General Model
In practice, testing measures are never perfectly consistent.Theories of test reliability have been developed to estimate the effects of inconsistency on the accuracy of measurement. The basic starting point for almost all theories of test reliability is the idea that test scores reflect the influence of two sorts of factors:
1. Factors that contribute to consistency: stable characteristics of the individual or the attribute that one is trying to measure
2. Factors that contribute to inconsistency: features of the individual or the situation that can affect test scores but have nothing to do with the attribute being measured
Some of these inconsistencies include:
- Temporary but general characteristics of the individual: health, fatigue, motivation, emotional strain
- Temporary and specific characteristics of individual: comprehension of the specific test task, specific tricks or techniques of dealing with the particular test materials, fluctuations of memory, attention or accuracy
- Aspects of the testing situation: freedom from distractions, clarity of instructions, interaction of personality, sex, or race of examiner
- Chance factors: luck in selection of answers by sheer guessing, momentary distractions
The goal of estimating reliability is to determine how much of the variability in test scores is due to errors in measurement and how much is due to variability in true scores.
A true score is the replicable feature of the concept being measured. It is the part of the observed score that would recur across different measurement occasions in the absence of error.
Errors of measurement are composed of both random error and systematic error. It represents the discrepancies between scores obtained on tests and the corresponding true scores.
This conceptual breakdown is typically represented by the simple equation:
- Observed test score = true score + errors of measurement
Read more about this topic: Reliability (psychometrics)
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