This analogy demonstrates that the validation process involves comparison with other measures of the same entity. This is not illustrated in the target shooting analogy because accuracy is defined by proximity to a visible standard. Such clear standards do not always exist vis a vis the measurement of health status. Reliability Reliability refers to the accuracy, consistency, stability over time and reproducibility of a measurement instrument. Each denotes different aspects of the reliability of a measuring instrument behaviour and all must be evaluated when determining reliability.
Internal consistency is determined by statistical examination of the results obtained.
It is assessed by administering the same measure to the same subject on two separate occasions. The two sets of scores are examined for their degree of correlation. IRR is assessed by different raters scoring the same patient at the same time and examining the degree of agreement between the scores. In some cases there is a need for two similar versions of a measure. This would be true when measuring memory, for example, when a learning effect from the initial administration of a memory test might bias the measurement of memory on the second occasion if the same measure was used.