.incidence-calculator-container {
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
max-width: 800px;
margin: 0 auto;
background: #fff;
padding: 20px;
border-radius: 8px;
box-shadow: 0 2px 10px rgba(0,0,0,0.1);
}
.calc-header {
background-color: #2c3e50;
color: white;
padding: 15px;
border-radius: 6px 6px 0 0;
text-align: center;
margin-bottom: 20px;
}
.calc-header h2 {
margin: 0;
font-size: 24px;
}
.form-group {
margin-bottom: 20px;
}
.form-group label {
display: block;
font-weight: 600;
margin-bottom: 8px;
color: #333;
}
.form-group input, .form-group select {
width: 100%;
padding: 12px;
border: 1px solid #ddd;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.form-group .help-text {
font-size: 12px;
color: #666;
margin-top: 5px;
}
.calc-btn {
background-color: #3498db;
color: white;
border: none;
padding: 15px 30px;
font-size: 18px;
border-radius: 4px;
cursor: pointer;
width: 100%;
font-weight: bold;
transition: background 0.3s;
}
.calc-btn:hover {
background-color: #2980b9;
}
#result-area {
margin-top: 30px;
padding: 20px;
background-color: #f8f9fa;
border: 1px solid #e9ecef;
border-radius: 4px;
display: none;
}
.result-value {
font-size: 32px;
color: #2c3e50;
font-weight: bold;
text-align: center;
margin-bottom: 10px;
}
.result-label {
text-align: center;
color: #7f8c8d;
margin-bottom: 20px;
}
.result-details {
border-top: 1px solid #ddd;
padding-top: 15px;
font-size: 14px;
line-height: 1.6;
}
.content-section {
margin-top: 40px;
line-height: 1.6;
color: #333;
}
.content-section h2 {
color: #2c3e50;
border-bottom: 2px solid #3498db;
padding-bottom: 10px;
margin-top: 30px;
}
.content-section h3 {
color: #2c3e50;
margin-top: 20px;
}
.content-section p, .content-section li {
font-size: 16px;
margin-bottom: 15px;
}
.formula-box {
background: #f1f8ff;
padding: 15px;
border-left: 4px solid #3498db;
font-family: monospace;
margin: 20px 0;
}
function calculateIncidenceRate() {
var cases = document.getElementById('newCases').value;
var time = document.getElementById('personTime').value;
var multiplier = document.getElementById('multiplier').value;
var unitSelect = document.getElementById('timeUnit');
var unitText = unitSelect.options[unitSelect.selectedIndex].text.toLowerCase();
// Clean inputs
var nCases = parseFloat(cases);
var nTime = parseFloat(time);
var nMult = parseFloat(multiplier);
// Validation
if (isNaN(nCases) || isNaN(nTime) || nCases < 0 || nTime <= 0) {
alert("Please enter valid positive numbers for Cases and Person-Time.");
return;
}
// Calculation
var rawRate = nCases / nTime;
var adjustedRate = rawRate * nMult;
// Formatting
var decimals = 2;
if (adjustedRate < 0.1) decimals = 4;
else if (adjustedRate < 1) decimals = 3;
var finalString = adjustedRate.toFixed(decimals);
// Display Logic
document.getElementById('result-area').style.display = 'block';
document.getElementById('finalRate').innerHTML = finalString + "
";
var unitSuffix = "person-years";
if (unitText.includes("months")) unitSuffix = "person-months";
if (unitText.includes("days")) unitSuffix = "person-days";
var interpretationHtml = "
" +
"(" + nCases + " cases ÷ " + nTime + " " + unitSuffix + ") × " + nMult.toLocaleString() + " = " + finalString;
document.getElementById('interpretation').innerHTML = interpretationHtml;
}
What is Person-Years Incidence Rate?
The Incidence Rate (often called Incidence Density) is a fundamental measure in epidemiology that calculates the frequency with which a disease or other incident event occurs over a specified period. Unlike cumulative incidence, which measures the proportion of people who develop a disease, the incidence rate explicitly incorporates time into the denominator.
This method is particularly useful in dynamic cohorts (populations where people enter and leave the study at different times) or when study participants are followed for varying lengths of time. The denominator uses "person-time" (most commonly person-years), representing the sum of the time periods that all individuals in the study were at risk of developing the disease.
How to Calculate Person-Years Incidence Rate
To calculate the incidence rate, you need two primary pieces of data: the count of new cases and the total time the population was at risk.
Incidence Rate = (Number of New Cases / Total Person-Time at Risk) × Multiplier
Step-by-Step Guide
- Identify New Cases: Count the number of individuals who developed the condition during the study period. Do not include existing cases.
- Calculate Person-Time: Sum the amount of time each individual contributed to the study while they were healthy and at risk. If a person develops the disease after 2 years in a 5-year study, they contribute only 2 person-years.
- Divide: Divide the number of cases by the total person-time.
- Standardize: Multiply the result by a standard base (like 1,000 or 100,000) to make the number readable and comparable.
Example Calculation
Imagine a study following 500 nurses to track the incidence of a specific occupational injury.
- Scenario: The study lasts for 5 years. However, not everyone stays for the full 5 years. Some leave, and some get injured.
- Total Person-Time: After summing the time for all 500 nurses, the researchers calculate a total of 2,200 person-years at risk.
- New Cases: During this time, 15 nurses sustain the injury.
Using the formula:
Rate = 15 / 2,200 = 0.006818
To make this readable, we multiply by 1,000:
Rate = 6.82 cases per 1,000 person-years.
Why Use Person-Years Instead of Just Population?
Using a simple population count (Cumulative Incidence) assumes everyone was observed for the exact same amount of time. In reality, people drop out of studies, pass away from other causes, or enter the study late.
Person-Years accounts for these discrepancies, providing a more accurate reflection of the "force of morbidity" or how fast new cases are occurring in the population relative to the actual time observed.