TABLE 1.
Specificity of MRI analyses: false positive rates in one‐sample and two‐sample analyses
| GM | WM | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| One sample t test | OLS | 0.02/0.022 | 0.029/0.023 | ||||||||
| WLS | 0.027/0.022 | 0.026/0.013 | |||||||||
| Two sample t tests | N 1 = 2 | N 1 = 5 | N 1 = 10 | N 1 = 30 | N 1 = 60 | N 1 = 2 | N 1 = 5 | N 1 = 10 | N 1 = 30 | N 1 = 60 | |
| OLS | 0.860/0.245 | 0.509/0.104 | 0.203/0.055 | 0.074/0.049 | 0.051/0.042 | 0.618/0.182 | 0.254/0.066 | 0.132/0.043 | 0.076/0.021 | 0.072/0.032 | |
| WLS | 0.895/0.268 | 0.601/0.112 | 0.332/0.061 | 0.102/0.047 | 0.075/0.046 | 0.592/0.167 | 0.256/0.05 | 0.117/0.03 | 0.068/0.024 | 0.071/0.033 | |
Note: The false positive rates are reported at the voxel/cluster‐levels. For one‐sample t tests, the false positive rate was below 0.05, the p value used for statistical significance, for both OLS and WLS analyses. For two‐sample t tests the false positive rate, reported for different numbers of subjects N 1 in the first group, increased similarly for OLS and WLS analyses in unbalanced group comparisons (Salmond et al., 2002). The higher number of false positives in grey matter arises from cortical regions affected by magnetic field inhomogeneities leading to bias on the R2* estimates (see Figure S4).