40.3.1  Counting the number, percentage and types of cases

In order to count the number of cases of a particular disease or disorder, you need to be able to decide if a person really has that condition or not. You make your diagnosis based on criteria given in a case definition, i.e. a set of standard descriptions of the disease. You will learn about case definitions in detail in Study Session 41.

To calculate the percentage of cases that are due to a particular disease or disorder, you divide the number of cases of that condition (e.g. malaria) by the total number of cases of all diseases and disorders combined, and multiply the result by 100.

For example, there were four cases of malaria in the three days covered by the data in Table 40.1, and 18 cases in total of all diseases or disorders. The percentage of malaria cases is therefore:

4 ÷ 18 × 100 = 22.2%.

So 22.2% of all cases seen in those three days in Zemen Health Post were due to malaria.

Now complete Activity 40.1.

Activity 40.1  Ranking causes of disease/disorder in order of magnitude

Take a pen or pencil and a sheet of paper, and draw a table ranking the causes of disease/disorder shown in Table 40.1 according to their magnitude (i.e. the largest number at the top and the smallest number at the bottom). For each disease/disorder listed in your table, give the number of cases seen, and calculate the percentage of the total cases that were due to this cause.


Your table should look like Table 40.2 below.

  • Use your version of Table 40.2 to answer the following question. What is the highest ranking (i.e. most common) disease/disorder among people attending the Zemen Health Post during the three days of data collection, and what is the second most common diagnosis?

  • Malaria is the most common disease/disorder and pneumonia is the second most common diagnosis, as Table 40.2 shows.

Table 40.2  List of causes of disease/disorder seen at Zemen Health Centre during three days, ranked in order of magnitude (based on data in Table 40.1).

Disease/disorder Number of casesPercent of total cases
Malaria422.2% (4/18)
Pneumonia313.6% (3/18)
Measles211.1% (2/18)
Injury (laceration, fracture)211.1% (2/18)
Meningitis211.1% (2/18)
Diarrhoea211.1% (2/18)
Tuberculosis15.5% (1/18)
Polio15.5% (1/18)
Abortion15.5% (1/18)

40.3  Analysis and interpretation of public health data

40.3.2  Incidence rate