Lesson 6: Practical application of incidence Copy

Examining the practical application of incidence data

Read the following abstract of a study which investigated the incidence of retinopathy of prematurity (ROP) in England. As you read, critically examine and consider:

  • The sources of data used for the numerator and denominator in this study.
  • The possible problems that might occur when following up a cohort in incidence studies.
  • The practical uses of incidence data for eye health service provision in the UK, the study setting.
  • The guidelines and case definition influence on the incidence of disease or treatment.

Incidence and treatment of retinopathy of prematurity in England between 1990 and 2011: database study, Painter et al. BJO 2014

Summary: The study analysed English National Hospital Episode Statistics (HES) to obtain annual rates of diagnosis of, and treatment for, babies with ROP. The researchers used national data on low birth weight (LBW) babies as the denominator to calculate the rate of ROP per 1000 babies at risk. LBW babies were defined as weighing less than 1500g at birth and therefore eligible for ROP screening.

The study found that the recorded incidence of ROP increased tenfold, from 12.8 per LBW babies in 1990 to 125.5 per 1000 LBW babies in 2011.

Study methods: The researchers examined routinely collected statistical records of babies under one year of age, in hospital in England between 1990 and 2011, using Hospital Episode Statistics (HES). HES is the routine administrative dataset for England that includes statistical information about all National Health Service inpatient events and admissions for day-case care. Records with a diagnosis of any stage of ROP, treatment of
ROP with cryotherapy or laser, were identified.

Screening for ROP: Current UK guidelines advise ROP screening for all babies born before 32 weeks’ gestation and/or under 1500g.

Data sources: There are data on birth weight, available to the public, from the Office of Population Censuses and Surveys (OPCS) for England & Wales (until 1996 called the Office for National Statistics). The study used these data to identify babies eligible for screening. HES data are available for England, whereas birth weight data are for both England and Wales. Live births in Wales accounted for 5.1% of births in England and Wales between
1999 and 2011. As this proportion was very small and stable, the discrepancy between numerators and denominators was very small compared with the large changes in diagnosis and treatment over time.

Below are the results.


  • Episode based rate = counting each baby for each admission with ROP
  • Person based rate = counting each baby with ROP only once.

Discussion points to reflect on

  • Using Table 1, describe the incidence of low birthweight babies born between 1990 and 2011.
  • From Figure 1: What was the increase in the incidence of diagnosed ROP for 2000 compared to 1990, 2005 compared to 2000, and 2010 compared to 1990?
    a. What happened after 2010?
    b. What could be the contributing factors (ophthalmic and non-ophthalmic) to the increase in incidence?
  • Using Figure 2, describe the incidence of treatment for ROP between 1990 and 2005.