XNN001 Introductory epidemiological concepts - Study design


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  • Descriptive – describes occurrences of disease or exposure. Most likely to be used to look for patterns of disease and to measure the occurrence of disease or risk factors.Analytic studies – involves planned comparisons between people with/ without disease or exposed/ not exposed groups.
  • Time point for prevalence should always be reported.Long lasting disease may have high prevalence, but low incidence. Disease that rapidly resolves may have low prevalence, but high incidence. To measure incidence need to start with cohort of people who are disease free.
  • Dietary behaviours are complex and are the culmination of a range of factors including access, availability and affordability to foods that, arguably, are beyond the realm of the individual and are characteristics of the food supply. This contrasts with physical activity, where there may be fewer contextual factors that may influence an individual’s behaviour. In understanding the epidemiology of dietary behaviours, nutrition-related health outcomes and trends among populations, an understanding of the food supply with which the population interacts is important. In bringing about better health among populations, the scope of public health nutrition extends beyond health promotion and policy strategies that encourage individuals to make more healthy dietary choices to ensuring that the available food supply also promotes these choices.
  • In Australia, information about household expenditure on food is collected periodically as part of the Australian Bureau of Statistics’ Household Expenditure Survey. Limitations of this data are that it does not collect information on the quantity of foods purchased and the descriptions of foods purchased are not sufficient to allow for analyses of nutrient contents. Information on food supply can take a variety of factors into account, including production, imports, export, industrial and animal use of food, and waste.Food Balance Sheets Provide information about the amount of food available for human consumption in a country in a given year from the FAOtakes into account production, changes in stocks, imports & exports, agricultural & industrial use of foodstuffsrelates only to primary producegive trends in food supply over time both within & between countries Relates to national average, provides estimates per kg of food per head per year OR per grams of food per head per day.Important– food availability data is not the same as food consumption data (i.e. Information on the availability of food does not tell us how much food is actually eaten).
  • To compare trends in ‘apparent’ food consumption over time: Still an estimate as not measuring actual intake at the household levelApparent Consumption Data Derived by ABS in way similar to food balance sheets, however reporting is not limited to primary produce Not used for all food products e.g. if there was a better way of deriving consumption or for foods for which all components of the equation were not available (some milk products, beer, eggs, wine, cheese etc)Gives overall trends & correlationsData is useful in nutrition planningLong term data minimizes daily & seasonal variationsLarge sample populations increase validity of conclusionsNo placebo effectNo participant error
  • XNN001 Introductory epidemiological concepts - Study design

    1. 1. INTRODUCTORY EPIDEMIOLOGICAL CONCEPTS – STUDY DESIGN XNN001 Population nutrition and physical activity assessment
    2. 2. Refresh... why is epidemiology important?  What is epidemiology?  Inform decisions  Monitor change  Equity
    3. 3. Why do we collect data?  Monitoring and surveillance  Identification of outbreaks  Identification of areas of public health significance  Effectiveness of programs  Linking exposure and disease  To identify causes of disease/ states of health  Provides indication of risk of disease based on exposure
    4. 4. Study design Study design Observationa l Experiment al Analytic - Ecological - Cross-sectional - Case-control - Cohort - Randomised controlled trial Descriptive - Case reports - Case series
    5. 5. Epidemiological approaches are applied in situations in which we are interested in upstream factors i.e. Personal, community or environmental risk factors known to affect disease rates
    6. 6. What are we measuring?  Need to have clear idea about what we are trying to determine  Prevalence vs Incidence Prevalence - proportion of population that has disease at given time (i.e. Existing cases) Number of people with disease at given point in time ___________________________________________ Total number of people in population Incidence – rate at which people develop disease/ health outcome (i.e. New cases) Number of people who develop disease in given time period _________________________________________________
    7. 7. Systematic review Randomised controlled trial Cohort study Case-control study Cross-sectional study Ecological study Case report, case series Editorials, expert opinion
    8. 8. Case reports and case series  Beginning point for scientific study – identify potential health outcomes, stimulate interest in area, potential to advance knowledge  Detailed descriptions of one/ more cases that are unusual  Selective nature and limited amount of information provided  provide little evidence of causality  cannot provide much information about patterns of disease
    9. 9. Ecological study design  First step in determining whether association exists  Studies of differences in health status (death rates, disease patterns, health-risk behaviours) between countries and regions, or within the same region at different times  Use existing data sources  Aim to generate hypotheses about environmental or behavioural exposures and disease
    10. 10.  The group, rather than the individual, is the unit of analysis  Advantages 1. Data easily available - studies inexpensive 2. Hypothesis-generating – suggest avenues for research  Disadvantages 1. Ecological fallacy: might not accurately represent the exposure – disease relationship at the individual level 2. Cannot directly link “exposure” with the “outcome”
    11. 11. Extending beyond surveys of individuals  Food supply data  Nutrient composition of foods  Australian Household Expenditure Survey  International comparisons
    12. 12. Food supply data  To calculate „available food for consumption‟ – Food balance sheets  Information on amounts of food (raw commodities) available for consumption per year Food available for use = production + imports – exports  Food available for consumption =  production + imports – exports – industrial use – animal use  Important – food available for consumption does NOT tell us how much food is actually eaten!
    13. 13. Food supply data  To compare food trends within Australia Apparent food consumption data Apparent consumption = (commercial production + estimated home production + imports + opening stocks) (exports + usage for processed foods + non-food usage + wastage + closing stocks) MINUS
    14. 14. Cross-sectional study design Begin with a defined population Exposed + have disease Exposed + do not have disease Not exposed + have disease Not exposed + do not have disease Gather data on exposure and disease
    15. 15. Advantages of cross-sectional study design 1. Relatively inexpensive 2. No follow-up required 3. Less participant burden Disadvantages of cross-sectional study design 1. Cannot assess temprality 2. Can establish associations, but not causation 3. Considerable potential for confounding 4. Neyman (prevalence) bias (diseases or exposures that are longer-lasting will be over-represented relative to those of shorter duration)
    16. 16. Australian Health Survey  http://www.abs.gov.au/websitedbs/D3310114.n sf/home/Australian+Health+Survey+- +Frequently+Asked+Questions
    17. 17. Case-control study design  Observational, retrospective & focused on individuals  Comparison of exposure frequencies among diseased (case) and non-diseased (Control) groups
    18. 18. NOT EXPOSED EXPOSED NOT EXPOSED EXPOSED Population TIME Adapted From: Fletcher et al (1996) Clinical Epidemiology: the essentials. Baltimore: Williams and Wilkins. CASES (people with disease) TIME Direction of inquiry CONTROLS (people without disease) Design of case-control study
    19. 19. Advantages of case-control study design 1. Suitable for rare diseases 2. Possible to evaluate a large number of potential causes/risk factors 3. Efficient design, requiring less time and more modest costs than prospective studies 4. Higher on hierarchy of study designs
    20. 20. Disadvantages of case-control study design 1. Potentially difficult to distinguish exposures that precede disease (antecedent causes) from concurrent associated factors 2. Requires representative samples of cases and controls 3. Selection of appropriate controls can be difficult 4. Generalisability of findings  Refusals  Representation of original populations 5. Results potentially flawed due to  Reliance upon accurate historical information about exposure, in particular recall may differ between cases and controls (recall bias)  Unsuitability of controls (lack of comparability with cases or overmatching) 6. Still cannot be CERTAIN that if exposure preceded the disease, then it is in fact causal
    21. 21. Cohort study  Observational, prospective & focused on individuals  Essence is comparison of disease frequencies among exposed and non- exposed groups
    22. 22. Population People without the disease TIME & Direction of inquiry Disease Disease No disease No disease Adapted From: Fletcher et al (1996) Clinical Epidemiology: the essentials. Baltimore: Williams and Wilkins. Exposed Not Exposed Design of cohort study
    23. 23. Advantages of a cohort study 1. Possible to establish temporality 2. Possible to study several outcomes from exposure to same hazard 3. Bias - less of a problem than case-control or cross-sectional studies
    24. 24. Disadvantages of a cohort study 1. Expensive in terms of time and money 2. Large, representative sample required 3. Higher risk of attrition bias 4. Measures used in early steps of study may change (eg, due to technological improvements) and so results at different points in time may not be directly comparable
    25. 25. Randomised controlled trials (RCT)  Experimental, prospective  Types  Clinical trials – patients  Prevention trials – healthy people  Community trials – community level
    26. 26. The structure of a clinical trial Population of patients with the condition SAMPLE TIME Improved Improved Not Improved Not Improved Experimental intervention Comparison intervention
    27. 27. Randomised Trials Participants Non- participants Experimental Population Reference Population ASSIGNMENT Randomisation Study Group Comparison Group Improved Not improved Improved Not improved
    28. 28. Advantages of RCT Advantages of RCT 1. If randomisation is successful, considered strongest design for causal inference 2. Researchers control the exposure – specific and accurate Disadvantages of RCT 1. Expensive and time consuming 2. Attrition 3. Compliance
    29. 29. http://www.health.qut.edu.au/ens/research/nourish.
    30. 30. Hierarchy of study designs Randomised control trial Cohort study Case-control study Cross-sectional study Ecological study Experimental Observational Increasingcostandevidence
    31. 31. Systematic review Randomised controlled trial Cohort study Case-control study Cross-sectional study Ecological study Case report, case series Editorials, expert opinion