# Cumulative incidence

(Redirected from Incidence proportions)

Cumulative incidence or incidence proportion is a measure of frequency, as in epidemiology, where it is a measure of disease frequency during a period of time. Where the period of time considered is an entire lifetime, the incidence proportion is called lifetime risk.[1]

Cumulative incidence is defined as the probability that a particular event, such as occurrence of a particular disease, has occurred before a given time.[2] It is equivalent to the incidence, calculated using a period of time during which all of the individuals in the population are considered to be at risk for the outcome. It is sometimes also referred to as the incidence proportion.

Cumulative incidence is calculated by the number of new cases during a period divided by the number of subjects at risk in the population at the beginning of the study.

It may also be calculated by the incidence rate multiplied by duration:[3]

${\displaystyle CI(t)=1-e^{-IR(t)\cdot D}\,.}$

In order to examine progression from less severe chronic kidney disease to end-stage kidney disease (ESKD), in 2010, researchers recruited a group of 8,808 Indigenous Australians, aged between 45 to 64, who currently have chronic kidney disease. When tested at baseline (at the start of the study in 2010), 326 participants were found to have end-stage kidney disease (ESKD). Participants were followed up and tested again in 2015 (5 years later) (note: only those who previously had tested negative for ESKD were tested again at follow up). A further 248 cases of ESKD were identified during this 5 year period (either diagnosed during the interim or diagnosed at the time of the final study assessment). Data from this study are presented in the following table (Table 1): Table 1 Number of participants recruited Existing cases of ESKD identified at baseline (2010) Existing cases of ESKD (from baseline) remaining at follow-up (2015)* Number lost to follow-up in those who tested negative to ESKD at baseline New cases of ESKD at follow-up (2015) 8,808 326 251 401 248

• It is assumed that once participants have developed ESKD, they continue to have

ESKD, and that over the study follow-up period some of these cases detected at baseline were lost to follow-up a) What type of study design is this study? [1 mark] b) What was the prevalence of ESKD at the start of the study in 2010? [1.5 marks] c) What was the prevalence of ESKD in 2015 (at the end of the study)? [2.5 marks] d) What was the cumulative incidence of ESKD over the 5-year study period? [2

## References

1. ^ Rychetnik L, Hawe P, Waters E, Barratt A, Frommer M (July 2004). "A glossary for evidence based public health". J Epidemiol Community Health. 58 (7): 538–45. PMC . PMID 15194712. doi:10.1136/jech.2003.011585.
2. ^ Dodge, Y. (2003). The Oxford Dictionary of Statistical Terms. OUP. ISBN 0-19-920613-9.
3. ^ Bouyer, Jean; Hémon, Denis; Cordier, Sylvaine; Derriennic, Francis; Stücker, Isabelle; Stengel, Bénédicte; Clavel, Jacqueline (2009). Épidemiologie principes et méthodes quantitatives. Paris: Lavoisier.