Internal reliability of a test instrument is defined as

Correct option is B

Internal reliability refers to the consistency of a measurement tool over time in terms of how reliably it can produce the same results when administered at different points in time under similar conditions. Option (b) — "The consistency of a measure taken at two different points of time" — rightly reflects this aspect of reliability in testing.

In psychometrics, internal reliability is a key form of reliability that ensures that the instrument or test consistently measures the same construct. When a test produces stable and consistent results over multiple applications, it demonstrates that its internal structure — including its items and scoring — is sound and free from random measurement error. This reliability is especially crucial in psychological and educational testing, where individual items must work together to reflect the same underlying trait or concept.

One commonly used method to assess internal reliability is split-half reliability, where a test is divided into two halves, and the scores are compared. Other methods include Cronbach's alpha, which measures the average correlation among all the items in the test.

Information Booster:

• Internal reliability ensures measurement consistency across items or time within the same test.

• It is a type of psychometric reliability that validates the stability of an instrument.

• Ensures that observed scores are dependable and not affected by random error.

• Commonly assessed using Cronbach’s Alpha, split-half, or Kuder-Richardson formulas.

• High internal reliability means items are internally consistent and measure the same construct.

• Important for research tools, questionnaires, psychological scales, etc.

• A reliable test increases the credibility of research findings.

Additional Knowledge:

(a) The increase and decrease in scores measured over time produce a variance of less than 50%
This is an incorrect and ambiguous statement. It mixes the concept of reliability with variance. Internal reliability has nothing to do with the % of score variance over time.

(c) The identical observation of single individual's scores
This resembles test-retest reliability or individual-level consistency, but it lacks clarity about item-level consistency within a test.

(d) The property of a distribution to achieve mean of 0 and variance of 1
This refers to standardized (normal) distributions, particularly z-scores, and is unrelated to reliability or consistency of measurement instruments.