Reliability Essay

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Reliability refers, at a general level, to consistency of measurement. Consistency can be conceptualized differently for different forms of reliability estimation, but in all cases reliability is focused on whether a measurement yields consistent results.

Such consistency is critical to research practice, where variables must be operationalized and measured. For example, socioeconomic status can be measured as average family income, whether a child receives a reduced lunch rate, education level of parent, or by other variables. Regardless of method, the measurement must reflect dependable characterizations of the units of observation (e.g., people, families) on the variable. One head of house-hold might guess at his or her annual family income while another provides an accurate amount. In such a case, the variable is not being consistently measured across the units of observation.

There are three dominant measurement theories that can be used to conceptualize reliability of scores: classical test theory, generalizability theory, and item response theory. In research practice, however, it is much more common for researchers to employ the classical test theory framework than the other two methods, at least in part due to ease of use and historical precedence.

In classical test theory, sometimes called true score theory, a score is perfectly reliable only when the obtained score is measured without error. Theoretically, then, the true score (T) is a function of both the obtained score (O) and some degree of error, as indicated by:

XT = Xo + error.

Reliability can estimated in a variety of ways to account for different types of measurement error, including but not limited to, test-retest (stability), alternate forms, internal consistency, and interrater reliability.

The most common reliability statistic is coefficient alpha, as introduced by Lee Cronbach in 1951. The intraclass correlation (ICC) can also be valuable to estimate reliability in a number of situations.


  • Henson, R. K. (2001) Understanding internal consistency reliability estimates: a conceptual primer on coefficient alpha. Measurement and Evaluation in Counseling and Development 34: 177—89.

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