A reliability of 0.5 means that there is equal measurement- and error-related variance: this means that an individual obtaining a score equal to the mean of the group could have the highest, or the lowest, underlying true value of the outcome in the group. Accordingly, a reliability of 1 means that all variability is attributable to true differences and there is no measurement error, while a reliability of 0 means that all variability is accounted for by measurement error.
It is an approximation of the fraction of the total variance which is not accounted for by measurement error (i.e.
Reliability is a measure of the accuracy or consistency of an outcome, the distinguishability of individual measurements, and a measure of the signal-to-noise ratio in a set of data. In the assessment of individual differences, reliability is typically assessed using test-retest reliability, inter-rater reliability or internal consistency. In summary, these methods and tools will allow researchers to avoid performing costly studies which are, by virtue of their design, unlikely to yield informative conclusions. An R package called relfeas also accompanies this article for performing these calculations. In this way, the feasibility of a new study can be assessed during planning stages, and before collecting any new data. I present a new method and tools for using summary statistics from previously published test-retest studies to approximate the reliability of outcomes in new samples. Reliability is usually assessed in validation studies using healthy participants, however these results are often not easily applicable to clinical studies examining different populations. This problem is exacerbated by the use of outcome measures whose accuracy is sometimes insufficient to answer the scientific questions posed.
These constraints limit the possibility of large-sample experimental designs, and often lead to statistically underpowered studies. Positron emission tomography (PET), along with many other fields of clinical research, is both timeconsuming and expensive, and recruitable patients can be scarce.
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