Analytic Epidemiology: types of study designs

experimental designs
experimenter has control over the exposure and participants are randomized

Ecologic Study
study in which the units of analysis are populations or groups. Comparing several different populations.
information about both exposures (explanatory variables) and outcomes is collected.
use group rather than individual because indiv. measurements might not be available but group ones are (archives, )
Common outcome variables: mortality, cause specific mortality, types of morbidity, etc.

Intervention Study
an investigation involving intentional change in some aspect of the status of the subjects
ex. introduction of a preventive or therapeutic regimen or an intervention designed to test a hypothesized relationship
randomized control trails

a population group or subset that is followed over a period of time. comparing the cases of individuals
Birth or age cohort (baby boom generation)
work cohort (employers studied for occupational exposures)
School/education Cohort

Quasi-experimental study
the investigator manipulates the exposure factor but does not randomly assign the individuals to groups.

Observational studies
lots of research.
the investigator does not have control over the exposure factor.
the experimenter is not able to randomly assign subjects

Retrospective Cohort study
one that makes use of historical data to determine exposure level at some baseline in the past. followup for subsequent occurrences of disease between baseline and the present is performed

historical perspective study
combines retrospective and prospective approaches

Case control Study
subjects are defined on the basis of the presence or absence of an outcome of interest. Cases = indiv. who have disease or outcome
control group = do not have the disease
can only examine a single outcome or a limited set of outcomes
experimenter interviews cases and controls about past exposures

Randomized control trails
experiment in which subjects in the population are randomly allocated into groups (referred to as study and control groups) to receive or not to receive an experimental preventative or therapeutic procedure, or intervention. aka placebo. the results are accessed by rigorous comparisons of rates of disease, death, recovery, or other appropriate outcome in the study and control groups.

Prospective Cohort study
investigator starts with disease free groups for which exposures are determined first. the groups are then followed to see if development of disease will occur.

Case Control Advantages/Disadvantages
1. can be used to study low prevalence conditions
2. relatively quick and easy to complete
3. usually inexpensive
4. involve smaller number of subjects
1. measurement of exposure may be inaccurate
2. representativeness of cases and controls may be unknown
3. provide indirect estimates of risk
4. the temporal relationship between exposure factor and outcome cannot always be ascertained

Cohort Studies Advantages/Disadvantages
Permit direct observation of risk
expose factor is well defined
can study exposures that are uncommon in the population
the temporal relationship between factor and outcome is known
expensive and time consuming
complicated and difficult to carry out
subjects may be lost to followup during the case of the study
exposures can be misclassified

Ecologic Comparison study
an assessment of the association between exposure rates and disease rates during the same time period.

Matched case-control study
the cases and the controls have been matched to a certain set of criteria such as age, sex, race, etc.

Ecologic Study Advantages/disadvantages
may provide information about the context of health
can be performed when indv. measurements not available
can be conducted rapidly and with minimal resources
the ecologic fallacy
imprecise measurement of exposure

Seven factors that characterize study design
1. who manipulates the exposure factor
2. How many observations are made?
3. what is the directionality of exposure?
4. What are the methods of data collection?
5. What is the timing of data collection
6. What is the unit of observation?
7. How available are the study subjects?

Number of observations made
sometimes, observations of subjects is only once. this is the approach of cross-sectional studies, many ecologic studies, and most case control studies.
Sometimes more observations are made. Cohort studies and experimental studies

Directionality of Exposure – Retrospective approach
obtaining information about exposures that happened in the past. this method used in case control studies.
investigator starts out with someone that already has the disease and asks them about exposures that may have led to disease

Directionality of Exposure – A single point in time
the study is referenced about a single point in time, in a survey. (used in cross sectional studies)

Directionality of Exposure – Prospective Approach
information about the study outcome is collected in the future. experimental studies and cohort studies

Data Collection Methods
some studies use only existing, previously collected data while other studies need to collect new data.
Ecologic Studies use existing data

Timing of Data collection
in some studies information is collected about exposure from the past. if long periods of time have elapsed between the time the measurement of the exposure was taken and occurrence of disease, questions might be raised about the quality and applicability of the disease (person might not remember exposure to disease, therefore info is not valid)

in other studies, subjects may be followed (into the future) over a period of time. If subjects drop out of the study the outcome variable may be lost.

Unit of observation
the individual or an entire group. most epidemiologic study designs use the individual as the unit of observation. ecologic studies use the group as the unit of observation

Availability of Subjects
Certain groups of subjects may not be available for research (ex. for ethical issues)

Ecologic Correlation
an association between two variables measured at the group level

explanatory variables
those studied as correlates of outcome variables
ex. sex, socioeconomic level, age, income inequality, race, prevalence of physicians, unemployment, etc

Census tract
a small permanent statistical subdivision of a country, or equivalent. contains 1,000 and 8,000 people.

Census Block
an area bounded on all sides by visible and/or nonvisible features shown on a map prepared by the census bureau. smallest

Ecologic Fallacy
an erroneous inference that may occur because an association observed between variables on an aggregate level does not necessarily represent or reflect the association that exists on an individual level

Odds ratio
(an indirect measure of risk) a measure of the association between frequency of exposure and frequency of outcomes used in case control studies. the risk of an outcome associated with an exposure is estimated by calculating the odds of exposure among the cases and controls
(AD)/(BC) = OR
OR of more than one suggests a positive association
when it is 1.0 there is no association

Relative Risk
the measure of association used in cohort studies.
the ratio of the incidence rate (of the disease or health outcome) in an exposed group to the incidence rate (of the disease or health outcome) in a non exposed group.

incidence = risk of occurrence of an outcome

RR= (A/A+B)/(C/C+D)

Attributable Risk
in a cohort study refers to the difference between the incidence rate of a disease in the exposed group and the incidence rate in the non exposed group

Population Risk Difference
provides an indication of the benefit to the population derived by modifying a risk factor. This measure is the difference between the rate of disease in the non exposed segment of the population and the overall rate in the population.

PRD = incidence in the total population – incidence in the non-exposed segment

prophylactic trails
designed to test preventative measures

therapeutic trails
evaluate new treatment methods

clinical trail
the research activity that involves the administration of a test regimen to humans to evaluate its efficacy and safety

Single Blind Study/double blind study
the participants don’t know which group they are in. / the experimenters and the participants don’t know

Cross over design
participants may be switched between treatment groups

Community intervention (trails)
is an intervention designed for the purpose of educational and behavioral changes at the population level.
community trails are expensive, complex and time consuming

Program evaluation
the determination of whether the program meets stated goals and is justified economically

External Validity
ones ability to generalize from the results of the study to the external population
some studies may select subjects by taking a sample of convenience – or by using random samples +

sampling error
type of error that arises when values obtained for a sample differ from the values of the parent population (well defined selection criteria)

Internal Validity
the degree to which the study has used methodologically sound procedures.
ex. subjects need to be randomized, appropriate and reliable measurements need to be taken.

Hawthorne effect
participants behavioral changes as a result of their knowledge of them being in a study

Recall Bais
the fact that cases (participants in the study) may remember an exposure more clearly than controls

Selection Bias
distortions that result from procedures used to select subjects and from factors that influence participation in the study

Healthy worker effect
observation that employed populations tend to have lower mortality experience than the general population.

the distortion of a measure of the effect of an exposure on an outcome due to the association of the exposure with other factors that influence the occurrence of the outcome.

A- subjects with the disease that are exposed
B- no disease, exposed
C- with disease not exposed
D- no disease, not exposed

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