Data and functions for analyzing and simulating illusory truth datasets, developed as part of a longitudinal study by Henderson, Barr, and Simons (2020). The illusory truth effect is the observation that people rate repeated statements as more likely to be true than novel statements. We tested the trajectory of the illusory truth effect by collecting truth ratings for statements repeated across four time intervals: immediately, one day, one week, and one month following initial presentation. The package contains the anonymized data from the study along with stimulus materials, as well as functions for analyzing the data, running simulations, and calculating power. Further details about the project are available at < https://osf.io/nvugt/>, which includes Stage 1 of the Registered Report at the Journal of Cognition (< https://osf.io/vqnx2/>).