25 biostats equations

[Kruno]

not all of these equations are important for the usmle, but its helpful to learn a few. and if you know a little biostats and can visualize a two by two sample table, then you can probably figure out most of them.

Sensitivity = A / (A+C)
False Negative Rate = C / (A+C)
Specificity = D / (B+D)
False Positive Rate = B / (B+D)

Positive Predictive Value = A / (A+B)
False Discovery Rate = B / (B+D)
Negative Predictive Value = D / (C+D)
False Omission Rate = C / (C+D)

Positive Likelihood Ratio = [A / (A+C)] / [B / (B+D)]
Negative Likelihood Ratio = [C / (A+C)] / [D / (B+D)]
Informedness = [A / (A+C)] + [D / (B+D)] - 1
Markedness = [A / (A+B)] + [D / (C+D)] - 1

Accuracy = (A+D) / (A+B+C+D)
Precision = A / (A+B)

Relative Risk = [A / (A+B)] / [C / (C+D)]
Relative Risk Reduction = 1 - [A / (A+B)] / [C / (C+D)]
Absolute Risk Reduction = [C / (C+D)] - [A / (A+B)]
Number Needed To Treat = 1 / [C / (C+D)] - [A / (A+B)]
Attributable Risk = [A / (A+B)] - [C / (C+D)]
Number Needed To Harm = 1 / [A / (A+B)] - [C / (C+D)]

Odds of Exposure (Cases) = A / C
Odds of Exposure (Controls) = B / D
Odds Ratio = (A / C) / (B / D)

Prevalence = [(A+C) / (A+B+C+D)]
Incidence = [(A+C) / (A+B+C+D)] / Time Period


don't try to memorize these equations... the list is here to help illustrate the pattern. i tried to organize these equations in a way that's easy to follow, but if you would like a better visual presentation, check out my youtube video... List of Equations

[Kruno]

How To Calculate

Sensitivity

...if you know the number of true positives and the total number of people with the disease

...if you know the prevalence of the disease, the number of positive tests, and the positive predictive value

Specificity

...if you know the number of true negatives and the total number of people without the disease

...if you know the prevalence of the disease, the number of negative tests, and the negative predictive value

False negative rate

...if you know the number of false negatives and the total number of people with the disease

False positive rate

...if you know the number of false positives and the total number of people without the disease

[Kruno]

Definitions and Equations

Here is the same list, but with a youtube link for each equation. These short videos show the definition and briefly explain how to make the calculation.

Sensitivity = A / (A+C)
False Negative Rate = C / (A+C)
Specificity = D / (B+D)
False Positive Rate = B / (B+D)

Positive Predictive Value = A / (A+B)
False Discovery Rate = B / (B+D)
Negative Predictive Value = D / (C+D)
False Omission Rate = C / (C+D)

Positive Likelihood Ratio = [A / (A+C)] / [B / (B+D)]
Negative Likelihood Ratio = [C / (A+C)] / [D / (B+D)]
Informedness = [A / (A+C)] + [D / (B+D)] - 1
Markedness = [A / (A+B)] + [D / (C+D)] - 1

Accuracy = (A+D) / (A+B+C+D)
Precision = A / (A+B)

Relative Risk = [A / (A+B)] / [C / (C+D)]
Relative Risk Reduction = 1 - [A / (A+B)] / [C / (C+D)]
Absolute Risk Reduction = [C / (C+D)] - [A / (A+B)]
Number Needed To Treat = 1 / [C / (C+D)] - [A / (A+B)]
Attributable Risk = [A / (A+B)] - [C / (C+D)]
Number Needed To Harm = 1 / [A / (A+B)] - [C / (C+D)]

Odds of Exposure (Cases) = A / C
Odds of Exposure (Controls) = B / D
Odds Ratio = (A / C) / (B / D)

Prevalence = [(A+C) / (A+B+C+D)]
Incidence = [(A+C) / (A+B+C+D)] / Time Period

[Kruno]

here is new playlist of quick calculations for the 25 biostats equations in my EpiStats channel. the videos use a biostats calculator (which has a sample table) to show each step that is required to make the calculation.

Absolute Risk Reduction

Accuracy

Attributable Risk

False Discovery Rate

False Negative Rate

False Omission Rate

False Positive Rate

Informedness

Incidence

Markedness

Negative Likelihood Ratio

Number Needed To Harm

Number Needed To Treat

Negative Predictive Value

Odds of Exposure (Cases)

Odds of Exposure (Controls)

Odds Ratio

Positive Likelihood Ratio

Positive Predictive Value

Precision

Prevalence

Relative Risk

Relative Risk Reduction

Sensitivity

Specificity

[Kruno]

i have almost completed a playlist of animated lectures which demonstrate how to make each calculation, but in the meantime.... here is the equations table from the quick info section of the EpiStats app