Transcript No Slide Title

Observational Research
Methods
Ivan J Perry
Department of Epidemiology &
Public Health
University College Cork
Scenario
It is suggested that occupational exposure to
volatile anaesthetic agents causes depression.
You wish to test this hypothesis.
Learning objectives
At the end of this lecture you should be able to:
•List and distinguish between the four major types of
observational analytical study designs, including the
strengths and limitations of each design
•Understand the difference between prevalence and incidence
•Given a research question you should be able to select the
appropriate study design
Observational
Descriptive
Analytical
Experimental
Type of
Study
Alternative
Name
Unit of
Study
Ecological
Correlational
Populations
Cross-sectional
Prevalence
Individuals
Case-control
Case-reference
Individuals
Cohort
Follow-up
Individuals
Ecological studies often provide the first tentative evidence of
an association between a causal factor and disease
The units of analysis are populations or groups of people
rather than individuals
Pope et al., 1995
Advantages
•Ecological studies are simple to conduct, often using
pre-existing data collected for other purposes
•Data can be used from populations with widely differing
characteristics
Disadvantages
•Suitable exposure and outcome data may not be
available as the data is usually pre-existing
•Usually not possible to adjust for potential confounders
•Links between exposure and disease seen at the aggregate
(population) level may be spurious and not seen at the
individual level (ecological fallacy)
Objectives
•To examine health problem or disease
frequency
•To examine association between the exposure
and health problem or disease frequency
•Unit of analysis is individual
•Exposure and disease status of individual is
assessed at the same time
Cross-sectional Studies measure prevalence
Prevalence is the number of cases in a
defined population at a specified point in
time, expressed as a proportion of the total
population at risk for the condition. The
prevalence rate for a disease is calculated as
follows:
P = No. of people with disease at specified time
No. of people in population at risk at specified time
60
50
40
% 30
20
10
0
50-54
55-59
Male
60-64
65-69
Female
Cork and Kerry Diabetes & Heart Disease Study, 1998
Advantages
•A cross sectional study is short term,
easy and economical to conduct
•A cross sectional study generally starts
with a reference population and is
generalisable
•Causal inference can be made from cross
sectional data, provided it is known that the
exposure preceded the effect or disease
Disadvantages
•It is not possible to determine in some
cases whether the exposure preceded the
condition or disease
•Not suitable for investigation of rare diseases
•Generally requires large number of subjects
•The problem of selective survival may
be an issue
Type of
Study
Alternative
Name
Unit of
Study
Ecological
Correlational
Populations
Cross-sectional
Prevalence
Individuals
Case-control
Case-reference
Individuals
Cohort
Follow-up
Individuals
Doll and Hill’s Data
Lung cancer patients Controls Total
Smokers
647 622 1269
Non-smokers
2 27 29
Total
649 649 1298
Cases
Controls
Exposed
a
b
Not
Exposed
c
d
OR = a x d  b x c
Note: the odds ratio of the
Doll & Hill data shows
clearly how much smoking
increases the risk of lung
cancer.
Using Doll & Hill’s data:
OR = 647 x 27 = 14.04
622 x 2
Advantages
Well suited to the study of rare disease
Relatively quick and inexpensive to conduct
Requires comparatively few subjects
Existing records can be used in
some case-control studies
No loss to follow-up.
Allows study of multiple potential
causes of disease
May not be able to
establish sequence of
events
Unsuitable for the study
of rare exposure
Disadvantages
Selection of an
appropriate control
group may be difficult
Relies on record or recall for
information on past exposure
(potential for recall bias)
Incidence rate
Cumulative Incidence
Relative Risk
Incidence rate (IR)is the number of new cases arising in a
given period in a specified population. Incidence can be
measured as follows:
IR = No. people who get disease (given time period)
Sum of time each person remained under
observation and at risk of becoming a case
Usually expresses as number of cases per 1000 or per,
100,000 person years of follow-up.
Cumulative Incidence measures the denominator only at the
beginning of the study. Cumulative incidence can be measured as
follows:
No. people who get disease (given time period)
CI=
No. disease free people in population at risk at
the beginning of the period
Usually expresses as % risk during a defined period of
follow-up, e.g. over 1 year or 5 years
Ratio Measure
Incidence of disease in exposed
Incidence of disease in non-exposed
Note: in case-control studies relative
risk is derived indirectly from the
odds ratio.
Cigarette smoking and incidence rate of
stroke in a cohort of 118,539 women
Smoking category
No. stroke cases
Person-years
observation (>8)
Never smokes
Ex-smoker
Smoker
Total
70
65
139
274
395,594
232,712
280,141
908,447
Colditz et al., 1988
Stroke
incidence rate
(per 100,000
person-years)
17.7
27.9
49.6
30.2
Advantages
Suitable for the study of rare exposure
Can assess multiple outcomes (effects)
of single exposure
Can demonstrate temporal relationship
between exposure and disease
Allows direct measurement of incidence
of disease in the exposed and non-exposed
population
Changes in exposure over time can be
studied
Recall and selection bias are unlikely
Disadvantages
Not suitable for the study of a rare disease,
unless a large sample size is obtained
If prospective, can be expensive and
time consuming
If retrospective, requires the availability
of existing record
Validity of the result can be affected
by loss of follow up