1.2 Calculating disease incidence
In this section we will ask you to do some simple calculations of weekly disease incidence, which is made using the formula:
Here is a worked example:
In an office with 140 employees, during February 2019 (4 weeks), 17 contracted influenza. What is the weekly incidence of influenza in this office?
Incidence = 17/ (140 x 4) = 0.0304 cases per employee per week.
Activity 2 Calculating disease incidence
In a secondary school, during the Winter term lasting from January – March 2021 (13 weeks) a number of students were recorded as absent due to COVID-19 infection. The data was broken down according to individual year groups (Table 1). Calculate the weekly incidence of infection in each year group. Don’t forget the units.
| Year group | Number of students | Number of cases |
| 9 | 196 | 16 |
| 10 | 229 | 42 |
| 11 | 168 | 34 |
| 12 | 154 | 24 |
Which year group shows the highest incidence of COVID-19 infection?
Answer
Did you see that year 11 has the highest incidence (0.0156 cases per student per week), even though the total number of cases was higher in year 10.