Source: https://www.researchgate.net/profile/Mdjulkar_Nayeen_Mahi/post/How_to_calculate_RMSSTD_root_mean_square_standard_deviation_for_Text_cluster_analysis/attachment/59d640d379197b807799cf4d/AS%3A432280024948736%401480074997073/download/Presentation_Malo.pdf