Evidence-based medicine
is based on research which comes into account through several different study
designs. Different study designs give different outcomes. In other words, in
different circumstances, the researcher requires to use different but the best
design to carry out a study. At the same time, each study design has strengths
and limitations. However, the researcher tries to use the design where
advantages overweigh the disadvantages. Study designs are usually based on:
- The prevalence of health problems
- Health problems
- Evaluation of therapy, including treatment and prevention
Qualitative research:
In qualitative research, the researcher determines the relationship between one
thing (an independent variable) and another (dependent or outcome variable) in
a population that is selected for the study purpose.
Goals: To explore the depth and complexity of a
phenomenon
Quantitative research:
It is a formal, objective, systemic way of obtaining information about the
world. Here, a method used to describe test and examine cause-effect
relationship. Quantitative studies are then divided into two experimental and
observational.
Goals:
To test relationships, describe and examine cause-effect relations
Types of study designs:
Observational studies
The researcher just observes and collects information, but does not try to change the people (or animals or reagents) being observed or the outcome.
Examples
- A survey of smoking habits among the factory workers
- A researcher joins to study life style of a gang (the researcher dose not try to change his style or behavior, this is observational study)
- You take blood samples to measure blood alcohol levels during specific days (yes, you are intervene to take the blood, but you do not alter the outcome).
Observational Designs
Cross-sectional surveys:
Cross-sectional surveys determine the frequency of a particular attribute, for instance, a specific exposure or disease or any other health-related event, in the study population at a particular point in time.
Cohort, or longitudinal, or prospective studies:
It is a forward study that may run over a long time. It is type of clinical or observational study in which the researchers analyze a risk factor or cause in a particular groups of un-affected people e.g. people exposed to pollution, taking same type of vaccination or drugs etc. In other words, comparison between two groups is done. Subgroups in a cohort may be compared with each other.
Case-Control study:
It
is the type of observational study where the researcher selects two groups from
the same population; one subgroup having specific disease and the other
subgroup does not. Both subgroups are exposed to same circumstances. It leads
to the conclusion where the researcher can recommend and control the outcome of
the specific diseases in the population.
Example:
A
group is taken from American whites who are equally exposed to HIV some people
belong to this group are affected by HIV and some are not. So it will give the
way to researcher why some are affected and some are not.
Experimental studies
The researcher intervenes to change something (e.g. gives some patients a drug) and then observes what happens. In an observational study there is non-intervention.
Examples
- Offer a beer to law students and see whether they argue better or not when they are drunk.
- Encourage one group of bikers to stop smoking the funny-looking cigarettes to check whether they get less or more belligerent.
- Warn one group of students who drink that they would raise their blood alcohol levels next Monday and then compare level of blood alcohol levels of this group with those who were not warned.
- A school teacher gave two different types of pens to two groups of the students. He informed the one group that it will improve your writing skills and he did not inform the other group. Next week he observed that there was no improvement in the handwriting of the students in either group. It means that the pens had no effect on the writing skills.
Experimental Designs
Randomized controlled trial (RCT):
The RCT is one of the simplest
but most powerful tools in the research. Kane and
Radosevich state as “Clinical research worships at the shrine of
RCT”. It refers to a study where the elements are randomly selected to
receive one of several clinical interventions. The term “intervention” used in
RCTs usually refers to treatment; however, it can be used in much wider sense referring
to clinical maneuver/s that may have an effect on the health status of the
participants. The clinical maneuvers may include:
· Prevention strategies
· Screening programs
· Diagnostic tests
· Interventional procedures
· The setting in which health care is provided
· Educational models
In radiology, RCT can play an important role in the assessment of
screening programs, diagnostic tests and procedures.
Example:
RCT
has studied effect of raloxifene on fracture risk in postmenopausal women and
reported that the women who used raloxifene drug over the same five year period
of time as the women who did not take the drug reduced their risk of clinical
vertebrate fracture.
Quasi-experimental designs:
Quasi study design estimates the
causal impact of an intervention on its target population or study population. It
has many similarities with traditional experimental design or RCTs, but they lack randomization. Quasi-experimental
design allows the researcher to control the assignment to the treatment
condition without random assignment (e.g. an eligibility cutoff mark). In some cases, the researcher may have
no control over assignment to treatment condition.
Advantages and disadvantaged of study designs:
Advantages and Disadvantages of study
designs
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Study Design
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Advantages
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Disadvantages
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Cross
sectional
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Quick,
covers whole population and offers representativeness
|
Causes
biases, diagnostic inaccuracy and no causal sequence
|
Cohort
|
Prospective, establish casual sequence and
estimates incidence
|
Costly and time-consuming
|
Case
control
|
Economical
to focus casual effect
|
Need
recall of past events and cases may not be equal to controls
|
RCT
|
Removes biases, offers etiologic and
evaluative research
|
Ethical issues and generalizability
|
Quasi
experimental
|
Can
use natural experiments and may be more effective than RCT
|
Significant
bias
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