Abstract Reliability is central to how researchers approach measurement in standard, group‐based analyses of single‐time‐point data, yet this critical aspect is often overlooked in the analysis of repeated observations. Since its inception, reliability has been a between‐person concept, but we redevelop this notion for within‐person designs by proposing a new coefficient of reliability for single‐subject designs. This coefficient shares the same general definition of reliability as former coefficients—the ratio of the true score variance to the total variance—but applies to time‐dependent within‐person variability rather than independent between‐person variability. Coefficient begins with a latent variable time series model called a state space model, and is then extended to a state space model for multiple subjects with continuous or discrete variation across people. Using analytic methods, we derive coefficient and prove its relations to other coefficients of reliability.
Peter Woodruff (Wed,) studied this question.