1. Natasha Roberts How do we measure the health of the public? An Introduction to Epidemiology

2. Acknowledgements  This presentation has been adapted from the original presentation provided by the following contributors Paul Scott Paul Pilkington Sara Roberts Alice Walsh

3.  Descriptive epidemiology Incidence and prevalence Qualitative information o Analytical epidemiology Types of studies Association and causation What is epidemiology and what are its uses?

4. What is Epidemiology? “the study of the frequency, distribution and determinants of health problems and disease in human populations” The unit of interest is the population

5. Purpose of epidemiology “To obtain, interpret and use health information to promote health and reduce disease”

6. In the news… No evidence to support the role of antioxidant supplements in increasing the lifespan of healthy people or patients with various diseases Diagnostic heart tests are underused in older people, women, south Asians, and people from deprived areas Uptake of HPV vaccine by adolescent schoolgirls in Manchester is good overall, but lower in areas with a higher proportion of ethnic minority girls

7. How does Epidemiology help? (1) It allows the distribution of health and illness in a population to be described in terms of: WHERE and WHEN does this health problem occur in the population? WHAT is the problem and its frequency? WHO is affected? WHY does it occur in this particular population?

8. How does Epidemiology help? (2) Epidemiological information is used to: Prevent illness and promote health To help treat people with existing disease Evaluate existing health services

9. What is descriptive epidemiology? Frequency (of disease) (incidence & prevalence) + Distribution (of disease) = “descriptive epidemiology”

10. Descriptive epidemiology  Usually makes use of routinely collected data, (e.g. death certification data, hospital episode statistics, infectious disease notifications)  May require special surveys (usually cross sectional)  Can’t answer ‘why?’ but can raise a causal hypothesis  Can often provide sufficient information for public health action to be taken

11. TIME, PLACE, PERSON  Time: Trends, seasonal variations, cohort effects…  Place: Variations between geographical areas – local, national, international…  Person: Variations in health by age, sex, ethnicity, occupation, leisure interests...  All help us examine variations (inequalities) in health

12. Pneumococcal meningitis (source HPA surveillance data)

13. Public Health Action On 4 th September 2006 Pneumococcal vaccination introduced into childhood immunisation schedule!

14. International Breast Cancer Death Rates Why might death rates in the UK be high?

15. Descriptive epidemiology  By studying frequency and distribution we can describe patterns of disease  This can raise further questions and help us to generate hypotheses for the causes of disease  It also helps us to respond to public health problems

16. Measures of disease frequency There are 2 major types of measure of disease frequency: Incidence - The incidence is the number of NEW CASES of disease that develop, in a population of individuals at risk, during a specified time period Prevalence - Prevalence is the total number of EXISTING CASES of disease in a population at one point in time.

17. Incidence and prevalence Sick population (Prevalence) Healthy population Incidence (new cases) die (mortality) recover

18. Analytical epidemiology Descriptive epidemiology + Analysis of cause and effect = “analytical epidemiology”

19. John Snow  John Snow, physician (1813-1858)  Outbreaks of Cholera were common in London during the 19 th century  But what was causing the cholera? The popular theory at the time was that bad gases caused it (‘miasma’ theory)

20. What did he do? Analysis by place: he mapped the cases – most were near Broad Street (…miasma would predict even spread) Anecdote: People had complained that the water smelt bad. Cases from further afield had water delivered by cart from Broad Street.

21. Public health action He removed the handle from the Broad Street pump and the number of infections fell

22. What did he do? Recorded deaths by water supplier The Lambeth company had started to pump water from 20 miles upstream from the Thames Conclusion: Risk of infection is highest in people using water from the Southwark and Vauxhall water company.

23. Types of analytical study Observational studies Cross-sectional study (may be descriptive or analytical) Case control study Cohort study Intervention study (experimentation) Randomised controlled trial (RCT)

24. Cross-sectional study  Information on health status and other characteristics is collected from each subject at one point in time  Cross-sectional studies can be descriptive… (eg. the prevalence of cough in a population)  Or analytical… (eg. the association between cough and risk factors such as type of house lived in or whether person is a smoker)

25. EXERCISE Discuss in pairs or small groups examples of cross sectional studies you have come across.

26. Case-control study  Compares people with a condition (cases) to a similar group of people without the condition (controls)  The aim is to try and identify the risk factors which may have caused the cases to get the condition in the first place  Often used to investigate the source of an outbreak of disease.

27. EXERCISE Discuss in pairs or small groups examples of case control Studies you have come across.

28. Cohort Study  Follow up two groups of people over time and compare the occurrence of disease  One group is exposed to a possible risk factor for the disease, while the other is not (the control group)  The exposure is the starting point, the disease is the outcome of interest

29. EXERCISE Discuss in pairs or small groups examples of Cohort Studies you have come across.

30. Cohort Study (2)  Cohorts can be retrospective too  The starting point is still the EXPOSURE  Outbreak of salmonella amongst guests at a wedding  Use wedding menu to identify potential exposures and then survey the guests  Identify most likely source of the outbreak

31. Randomised Controlled Trial  Compares effectiveness of a new intervention against the best current alternative (or a placebo)  Can be for clinical or educational interventions

32. Randomised Controlled Trial  Select people with the same disease or characteristics (a defined target population)  Randomly allocate these people to ‘intervention’ or ‘control’ groups  Intervention group receives the new treatment, the control group receives the standard or placebo treatment  The benefits of each treatment are assessed by comparing the health gain in each group

33. RCT Didgeridoo playing as alternative treatment for obstructive sleep apnoea syndrome: randomised controlled trial. Reported in BMJ Dec 2005. 25 adults with obstructive sleep apnoea, randomised to didgeridoo instructions and daily practice for 4 months (14), or placing on the waiting list for lessons (11). Didgeridoo players reported less daytime sleepiness and their partners reported less night time disturbance, compared with waiting list group.

34. In the news…..BBC website Vitamin D ‘can lower cancer risk’ High doses of vitamin D can reduce the risk of developing some common cancers by as much as 50%, US scientists claim. Grapefruit 'may cut gum disease' Researchers found people with gum disease who ate two grapefruits a day for a fortnight showed significantly less bleeding from the gums. Oily fish is a source of vitamin D Grapefruit is full of vitamin C Sibling link to heart health risk Having a brother or sister with cardiovascular disease (CVD) is bad news for your own odds of developing problems, research has found. Heart disease may run in the family

35. Interpreting results of analytical studies  No association found  Association may be artifactual (false) Due to Chance Due to Bias in the study  Association may be real, but indirect Apparent relationship due to a confounding factor  Association is direct (causal, true)

36. Central dogma of epidemiology An ASSOCIATION between a risk factor (smoking) and a disease (lung cancer) DOES NOT INDICATE a CAUSAL relationship

37. Association is not proof of cause Bradford Hill’s Criteria for Causation  Strength of association  Temporal relationship  Geographical distribution  Dose-response relationship  Consistency of results  Biological plausibility (but remember John Snow)  Specificity (if a single causal agent)  Reversibility

38. Assessing the relationship between a possible cause and an outcome

39. Conclusions  Epidemiology is a core part of public health.  It allows the distribution of health and ill-health in a population to be described, and possible causal factors to be identified.  It enables public health professionals to understand health problems and take appropriate action.

40. What we have covered:  What is epidemiology and what are its uses?  Descriptive epidemiology. Incidence and prevalence  Analytical epidemiology – types of studies association and causation

41. References  Medical statistics at a glance – Petrie and Sabin. Blackwell.  Epidemiology in Medicine – Charles Hennekins. Little, Brown and Company.  Epidemiology for the uninitiated – G.Rose and D.Barker.  Health Knowledge website http://www.healthknowledge.org.uk/Epidemiology/Epide miology%201.htm