Figure 8. Data quality significantly mediates observed associations with age in youth.

Having found that data quality is associated with both age and cortical thickness, we evaluated whether data quality might systematically bias inference regarding brain development. To do this, a mediation analysis was performed for each cortical region (A), where we evaluated if the Euler number mediated the apparent relationship between age and cortical thickness. At each region, Sobel z-scores were calculated as the test statistic for the mediation analysis. A positive Sobel's value indicates that when controlling for data quality an increased effect of age was revealed; a negative Sobel's value indicates that when controlling for data quality a diminished association with age was present (B). This procedure was applied to both the training (C) and internal test set (D) from the PNC, which revealed consistent mediation effects in both samples. Data quality significantly mediated the relationship between age and cortical thickness in a bidirectional, regionally heterogeneous manner. After controlling for data quality, the apparent age effect was increased in many regions (regions in warm colors), where higher data quality was associated with thicker cortex (see Figure 7). However, in a subset of regions including the occipital and posterior cingulate cortex, controlling for data quality resulted in a diminished association with age (cool colors). Multiple comparisons were accounted for using FDR (q <0.05).