1. The Basics of Epidemiology

2. Epidemiology Definition Epidemiology is the study of the distribution and determinants of disease or conditions in human populations.

3. Associations Types of Associations: A.Non-statistical: Not quantifiable B.Statistical: 1. Non-causal: does not meet all criteria 2. Causal: a: Indirect: intervening variable b: Direct: no intervening variable

4. Causal Associations Postulates for establishing cause: 1.Strength 2.Time - cause before effect 3.Plausibility – biological 4.Confounding – controlling confounding effects

5. Causal Associations (cont’d) 5 ) Graduation – dose response – As the etiologic factor increases the disease or condition increases. 6) Consistency – comparison with other works-need for literature review. 7) Specificity – detailing the causal pathway.

6. Basic Measures of Associations

7. Epidemiologic Measures Frequency - a simple count of affected individuals

8. Epidemiologic Measures (continued) Proportion - a ratio in which x is part of the denominator x = —— x + y Note: often referred to as rate.

9. Epidemiologic Measures (continued) Rates - measure the occurrence of events in a population

10. Epidemiologic Measures (continued) Incidence - number of new events or cases of disease that develop in a population of individuals at risk during a specified time interval

11. Epidemiologic Measures (continued) Incidence rate = new cases occurring during a given time period population at risk during the same period

12. Epidemiologic Measures (continued) Prevalence – number of individuals in a population who have the disease at a specific point in time

13. Epidemiologic Measures (continued) Prevalence rate = number of existing cases of disease total population at a point in time

14. x = —— y Epidemiologic Measures (continued) Ratios - obtained by dividing one quantity by another

15. Relative Ratio (RR o ) The rate of two Incidence or Prevalence rates yields the RR o which suggests strength of association

16. Attributable Difference The subtraction of one Incidence or Prevalence rate from another yields the Attributable rate which suggests specificity

17. JanMayDe c ABCABC E D Fe b MarJunJulAprAugSe p OctNov Incidence and Prevalence Exercise

18. Example: Calculating Incidence and Prevalence Two groups of 100 each babies are observed for one year with respect to pneumonia cases. The following cases were observed over the time periods indicated: Babies whose mothers contracted Chlamydia during pregnancy Month: M01M02M03M04M05M06M07 M08M09M10M11M12 a----------------------- b-------------------------- c--------------------------------------------------------- d------------------------------------------------------------ e-------------------------------------------- f---------------------------------- g-------------------------- h------------------------------------ i---------------------------------------------------- j-------------------------------------- k----------------------------------------------- l------------------------------------ m------------------------------------------------------ n-------------------------------- o------------------------------------------------ p---------------------------------------------- Babies whose mothers did not contract Chlamydia during pregnancy: M01M02M03M04M05M06M07M08M09 M10M11M12 q-------------------------- r---------------------------------- s-------------------- -------------- t----------------------- Note1: the above chart describes episodes of illness. For example case ‘i’ started at the beginning of month 1 (M01) of the study year and lasted until the end of month 5. Note2: the first letter of the month corresponds to the first day, the second the tenth, the third to the 20th, and the fourth to the last day of the month. Assignment: Analyze these data.

19. 10.1 <5 5 - 14.9 15+ DE MA RI CT NJ MD DC NH VT 4.1 3.2 20.7 31.1 17.1 12.0 9.7 34.6 152.1 31.3 14.6 39.3 15.0 17.9 20.8 19.3 13.6 10.2 2.8 12.5 5.5 5.1 3.6 3.1 3.2 7.0 4.3 3.3 0.5 1.0 1.7 1.4 8.9 7.5 12.0 5.5 7.8 6.5 3.6 7.4 7.9 10.6 5.5 6.2 8.2 10.5 9.8 11.5 13.2 3.7 AIDS Rates, Reported in 2001 Rate (per 100,000 population) Guam 7.6 US Pacific Islands 0.3 US Virgin Islands 28.6 Puerto Rico 32.3

20. Basic Designs

21. Basic Designs of a Study The Epidemiologic Diagram Cs----------  Ds Cf The Independent Variable: The cause. The Etiologic factor of study The ‘ X ’ factor The Dependent Variable: The effect The outcome disease or condition The ‘ Y ’ factor

22. Basic Design of a Study The major categories of studies is define by control over the independent variable –Observational: Studying the existing relationship between the independent and the dependent variable without intervention –Experimental: Studying the relationship between the independent and dependent variables by affecting a status or change in the independent variable and observing a change in the dependent variable.

23. Basic Designs The Descriptive: Summarizes and presents information on the patterns of disease occurrence and the etiologic variable. The conclusions from this approach are the specification of research questions or the development of hypotheses. The Analytic: Compares and formulates information on the patterns of disease occurrence and the etiologic variable. The conclusions from this approach are the solutions to research questions or the acceptance of hypotheses.

24. Descriptive Studies Present the patterns of disease occurrence in human populations Provide observations on disease occurrences Suspect etiologic or confounding factors.

25. Analytic Studies Make Comparisons Test Hypothesis Establish Causality: a) Strength c) Time b) Dose d) Confounding Consistency Specificity

26. Major Descriptive Factors PERSON –Age (1) degree of exposure (2) susceptibility/immunity –Gender (Sex) –Ethnicity (Race) –Marital Status –Socioeconomic Status PLACE TIME

27. Basic Designs of a Study Three major types of analytic studies: As defined by the observational perspective. –The prospective: From a population defined by the etiologic factor, the occurrence of disease is observed. –The Cross-Sectional: Within a defined population information on the occurrence of independent and dependent variables are ascertained. –The Retrospective: From a population defined by the disease state, the history of exposure to the etiologic factor is observed.

28. ProspectiveCross- Sectional Retrospective Time/DirectionForward (From Ef) Bi-directionalBackward (From DS) CostExpensive--------------Inexpensive Epi DiagramEf Ds BiasLeast--------------Most StatisticsIc, RR, AdPc, RR, AD%Ef. RR, AD, Ro TermsGot, Get, RiskHaveHad Usual FormCohort Clinical Trial Experimental Survey Census Case-Control Case History A Review of Study Designs

29. Types of Bias ObserverRespondent Selection: -Selection-Response Bias -Loss/Follow Up -Confounder Bias Missclass- ification:-Observer -Selective Recall -Instrument -Regression