Please enter valid positive numbers. Events cannot exceed Total Subjects.
Control Event Rate (CER)
0%
0 / 0
Experimental Event Rate (EER)
0%
0 / 0
Comparative Analysis
Relative Risk (RR)
–
Rel. Risk Reduction (RRR)
–
Abs. Risk Reduction (ARR)
–
Number Needed to Treat (NNT)
–
function calculateEventRates() {
// Get Inputs
var cEvents = parseFloat(document.getElementById('controlEvents').value);
var cTotal = parseFloat(document.getElementById('controlTotal').value);
var eEvents = parseFloat(document.getElementById('expEvents').value);
var eTotal = parseFloat(document.getElementById('expTotal').value);
var errorMsg = document.getElementById('er-error-msg');
var resultsDiv = document.getElementById('er-results');
// Validation
if (isNaN(cEvents) || isNaN(cTotal) || isNaN(eEvents) || isNaN(eTotal) || cTotal <= 0 || eTotal cTotal || eEvents > eTotal) {
errorMsg.innerHTML = "Error: Number of events cannot be greater than total subjects.";
errorMsg.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
// Hide error if valid
errorMsg.style.display = 'none';
resultsDiv.style.display = 'block';
// Calculations
var cer = cEvents / cTotal;
var eer = eEvents / eTotal;
var rr = (cer === 0) ? 0 : eer / cer;
var arr = cer – eer;
var rrr = (cer === 0) ? 0 : (cer – eer) / cer;
// NNT Calculation (Inverse of ARR)
var nnt = 0;
var nntText = "-";
if (arr !== 0) {
nnt = 1 / arr;
// Usually NNT is rounded up to the next whole number for clinical relevance
nntText = Math.ceil(Math.abs(nnt));
if (arr < 0) {
nntText += " (NNH – Harm)";
}
} else {
nntText = "Infinity (No Difference)";
}
// Display Updates
document.getElementById('displayCER').innerHTML = (cer * 100).toFixed(2) + "%";
document.getElementById('displayCERFraction').innerHTML = cEvents + " events / " + cTotal + " subjects";
document.getElementById('displayEER').innerHTML = (eer * 100).toFixed(2) + "%";
document.getElementById('displayEERFraction').innerHTML = eEvents + " events / " + eTotal + " subjects";
document.getElementById('displayRR').innerHTML = rr.toFixed(4);
document.getElementById('displayRRR').innerHTML = (rrr * 100).toFixed(2) + "%";
document.getElementById('displayARR').innerHTML = (arr * 100).toFixed(2) + "%";
document.getElementById('displayNNT').innerHTML = nntText;
}
Understanding Event Rate Calculations
The Event Rate Calculator is an essential tool for epidemiology, clinical research, and evidence-based medicine. It allows researchers to quantify the frequency of specific outcomes (events) within a defined population over a specific period. By comparing the event rates of two distinct groups—typically a Control Group and an Experimental Group—we can derive critical statistical measures regarding the efficacy of a treatment or intervention.
Key Definitions
Control Event Rate (CER): The proportion of patients in the control group who experience the event. It acts as the baseline risk.
Experimental Event Rate (EER): The proportion of patients in the experimental (treatment) group who experience the event.
Relative Risk (RR): The ratio of the probability of an event occurring in the exposed group versus the non-exposed group (EER / CER).
Number Needed to Treat (NNT): The number of patients you need to treat to prevent one additional bad outcome. Ideally, this number should be low.
Formulas Used
Metric
Formula
Interpretation
Event Rate
Events / Total Subjects
Percentage of group experiencing the outcome.
Absolute Risk Reduction (ARR)
CER – EER
The actual difference in risk between groups.
Relative Risk Reduction (RRR)
(CER – EER) / CER
How much the risk is reduced relative to the baseline.
NNT
1 / ARR
Patients needed to be treated to benefit one person.
Example Calculation
Consider a clinical trial for a new heart medication:
Control Group: 100 patients given a placebo. 15 suffer a heart attack. (CER = 15/100 = 0.15 or 15%)
Experimental Group: 100 patients given the new drug. 10 suffer a heart attack. (EER = 10/100 = 0.10 or 10%)
Using the calculator above, we find:
ARR: 15% – 10% = 5% (0.05). The drug reduces absolute risk by 5%.
RRR: (15% – 10%) / 15% = 33.3%. The drug reduces relative risk by one-third.
NNT: 1 / 0.05 = 20. You need to treat 20 people with the drug to prevent 1 heart attack.
Why Distinction Matters
It is crucial to look at both Absolute and Relative reductions. A treatment might claim a "50% reduction in risk" (Relative Risk Reduction), but if the event is extremely rare (e.g., dropping from 0.0002% to 0.0001%), the Absolute Risk Reduction is minuscule, and the Number Needed to Treat would be massive. This calculator helps contextualize those statistics.