Dear HCP team, I have seen that the zstat values for tasks contrasts are very large in the HCP_S900_787_tfMRI_ALLTASKS_level3_zstat1_hp200_s2_MSMAll.dscalar.nii file, to the point that one can observe areas of activation in task contrasts by setting very high z value thresholds (e.g., a z threshold of +14). I think (please correct me if I'm wrong) that the z values of the S900 file are very large because the group is very large, therefore the standard deviation is very small (given that there will be less variability in a group if one takes a very large group of people rather than a small group of people), and if the standard deviation is very small then even small differences from the mean will lead to very large z values.
I was wondering what implication does this have in terms of statistical significance. A z value of 14 or larger would correspond to an extremely small p value, i.e. it would be extremely unlikely to observe by chance a measure which is 14 times the standard deviation away from the mean. Would it therefore be correct to assume that the areas that we can observe in the S900 tfMRI_ALLTASKS task contrasts with a very high zstat threshold (e.g., 14) are statistically significant, without having to worry about multiple comparisons or family structure? Thank you very much, Xavier. _______________________________________________ HCP-Users mailing list [email protected] http://lists.humanconnectome.org/mailman/listinfo/hcp-users
