This study investigates whether data from people with endometriosis (n = 58) and fibromyalgia (n = 58) exhibit what is called "ergodicity," meaning that results from analyses of aggregated group data can be used to support conclusions about the individuals within the groups. The variables studied here are commonly investigated in chronic pain: pain intensity, pain interference, depressive symptoms, psychological flexibility, and pain catastrophizing. Data were collected twice daily for 42 days from each participant and analyzed in 2 ways: as separate cross-sectional group studies using the timepoints as the separate data sets (between-person) and as individual longitudinal studies using each person's time series data (within person). To confirm ergodicity, the results from the 2 analyses should agree. However, this is not what was observed in several respects. The between-person data showed substantially less variability compared with within-person data. This was evident in both the summary statistics involving single variables and in the correlational analyses. Overall, between-person correlations were relatively restricted in range, while within-person correlations varied widely. These findings have potentially profound implications for the field of chronic pain research. Because ergodicity was not found, this raises doubts around the assumption that aggregated data collected from groups can accurately represent the range of individual experiences in chronic pain. These results advocate for a shift toward inclusion of more individual person-focused approaches as an addition to group-based approaches. This shift could lead to more personalized and effective treatments by better capturing and then clarifying the heterogeneous nature of chronic pain, including the processes that underlie it.