Determining reliability of daily measures: An illustration with data on teacher stress

Intensive longitudinal data is increasingly used to study state-like processes such as changes in daily stress. Measures aimed at collecting such data require the same level of scrutiny regarding scale reliability as traditional questionnaires. The most prevalent methods used to assess reliability of intensive longitudinal measures are based on the generalizability theory or a multilevel factor analytic approach. However, the application of recent improvements made for the factor analytic approach may not be readily applicable for all researchers. Therefore, this article illustrates a five-step approach for determining reliability of daily data, which is one type of intensive longitudinal data. First, we show how the proposed reliability equations are applied. Next, we illustrate how these equations are used as part of our five-step approach with empirical data, originating from a study investigating changes in daily stress of secondary school teachers. The results are a within-level (ω^w), between-level (ω^b) reliability score. Mplus syntax for these examples is included and discussed. As such, this paper anticipates on the need for comprehensive guides for the analysis of daily data.