1. Epidemiology MPH 531 Analytic Epidemiology
Mpundu MKC
MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

2. Analytic Epidemiology
Analytical studies are 2nd major type of epidemiological studies
Analytic epidemiology is concerned with the search for causes and effects, or the why and the how.
Describes association between Exposure and Outcome
btwn

3. Basic Question in Analytic Epidemiology
Are exposure and disease linked? E D
Basic designs in epidemiology examine if exposures are correlated with disease.
Exposure
Disease

4. Analytic Epidemiology
Descriptive epidemiology
identifies several characteristics of persons with disease and we may question whether these features are really unusual, but descriptive epidemiology does not answer that question.

5. Analytic Epidemiology
Analytic epidemiology provides a way to find the answer by comparing groups which provides baseline data.
Comparison group provides a baseline data, a key feature in analytic epidemiology

6. Analytic Epidemiology
e.g. In an out break of Typhoid
We may find that persons with a particular characteristic (e.g. poor hygiene) are more likely than those without the characteristic to develop a disease.
The characteristic is said to be associated with the disease.
The characteristic may be a demographic factor such as age, race, or sex , behaviour, life style, genetic, place etc.

7. Analytic Epidemiology
Identifying factors that are associated with disease helps to identify populations at increased risk of disease; then target public health prevention and control activities.
Identifying risk factors also provides clues to direct research activities into the causes of a disease.

8. Analytic Epidemiology
Analytic epidemiology (studies) are concerned with the search for causes and effects or the why and how
Analytic epidemiology quantifies the association btwn exposures and outcomes and test the hypotheses about the causal relationship

9. TerminologIES used in Studies
Exposure
Study factors
Predictors
Explanatory variables
Risk factors
Determinants
Characteristics e.g. Gender, age, ethnicity
Out-come/Disease
Health status e.g. Malaria, HIV, TB
Mortality
Disability

10. Types of epidemiological studies
Cross-sectional
Cohort
Observational
Case-control
Case Reports/Series
Studies
Clinical Trials
Experimental
Community Trials

11. Types of epidemiological Studies
Two broad types
1. Observational Studies
Does not use randomisation
Subject determines his/her exposure status
The investigator is role of simply observing exposure status and subsequent disease outcome

12. Types of epidemiological Studies
Observational studies take the form of:
Basic designs
Descriptive (cross sectional studies)
Analytic (case control, cohort)
Hybrid Designs (Nested Case-Control, Case-Cohort)
Incomplete Designs (Ecologic)

13. Types of epidemiological Studies
2. Experimental Studies
Uses randomisation to allocate subjects to different categories of the exposure
Experimental designs take the form of:
Individual randomised trial (RCT)
Cluster randomised trials (villages, communities, schools)

14. Types of epidemiological Studies
Directionality of Studies
1. Forward
Cohort studies & clinical trials
2. Backward
Case control studies
3. Non-directional
Cross sectional studies

15. Types of epidemiological Studies
Timing
1. Retrospective (Backward)
Health outcome has occurred before the study is initiated
2. Prospective (Forward)
Outcome occurs after onset of the study
Clinical trials are always prospective
Cohort and case-control studies may either be retrospective or prospective

16. Measures of
Association

17. 1. RELATIVE RISK (RISK RATIO )
Compares the risk of some health-related event such as disease or death in two groups.
The two groups are typically differentiated by:
Demographic factors such as sex (e.g., males vs females)
Exposure to a suspected risk factor (e.g., Oral contraceptive or not).

18. RELATIVE RISK/Risk Ratio
The group that is of primarily interest in the numerator; the group being compared with in the denominator
Risk for group of primary interest
Risk Ratio =
Risk for comparison group
For measure of disease risk = incidence or attack rate.

19. RISK RATIO/RELATIVE RISK
Interpretation of Risk Ratio
RR = 1:
identical risk in the two groups
no association between exposure & incidence
RR > 1:
higher risk for the numerator group
Suggests a positive association
RR < 1:
Suggests the exposed are at lower risk of developing disease compared to non-exposed (decreased risk for the numerator group).

20. Risk Ratio/RELATIVE Risk
e.g. of Risk Ratio
Pellagra is a disease caused by dietary deficiency of niacin and characterized by dermatitis, diarrhea, and dementia.
Data from a comparative study can be summarized in a two-by-two table.
Data from the pellagra study are shown in Table below. The totals for females and males are also shown.

21. PELLAGRA CASES BY SEX Female a =46 b =1,438 1,484 Male c= 18 d = 1,401
YES NO
TOTAL
Female
a =46
b =1,438
1,484
Male
c= 18
d = 1,401
1, 419

22. Risk Ratio/RELATIVE Risk
To calculate the risk ratio of pellagra for females versus males, we must first calculate the risk of illness among females and among males.
Risk of illness among females = 𝑎 𝑎+𝑏 = 𝟒𝟔 𝟏,𝟒𝟖𝟒 =
Risk of illness among males = 𝒄 𝒄+𝒅 = 𝟏𝟖 𝟏,𝟒𝟏𝟗 = 0.013

23. Risk Ratio/RELATIVE Risk
The risk of illness among females is or 3.1% and the risk of illness among males is or 1.3%.
In calculating the risk ratio for females versus males, females are the group of primary interest and males are the comparison group.

24. Risk Ratio/RELATIVE Risk
3.1%
Risk ratio = = 2.4
1.3%
The risk of pellagra in females is 2.4 times higher than the risk in males. or
Females are 2.4 times at risk of developing pellagra than men.

25. THE RELATIVE RISK(RISK RATIO)
The relative risk is a ratio of two risks.
Assume that among the 100 people at risk, 50 are men and 50 women, If 15 men and 5 women develop influenza.
Calculate the relative risk of developing influenza in men, as compared with women, is:
RR= Risk in exposed =a/a+b
Risk in unexposed = c/c+d

26. THE RELATIVE RISK(RISK RATIO)
Risk in men =15/50
Risk in women = 5/50
15/50 : 5/50 = 3.0
Interpretation
Men are 3.0 times at risk of developing influenza than women
The risk of developing influenza was 3.0 times higher in men than the risk in women

27. 2. ODDS RATIO
Odds are simply a ratio of the probability that an event will occur versus the probability that the event will not occur.
Odds ratios, therefore are simply a ratio of odds.
Refer to the ratio of the odds of an event occurring in the exposed group versus the unexposed group.

28. 2. ODDS RATIO
These groups might be men and women, an experimental group and a control group.
The odds ratio is a measure of effect size, describing the strength of association.

29. Odds Ratio Calculation
Odds Ratio is the number of events divided by number of non-events
The odds that an Exposed person develops disease = 𝒂 𝒃
The odds that Non exposed develops
disease = 𝒄 𝒅

30. Odds Ratio Calculation
Outcome
Exposure
Disease
No Disease
Exposed A B
Not Exposed C D
Odds Ratio
= (a/b) / (c/d) = ad / bc

31. Odds Ratio Calculation
Odds Ratio is the ratio of the odds disease in the exposed to the odds disease in the non- exposed
OR=odds that exposed person develops disease = 𝒂 𝒃
odds that non exposed person develops disease 𝒄 𝒅

32. ODDS Ratios Odds Ratio = 𝒂𝒅 𝒃𝒄 Odds Ratio = 𝒂 𝒃 𝒄 𝒅
Odds Ratio = 𝒂 𝒃 X 𝒅 𝒄
Odds Ratio = 𝒂𝒅 𝒃𝒄 a b c d

33. e.g. of Odds Ratio Death = (300)(340) / (89)(100) = 7.1 Yes NO
a b
Diabetes No
c d
OR = ad / bc
= (300)(340) / (89)(100)
= 7.1

34. e.g. of Odds Ratios
We may conclude that the odds of dying among those with diabetes is 7.1 times high than those who are not diabetic Or Those who have diabetes are 7.1 times more likely to die earlier than those who not have diabetes

35. ODDS RATIO
Interpretation of ODDS Ratio
OR = 1:
Indicates that the event is equally likely to occur in both groups.
OR > 1:
Indicates that the condition or event is more likely to occur in the first group.
OR < 1:
Indicates that the condition or event is less likely to occur in the first group.

36. MEASURES OF PUBLIC HEALTH IMPACT

37. MEASURES OF PUBLIC HEALTH IMPACT
These is a measure of the Public Health Impact of a causative factor.
Attributable risk
Attributable (risk) fraction
Population attributable risk
Population attributable (risk) fraction
These measures assume that association between Exposure and Disease has already been shown to be causal

38. 1. Attributable RISK (proportion)
The incidence of disease in the exposed population whose disease can be attributed to the exposure.
we assume that the occurrence of disease in a group not exposed to the factor under study represents the baseline or expected risk for that disease
we will attribute any risk above that level in the exposed group to their exposure.

39. Attributable RISK (proportion)
Thus, the attributable risk is the proportion of disease in an exposed group attributable to the exposure.
It represents the expected reduction in disease if the exposure could be removed (or never existed).
Incidence in Exposed – Incidence in unexposed
AR = Ie – Iu

40. Attributable Risk
For two specified subpopulations, identified as exposed or unexposed to a suspected risk factor, with risk of a health event recorded over a specified period
AR= (Risk exposed group) −(Risk unexposed group)
AR = a/(a + b) – c/(c + d)

41. Did not develop lung cancer
e.g. Attributable RISK
Cigarette Smoking
Developed lung cancer
Did not develop lung cancer
Total
Yes 70
6930
7000
(a)
(b)
(a+b) No 3
2997
3000
(c)
(d)
(c+d)

42. (Incidence in exposed) −(incidence in unexposed)
e.g. Attributable RISK
(Incidence in exposed) −(incidence in unexposed)
Incidence rates
a. among smokers
= 70/7000 = 10 per 1000.
b. Among non-smokers
= 3/3000 = 1 per 1000
Attributable Risk = 10 – 1
= 90%
90% of lung cancer was due to smoking.

43. 2. ATTRIBUTABLE RISK FRACTION (ARF)
The proportion of disease in the exposed population whose disease can be attributed to the exposure. ARF = (Ie - Iu)/Ie

44. 3. POPULATION ATTRIBUTABLE RISK (PAR)
The incidence of disease in the total
population whose disease can be
attributed to the exposure.
The incidence rate of disease in the total
population minus the incidence of disease
among those who were not exposed to the
suspected causal factor.
PAR = Ip - Iu

45. POPULATION ATTRIBUTABLE RISK (PAR)
The incidence rate of disease in the total population minus the incidence of disease among those who were not exposed to the suspected causal factor.
PAR= Incidence in pop −incidence in unexposed
PAR = Ip - Iu
Importance
Provides an estimate of the amount by which the disease could be reduced if the suspected factor was eliminated or modified.

46. 4. POPULATION ATTRIBUTABLE RISK FRACTION (PARF)
The proportion of disease in the total population whose disease can be attributed to the exposure.
PARF = (Ip - Iu)/Ip

47. Key Message
Thus a risk is a proportion
But an odds is a ratio