Observational study is divided into descriptive and analytical studies.
Non-experimental
Observational because there is no individual intervention
Treatment and exposures occur in a “non-controlled” environment
Individuals can be observed prospectively or retrospectively
COHORT STUDY- an “observational” design comparing individuals with a known risk factor or exposure with others without the risk factor or exposure.
looking for a difference in the risk (incidence) of a disease over time.
best observational design
data usually collected prospectively (some retrospective)
CASE CONTROL - EFFECT TO CAUSE
Retrospective
When disease is rare
.
5. Figure 9-3 Selection of study groups in experimental and observational
epidemiologic studies.
6. Basic Questions in Analytic Epidemiology
Look to link exposure and disease
1. What is the exposure?
2. Who are the exposed?
3. What are the potential health effects?
4. What approach will you take to study
the relationship between exposure and
effect?
8. Cohort Studies
an “observational” design comparing
individuals with a known risk factor or exposure
with others without the risk factor or exposure.
looking for a difference in the risk (incidence)
of a disease over time.
best observational design
data usually collected prospectively (some
retrospective)
12. •Prospective Study - looks forward, looks
to the future, examines future events,
follows a condition, concern or disease
into the future
time
Study begins here
13. •Retrospective Study - “to look back”,
looks back in time to study events that
have already occurred
time
Study begins here
24. Relative Risk
• The relative risk can be defined as the probability of an
event (developing a disease) occurring in exposed people
compared with the probability of the event in unexposed
people, or as the ratio of these two probabilities.
• RR= Risk in exposed
• Risk in unexposed
25.
26. Interpreting relative risk of a disease
If RR =1 Risk in exposed equal to risk in
unexposed(no association)
If RR >1 Risk in exposed greater than risk in
unexposed(positive association;possibly
causal)
If RR < 1 Risk in exposed less than risk in
unexposed (negative association;
possibly protective)
27. Cohort study strengths and weaknesses
Strengths weaknesses
Allows calculation of incidence Long calendar time
Examine multiple outcomes for a given
exposure
Not good for rare diseases
Clarity of temporal sequence Not good for diseases with a long latency
Good for investigating rare exposures Differential loss to follow up can
introduce bias
29. Elements of case control study
1. Selection of cases
2. Selection of controls
3. Information on exposure
4. Analysis
30. 1. Selection of cases
• All people in source population who develop the disease of
interest
• Clear definition of outcome studied
• Prevalent vs incident cases
32. 2. Selection of controls
• Population based
• Health care facility based
• Case based
33. 3. Collecting good data on exposure
• Objectively – reproducibility of exposure measurement
• Accurately – information reflecting as closely as possible the
effect of exposure
• Precisely – Quality management in exposure measurement
35. Odds ratio
• An odds ratio (OR) is a measure of association between an
exposure and an outcome.
• The OR represents the odds that an outcome will occur given
a particular exposure, compared to the odds of the outcome
occurring in the absence of that exposure.
36.
37. Interpreting odds ratio
• OR=1
• Odds of exposure among cases and controls are same
• Exposure is not associated with disease
• OR>1
• Odds of exposure among cases are higher than controls
• Exposure is positively associated with disease
• OR<1
• Odds of exposure among cases are lower than controls
• Exposure is negatively associated with disease
38. Case control study strengths and weaknesses
STRENGTHS WEAKNESSES
Good for rare outcomes Susceptible to recall bias
Relatively quick to conduct Selection of an appropriate comparison
group may be difficult
Requires comparatively few subjects Rates of disease in exposed and
unexposed individuals cannot be
determined
Multiple exposures or risk factors can be
examined
39. Example 1
Thyroid hormones, namely triiodothyronine (Free T3), thyroxine (Free
T4) and thyroid stimulating hormone (TSH) were evaluated at the time
of diagnosis of preeclampsia in 82 pregnant women and equal number
of matched controls. (Kumar et al.)
Case Control
40. Example 2
46,112 never users of oral contraception and women 819,175 ever users
were followed for 39 years to ascertain mortality risk. (Hannaford et al.)
Cohort
41. Example 3
275 women attending the antenatal clinic at Kilifi district hospital,
Kenya, were recruited in November 1993 and tested for malaria in order
to calculate the prevalence. (Shulman et al.)
Cross-Sectional
42. Example 4
270 wards randomised to 3 groups of 90 each for women to receive
weekly a single oral supplement of placebo, vitamin A or â carotene for
over 3.5 years and followed to determine pregnancy-related mortality.
(West et al.)
Clinical Trial
43. Example 5
A survey among second trimester pregnant women 18-44 took place
between April 2003 and November 2003 to determine the prevalence of
anemia and hookworm. (Larocque et al.)
Cross-Sectional
44. Example 6
431 women were enrolled in a study within 21 days of conception and
monitored throughout pregnancy to determine caffeine exposure and
pregnancy outcome. (Mills et al.)
Cohort
