The first step is the definition of the sample group. Each subject must have the potential to develop the outcome of interest (that is, asthmatics should not be included in a cohort designed to study lung cancer). Furthermore, the sample population must be representative of the general population if the study is primarily looking at the incidence and natural history of the condition (descriptive).
If however the aim is to analyze the relation between predictor variables and outcomes (analytical) then the sample should contain as many patients likely to develop the outcome as possible, otherwise much time and expense will be spent collecting information of little value.
Key points
Cohort studies
- Cohort studies describe incidence or natural history.
- They analyze predictors (risk factors) thereby enabling calculation of relative risk.
- Cohort studies measure events in temporal sequence thereby distinguishing causes from effects.
- Retrospective cohorts where available are cheaper and quicker.
- Confounding variables are the major problem in analyzing cohort studies.
- Subject selection and loss to follow up is a major potential cause of bias.
These are primarily used to determine prevalence. Prevalence
equals the number of cases in a population at a given point in time. All the
measurements on each person are made at one point in time. Prevalence is
vitally important to the clinician because it influences considerably the
likelihood of any particular diagnosis and the predictive value of any
investigation.
Cross sectional studies are also used to infer causation. At
one point in time the subjects are assessed to determine whether they were
exposed to the relevant agent and whether they have the outcome of interest.
Some of the subjects will not have been exposed nor have the outcome of
interest. This clearly distinguishes this type of study from the other
observational studies (cohort and case controlled) where reference to either
exposure and/or outcome is made.
How to run a cross sectional study
Formulate the research question(s) and choose the sample
population. Then decide what variables of the study population are relevant to
the research question. A method for contacting sample subjects must be devised
and then implemented. In this way the data are collected and can then be analyzed
Advantages
·
The
most important advantage of cross sectional studies is that in general they are
quick and cheap. As there is no follow up, fewer resources are required to run
the study.
·
Data
are collected only once and multiple outcomes can be studied.
·
The
subjects are neither deliberately exposed, treated, nor not treated and hence
there are seldom ethical difficulties.
·
Cross
sectional studies are the best way to determine prevalence and are useful at
identifying associations that can then be more rigorously studied using a
cohort study or randomized controlled study.
Disadvantages
·
Rare
conditions cannot efficiently be studied using cross sectional studies because
even in large samples there may be no one with the disease. In this situation
it is better to study a cross sectional sample of patients who already have the
disease (a case series).
CASE-CONTROL
STUDIES
In contrast with cohort and cross
sectional studies, case-control studies are usually retrospective. People with
the outcome of interest are matched with a control group who do not. Retrospectively
the researcher determines which individuals were exposed to the agent or
treatment or the prevalence of a variable in each of the study groups. Where
the outcome is rare, case-control studies may be the only feasible approach.
As some of the subjects have been
deliberately chosen because they have the disease in question case-control
studies are much more cost efficient than cohort and cross sectional
studies—that is, a higher percentage of cases per study.
Case-control studies determine the
relative importance of a predictor variable in relation to the presence or
absence of the disease. Case-control studies are retrospective and cannot
therefore be used to calculate the relative risk; this a prospective cohort
study. Case-control studies can however be used to calculate odds ratios, which
in turn, usually approximate to the relative risk.
How to run a case-control study
Decide on the research question to be answered. Formulate a
hypothesis and then decide what will be measured and how. Specify the characteristics
of the study group and decide how to construct a valid control group. Then
compare the “exposure” of the two groups to each variable.
Advantages
·
When
conditions are uncommon, case-control studies generate a lot of information
from relatively few subjects.
·
When
there is a long latent period between an exposure and the disease, case-control
studies are the only feasible option
·
This
type of study is therefore useful for generating hypotheses that can then be
tested using other types of study.
Disadvantages
·
In
case-control studies comparatively few subjects are required so more resources
are available for studying each. In consequence a huge number of variables can
be considered.
·
This
flexibility of the variables studied comes at the expense of the restricted
outcomes studied. The only outcome is the presence or absence of the disease or
whatever criteria were chosen to select the cases.
·
The
major problems with case-control studies are the familiar ones of confounding
variables and bias. Bias may take two major forms.
Key points
Case-control studies
·
Case-control
studies are simple to organize
·
Retrospectively
compare two groups
·
Aim
to identify predictors of an outcome
·
Permit
assessment of the influence of predictors on outcome via calculation of an odds
ratio
·
Useful
for hypothesis generation
·
Can
only look at one outcome
·
Bias
is a major problem
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