Incidence

The term 'incidence' may seem straightforward, but it can refer to three distinct metrics: the total number of new cases, cumulative incidence, and incidence density. The calculations of these metrics can vary in difficulty (1). As demonstrated by Havers-Borgersen et al. (2) mistakes are published; however, numerous errors remain probably undetected as a result of unclear methodological descriptions.

1. Incident numbers

In the most basic form, incidence begins with the number of new cases that occur in a population over a specified period. It does not convey any information regarding risk on its own: 500 new cases in a village of 5000 is clearly not the same as in a city of 5 million. To be useful, the number of new cases of a disease needs to be compared to the number of people who are at risk.

2. Cumulative Incidence

The cumulative incidence is a classic risk measure, the number of new cases divided by the number of people free of disease at the start of the follow-up period, over a fixed time window. The measure is a proportion, even if it sometimes is presented as per 1,000 persons, and it answers a specific question: What proportion of an initially disease-free group develops the disease during follow up? Because the measure is based on a head count, it is easy to understand. However, it can only be measured if everyone is followed for the same amount of time and no one is lost to follow-up.

3. Incidence Density

In general, real populations are more dynamic. People enter and leave a cohort, die of other causes, emigrate, or are simply followed for different lengths of time. To deal with this, incidence density divides new cases by the total person-time at risk rather than by the number of people: the sum of the time each individual actually spent under observation before developing disease, dying, being lost to follow-up, or study ending.

Person-time at risk

For population-level incidence rates, person-time is typically not calculated from individual follow-up but approximated using official demographic statistics. It is then calculated as the average number of people in the population during the period, multiplied by the length of that period. The average population (mid-year population) is calculated as the average of the population size at the beginning and end of the period.

References

1. Spronk, I., Korevaar, J.C., Poos, R. et al. Calculating incidence rates and prevalence proportions: not as simple as it seems. BMC Public Health 19, 512 (2019). https://doi.org/10.1186/s12889-019-6820-3.

2. Havers-Borgersen E, Butt JH, Johansen M, Petersen OB, Ekelund CK, Rode L, Olesen JB, Køber L, Fosbøl EL. Preeclampsia and Long-Term Risk of Venous Thromboembolism," JAMA Network Open, 2023;6(11):e2343804 — formal correction notice, "Miscalculation of Incidence Rates," JAMA Network Open, published Jan 11, 2024 (DOI: 10.1001/jamanetworkopen.2023.54306).