Basic Statistics Calc

Note: Although the calculator uses percentages for clinical context (e.g. 67% instead of 0.67), formulae are displayed below in ratios for simplicity. Diagnostic test: A = true positives B = false positives C = false negatives D = true negatives Prevalence (or pre-test probability) = (A + C) / (A + B + C + D) Sensitivity = A / (A + C) Specificity = D / (B + D) Negative likelihood ratio = (1 – sensitivity) / specificity Positive likelihood ratio = Sensitivity / (1 – specificity) Pre-test odds = Prevalence / (1 – prevalence) Post-test odds = Prevalence × likelihood ratio Post-test probability = Post-test odds / (1 + post-test odds) Positive predictive value = A / (A + B) Or, (sensitivity × prevalence) / [(sensitivity × prevalence) + (1 – specificity) × (1 – prevalence) Negative predictive value = D / (C + D) Or, specificity × (1 – prevalence) / [(1 – sensitivity) × prevalence + specificity × (1 – prevalence)] Treatment : Experimental event rate*, EER = A / (A + B) Control event rate**, CER = C / (C + D) Relative risk, RR = EER / CER Odds ratio, OR = (A / B) / (C / D) Absolute risk reduction, ARR = EER – CER Number needed to treat, NNT = 1 / ARR Relative risk reduction, RRR = ARR / CER *Risk of outcome (absolute risk) in the experimental group **Risk of outcome (absolute risk) in the control group. If ARR <0, then it is an absolute risk increase . If NNT <0, then it is the number needed to harm .