types of study :
A good analysis must be
preceded by a good design .it should be emphasized that the form of the
statistical form of the statistical analysis is determined by the type of study design. depending on the existing state of knowledge about a
problem that is being studied.
Types of study:
· Non
intervention studies
· intervention
studies
Non intervention studies: Non-Interventional Study or Non-Interventional Trial is a study where
the medicinal product(s) is (are) prescribed in the usual manner in accordance
with the terms of the marketing authorization. The assignment of the patient to
a particular therapeutic strategy is not decided in advance by a trial protocol
but falls within current practice and the prescription of the medicine is
clearly separated from the decision to include the patient in the study.
·
Intervention studies: Interventional studies are often performed in laboratories and clinical
studies to establish beneficial effects of drugs or procedures. The present
section introduces the readers to randomised controlled study design.
·
Descriptive
studies:
A descriptive study is
one in which information is collected without changing the environment (i.e.,
nothing is manipulated). Sometimes these are referred to as “ correlational ”
or “ observational ” studies. The Office of Human Research Protections (OHRP)
defines a descriptive study as “Any study that is not truly experimental.” In
human research, a descriptive study can provide information about the naturally
occurring health status, behavior, attitudes or other characteristics of a
particular group. Descriptive studies are also conducted to demonstrate associations or relationships between things in the world around you.
Descriptive case studies: A descriptive case
study is one that is focused and detailed, in which propositions and questions
about a phenomenon are carefully scrutinized and articulated at the outset.
This articulation of what is already known about the phenomenon is called a descriptive
theory . It helps to specify the boundaries of the case, and it contributes
significantly to the rigor of the finished case study. The power and promise of
a descriptive case study lie in its potential for mining for abstract
interpretations of data and theory development. The main goal of the
descriptive case study is to assess a sample in detail and in depth, based on
an articulation of a descriptive theory. This theory must respect the depth and
scope of the case under study, which is conveyed through robust propositions
and questions. If a descriptive theory cannot be developed easily before a case
study, then the researcher may .
Cross
sectional studies: Cross-sectional research
is a research method often used in developmental psychology,
but also utilized in many other areas including social science and education.
This type of study utilizes different groups of people who differ in the
variable of interest, but share other characteristics such as socioeconomic
status, educational background, and ethnicity.
For example, researchers studying developmental psychology might
select groups of people who are remarkably similar in most areas, but differ
only in age. By doing this, any differences between groups can presumably be
attributed to age differences rather than to other variables.
·
Comparative or analytical studies: An analytical study attempts to establish causes or risk
factors for certain problems. this is done by comparing two or more groups
,some of which have or develop the problem and some of which or not.
·
Cross sectional comparative studies: A cross-sectional study
is a descriptive study in which disease and exposure status are measured
simultaneously in a given population. Cross-sectional studies can be thought of
as providing a "snapshot" of the frequency and characteristics of a
disease in a population at a particular point in time. This type of data can be
used to assess the prevalence of acute or chronic conditions in a population.
However, since exposure and disease status are measured at the same point in
time, it may not be possible to distinguish whether the exposure preceded or
followed the disease, and thus cause and effect relationships are not certain.
·
Case control studies: It is a
type of observational study in which two existing groups differing in outcome
are identified and compared on the basis of some supposed causal attribute.
Case-control studies are often used to identify factors that may contribute to
a medical condition by comparing subjects who have that condition/disease (the
'cases') with patients who do not have the condition/disease but are otherwise
similar (the 'controls'). •
Usually less expensive •
Sampling from source population • Can usually calculate
only the ratio of incidence rates or risks • Convenient for studying
many exposure.
Cohort
studies: A cohort study is a form of
longitudinal observational study. It begins with a group of people who do not
have the disease, takes baseline measurements, then follows them over time to
determine whether uses correlations to determine the absolute risk of subject
contraction. A cohort is a group of people who share a common characteristic or
experience within a defined period (e.g., are born, are exposed to a drug or
vaccine or pollutant, or undergo a certain medical procedure). Thus a group of
people who were born on a day or in a particular period, say 1948, form a birth
cohort. The comparison group may be the general population from which the
cohort is drawn, or it may be another cohort of persons thought to have had
little or no exposure to the substance under investigation, but otherwise
similar. Alternatively, subgroups within the cohort may be compared with each
other. An example of an epidemiological question that can be answered by the
use of a cohort study is: does exposure to X (say, smoking) associate with
outcome Y (say, lung cancer)? Such a study would recruit a group of smokers and
a group of non-smokers (the unexposed group) and follow them for a set period
of time and note differences in the incidence of lung cancer between the groups
at the end of this time
·
• Usually very expensive
·
• Complete source
population denominator
·
• Can calculate
incidence rates or risks and their differences and ratios.
·
Convenient for
studying many disease.
Randomized
clinical trial: These are cohort studies where allocation to
treatment and control groups is achieved by a random process, akin to flipping
a coin. Random allocation is helpful for reducing ‘selection bias’ and
‘allocation bias’, especially when combined with allocation concealment.
Because they are experiments, randomized trials can also employ blinding of
participants and caregivers.
· Parallel
design:
·
Classical clinical trial approach „
·
Two study groups „
·
Randomized
assignment.
· Cross
–over design:
·
Address question
of major interest − Will this patient do better on drug A or drug B? „
·
Removes “patient
effect” thereby reducing variability and increasing precision of estimation „
·
Opportunity to receive both treatments (or be
assured of receiving active treatment at some point) is attractive to patients „
·
Under assumption of no carryover effect,
design provides more information than simple parallel design.
·
Disadvantage of cross over design:
·
Assumption of no
carryover effects is difficult to test „
·
May be difficult
to deter mine appropriate length of washout period so as to avoid carryover
effects „
·
There may be “period” effects in addition to
carryover effects –
·
Progression of
disease
·
Dropout
· Cluster
randomized trials: Groups or clusters randomly assigned, not
individuals − Examples: villages, classrooms, platoon.
· Factorial
trial:
·
Evaluates
multiple factors simultaneously „
·
2 X 2 most practical, but little used „
·
Sometimes a
combination cannot be given (incomplete factorial)
·
Evaluates two
interventions simultaneously
·
„ Four
possible treatment combinations „
·
Efficient
approach in some circumstances „
·
Potentially more informative approach „
·
Increases
proportion getting active treatment „
·
Major concern: interaction of interventions
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