Global Blindness: An introduction to epidemiology
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The word ‘epidemiology’ is of Greek origin: ‘Epi’, meaning ‘upon’, ‘Demos’, meaning ‘population’, and ‘Logos’, meaning ‘study’. This translates to ‘a study of that which is visited upon a population’. We now define epidemiology as the study of: - distribution of health outcomes or disease in a population - determinants that influence the occurrence of disease - application, which provides direction for public health action. In this presentation, we consider each of these components more thoroughly and discover some commonly used epidemiological terminology. As you read, consider how epidemiological data supports, or could support, eye care work. -- © 2015 International Centre for Eye Health, London School of Hygiene & Tropical Medicine. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license. This presentation is part of the free online course: Global Blindness: Planning and Managing Eye Care Services. We encourage its use and adaptation for teaching and learning. Please help us to evaluate it and find more eye care Open Educational Resources on our website: http://iceh.lshtm.ac.uk/oer
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Global Blindness: An introduction to epidemiology
- 1. Global Blindness: Planning and managing eye care services An introduction to epidemiology
- 2. Learning outcomes Describe the definition of epidemiology Understand how epidemiology supports public health activities
- 3. Definition of epidemiology epi - upon demos - population logos - study
- 4. Definition of epidemiology Distribution of health outcomes or disease Determinants influence occurrence of disease Application - direction for public health action
- 5. Distribution of disease Frequency Pattern of occurrence • by place • by time • by characteristics
- 6. Determinants that influence the occurrence of disease • Exposure • Behaviour • Genetics
- 7. Application
- 8. Epidemiology provides important information • To prevent disease • To prioritise planning • To promote use of evidence
- 9. Prevalence is the measure of how many people have a disease in a population at a specific time
- 11. To calculate prevalence Prevalent cases Study population denominator numerator We need to know two numbers:
- 12. x 100 = 5 50 10% To calculate prevalence: example
- 13. Prevalence data allows us to understand and quantify the disease burden • To allocate resources • To measure impact
- 14. Prevalence data can be mapped Prevalence of active trachoma ≥ 30%
- 15. Incidence refers to the number of people in a population who develop a disease over a specific time A specific time new cases
- 16. To calculate incidence New cases over the time period Study population followed We need to know two numbers: denominator numerator
- 17. Example: incidence 10,000 diabetics followed up for 6 years 800 new cases
- 18. Example: Incidence Risk of developing retinopathy in 6 years 800 10,000 8 per 100 diabetics= =
- 19. Summary Epidemiology provides the clues for the magnitude, distributions and determinants of diseases affecting a population.
- 20. Summary Prevalence data is very important for planning public health interventions. Incidence data identifies the probability that a health outcome will occur.
- 21. Written by Dr Daksha Patel © 2016 International Centre for Eye Health, London School of Hygiene & Tropical Medicine. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license. Taken from the free online course: Global Blindness: Planning and Managing Eye Care Services. We encourage the use and adaptation of this resource for teaching and learning. Find more Open Educational Resources for eye care on our website at http://iceh.lshtm.ac.uk/oer
Editor's Notes
- In this presentation we introduce epidemiology through two bite size learning outcomes. By the end of the presentation you should be able to: Describe the definition of epidemiology Understand how epidemiology supports public health activities
- The word epidemiology comes from a Greek origin: Epi meaning upon, Demos meaning population and Logos meaning study. This translates to: A study of that which is visited upon a population
- The definition that we now use is: Epidemiology is the study of distribution of health outcomes or disease in a population, determinants that influence the occurrence of disease, and application, which provides direction for public health action.
- The distribution of disease can be shown as the frequency and pattern of occurrence. Frequency can be a number or the rate of risk of disease in a population. The pattern of occurrence can be described: by time – for example an annual or a daily occurrence by place – for example an urban or rural location by characteristics – for example by age, gender, ethnicity
- Determinants – the risk factors or causes of occurrence of disease – relate to exposure, behaviour and genetic risk factors
- Application is the public health action that can be taken to improve health within a population. Application is also sometimes referred to as disease control strategies. Image: Robert Essel / International Trachoma Initiative CC BY-NC 2.0 flic.kr/p/9hocNj
- Epidemiology provides important information about a disease within a population, and about what the risk factors are, or could be. Health workers use this information to implement control strategies: To prevent disease from occurring or spreading. To provide the health system with priorities for planning and targeting health services To promote the use of evidence for effective clinical care and policy development.
- In order to understand epidemiological information you need to be familiar with some commonly used terminology, such as prevalence and incidence. Prevalence is the measure of how many people have a disease, or health state, in a given population at a specific time.
- A good analogy for this is a photograph of the population and knowing how many people in the photograph have a disease Image: Corporal Peter R. Miller, US Marine Corps http://commons.wikimedia.org/wiki/File:EinwohnerRangabaliBangladesch.jpg
- To calculate the prevalence of a disease we need to know two numbers: Firstly, the study population, that is the total number of people in the population we are studying The number of prevalent cases, that is the number of people with disease at a specific time. To calculate the prevalence we divide the prevalent cases (the numerator) by the study population (the denominator). We can then multiply by 100 to get a percentage.
- For example, let us consider a school with 50 children, and of those 5 pupils wear spectacles. To calculate the prevalence we divide 5 by 50 and then multiply by 100 to get a percentage. This gives us a prevalence of 10% for spectacle wearing children at this school.
- Prevalence data allows us to understand and quantify the disease burden and health outcome. Health workers can use this information to allocate resources appropriately. They can also use it to measure the impact of health services by collecting prevalence data before, and after, a health intervention. For example, how much of a change did providing cataract surgery have on reducing blindness in a given population?
- Prevalence information can be mapped to show where the need is the greatest. For example, as shown in this trachoma map, the areas where active trachoma is most prevalent will benefit from mass distribution of azithromycin. Map: Trachoma Atlas Project CC BY-NC-ND www.trachomaatlas.org
- Incidence refers to the number of individuals in a population who develop a disease over a specific time. This is a measure of risk that the disease will occur.
- To calculate the incidence of a disease we need to know two numbers: The number of incident cases, that is the number of new cases of disease during a specific time period. And the study population, the total number of people in the population we are studying To calculate incidence we divide the number of new cases (the numerator) by the study population (the denominator).
- In this example we see how incidence can help the medical professionals predict the probability of developing a complication. 10,000 insulin dependent diabetics over the age of 40 were followed up for six years, to see if they developed background changes in their retina in keeping with background retinopathy. 800 new cases were identified in that period.
- To calculate the risk, or incidence, of developing retinopathy amongst diabetics in this population we take number of new cases, 800, and divide by 10,000 the population being followed up. This gives us a risk of risk of 0.08 or eight new cases in every 100 diabetics who would develop background changes in their retina in six years. This is called a probability.
- In summary, Epidemiology provides the clues for the magnitude, distributions and determinants of diseases affecting a population.
- Prevalence data is very important for planning public health interventions Incidence data identifies the probability that a health outcome will occur