Fragility Index, Fagan Nomogram & 2×2 Tools

JournalFeed Free Evidence-Based Medicine Tools

Fragility Index (Randomized Trial)

Enter a 2×2 table. We use two-sided Fisher’s exact test. FI adds events to the arm with fewer events until p ≥ 0.05.

Fragility Index

—

p-value (Fisher, two-sided)

—

p ≥ 0.05 is non-significant

Fragility Quotient

—

FI ÷ total sample size

Effect (RR)

—

Treatment Events (a)

Treatment Non-events (b)

Control Events (c)

Control Non-events (d)

Continuity correction for zero cells ⓘ

Alpha (for display only)

	Treatment	Control	Total
Event	20	35	55
No event	80	65	145
Total	100	100	200

Definition. The Fragility Index (FI) is the smallest number of participants whose outcome status would need to change from non-event to event in the arm with fewer events for the result to become non-significant (two-sided Fisher’s p ≥ α). The Fragility Quotient (FQ) is FI divided by the total sample size, which allows for normalization across studies.

Fagan Nomogram (Interactive)

Drag the left dot (pre‑test) or middle dot (LR), or use the inputs below.

Pre‑test probability

—

(drag left axis or edit)

Likelihood ratio

—

LR > 0 for +/− tests

Post‑test probability

—

(computed)

Pre‑test probability (%)

Input mode

Likelihood ratio (LR)

Tip: LR+ often >1; LR− between 0 and 1. Enter any LR > 0.

Pre‑test LR anchor Post‑test

How it works: Post‑test odds = Pre‑test odds × LR. Probabilities convert to odds via p/(1−p). Odds convert back to probability via odds/(1+odds). The diagram is for intuition; the numeric values are the source of truth.

EBM Calculator Suite

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Input Data

True Positives (TP)

False Positives (FP)

False Negatives (FN)

True Negatives (TN)

2×2 Contingency Table

Diagnostic Test Performance

	Disease +	Disease -	Total
Test +			100
Test -			100
Total	95	105	200

Quick Interpretation

This 2×2 table shows the performance of a diagnostic test. Values update automatically as you enter data.

Diagnostic Performance

Sensitivity

89.5%

True positive rate

Specificity

85.7%

True negative rate

PPV

85.0%

Positive predictive value

NPV

90.0%

Negative predictive value

LR+

6.3

Positive likelihood ratio

LR-

0.12

Negative likelihood ratio

Accuracy

87.5%

Overall correct rate

Prevalence

47.5%

Disease prevalence