given are within an acceptable margin of error.

Some of the highly accurate balances can give false results if they are not placed upon a completely level surface, so this calibration process is the best way to avoid this.

In the non-physical sciences, the definition of an instrument is much broader, encompassing everything from a set of survey questions to an intelligence test. A survey to measure reading ability in children must produce reliable and consistent results if it is to be taken seriously.

Cronbach’s alpha

Internal consistency is usually measured with Cronbach’s alpha, a statistic calculated from the pairwise correlations between items. Internal consistency ranges between negative infinity and one. Coefficient alpha will be negative whenever there is greater within-subject variability than between-subject variability.

A commonly accepted rule of thumb for describing internal consistency is          

Cronbach’s alpha Internal consistency
α ≥ 0.9 Excellent
0.9 > α ≥ 0.8 Good
0.8 > α ≥ 0.7 Acceptable
0.7 > α ≥ 0.6           Questionable
0.6 > α ≥ 0.5 Poor
0.5 > α Unacceptable


Internal consistency reliability defines the consistency of the results delivered in a test, ensuring that the various items measuring the different constructs deliver consistent scores.

For example:- an English test is divided into vocabulary, spelling, punctuation and grammar. The internal consistency reliability test provides a measure that each of these particular aptitudes is measured correctly and reliably.

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