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Observational studiesExperimental or intervention studies
The study use existing phenomena to understand aspects of health or illness
Investigators study people and exposures “in nature
The study test the effect of some intervention on a certain aspect of health or illness.
Investigator assigns (intervene) treatment or exposure by Random allocation of study subjects
Observational studies Experimental or intervention studies
The investigator neither controls the population nor factors exposed
The investigator can control both the population & factors exposed
Observational
studies
Descriptive
Look at Distribution of
disease+ no
Comparison group
Analytic
Look at Determinants
of disease + have a
comparison group
Experimental
(intervention) or Quasi-
experimental Studies
Randomized
controlled Trial,
Community trial &
lab.trial
1. Health care planning
2. Hypothesis generation (constitutes an important
first step in the search for determinants or risk factors)
3. Trend Analysis
Descriptive
study
Case report+
case series
Cross
sectional
study
Ecological
Correlational
study
Analytic
study
Case control
study
Cohort
study
Observational
No comparison Group
Have a comparisonGroup
1-Case Report 2-Case series
Report a new or uniquefinding in a disease e.g. Previously un-described diseaseUnexpected link between diseasesUnexpected new therapeutic effectA rare feature in a disease
Describe Unusual characteristics of a number of patients with a given disease
1- Case Report 2-Case series
Example:
Pulmonary embolism among premenopausal female.
Example:
Kaposi sarcoma a disease of old age has been reported in young adult homosexual men .
1-Case Report 2-Case series
Strengths and limitation:It provides the first report of unexpectedevent, It Generate hypotheses for testing and for further study.
Strengths and limitations:It describe unusualvariation of a diseaseIt can generatehypothesis.
1-Case report 2- Case series
Generalization of the results are rarely
Generalization of the results is limited because:1-Selection of study subjects is unrepresentative2-No comparison group
An ecological study is an epidemiologicalstudy in which the unit of analysis is a population rather than an individual.
(uses data from the entire population to compare disease frequencies in relation to certain exposure during the same period of time or at different points in time.
It correlates between variables
Example: the relation between exposure to black cloud and chest infection in different citiesLack of water sources flourides and dental cares in different communities or countriesThe association between industrialization and
obstructive Pulmonary diseases or lung cancer in different countriescomparing several governorates based on state-wide average air pollution and state-wide average prevalence of respiratory diseases)
They cannot prove that one variable causes a change in another variable.
(disease causation)
Does not allow for scientific control of variables.
Data of exposure/health is Unknown for
each individual
This is the observation of a definedpopulation at a single point in time or time interval. Exposure and outcome are measured at the same time point.It Collect data on a health problem from a group of subjects at a single point of time .
Study question :
Occurrence of disease in a single point of time
Example :
Number of female using contraceptive pills among those attending maternal and child health center
Advantages Advantages
SerengthsEasy, quick and notexpensive.
Estimate the overall prevalence of a diseaseEstimate exposure proportion in the population
Suitable to study prevalence of chronic disease.The first step to develop evidence for causal association.
Limitations
Not useful for establishing causal relationships.
It does not allow us to answer the question
,which came first(exposure or the disease)
It may be a coincidence
Data are susceptible to distortion through the bias انحياز وانحراف )in the research during sampling, questionnaire and interviewing.
Limitations
The population census.Market research organizations often use cross sectional studies (for example, opinion polls), the type of sampleused here may be a convenience sample or a quota sample.The prevalence study of skull fractures in childrenadmitted to hospital in Edinburgh from 1983 to 1989.Note that although the study period was seven years it was not a longitudinal or cohort study because information about each subject was recorded at a single point in time.
Descriptive
study
Case report+
case series
Cross
sectional
study
Ecological
Correlational
study
Analytic
study
Case control
study
Cohort
study
Observational
No comparison Group
Have comparisonGroup
What is the study?
It is an observational study that Provide a simple way to investigate causes of diseases,
especially rare ones. (behcet syndrome),
Arthritis, Juvenile reumatoid,( cystic fibrosis)
Study question:
Does the present outcome (disease), occurred due to certain exposure
In this study the investigator starts with the outcome and look backward to detect the history of exposure to the suspected cause.
Identify the cases according to standard criteria for diagnosis (study group) (case definition)
Control subjects must be:Free from the studied disease and should be Representative of the source population from which cases were derived.
They are Matched with cases especially for characteristics that may affect the disease occurrence e.g. age, sex, social class…..Ex, a case control study of asthma among high school students .
Advantages of case control study:
Disadvantages of case control study:
Easy, quick, and cheap.
Useful in studying rarediseases (breast cancer in men)Allows the study of several risk factorsCan prove that an exposure is related to an outcome
Can’t calculate ratesNot useful in rareexposure.
Recall bias as it depends on retrospective history.
A measure of association Can be used to:Measure effect size or quantify the strength of associationbetween a potential risk or protective factor (exposure) and an outcome.
Odds ratio (O.R):O.R. = Odds of exposure among cases / Odds of exposure among control
The result of dividing the proportion of individuals with an attribute by the proportion of individuals without the attribute in a population .
The probability of occurrence of an event as compared with (divided by) the probability of non-occurrence of the event in a population.
So, ex, among cases the odds would be
A/A+C ÷C/A+C = A/C
So, ex, among control the odds would be
B /B+D ÷ D/ B+D = B /D
a/ c
O.R. = _______ = AxD÷ BxC
b/ d=1 0r > 1 or < 1
OR=(a × d)/(b × c)O.R=1 means that the odds of exposure among the cases equal the odds of exposure among the control, so no evidence of association between exposure and outcome.
O.R >1 means that the cases have a higher odds of exposure to the risk more than the control.
Ex.,People who ate food from the bakery had 3 timesthe risk of developing hepatitis A compared to people who did not eat food from the bakery
1- Study of obesity as a risk factor for DM.
2- The effect of high salt intake on Hypertensive adults
3-The study of atrial fibrillation in middle aged men during exercise.
Cohort means a group of people having a common characteristic, e.g.Example: \
A group of female pharmacists working together in a lab and some of them work on certain chemicals(exposed) and others are not (unexposed). They are followed up for a period of time and studying disease(s) occurrence or unfavorable outcomes of pregnancy for example.Ex., A study of cancers and anomalies after atomic reactions
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)?
The exposure has already occurred
A cohort of people- with and without the exposure-are followed up over a period of time to see who develops the condition(s) of interest
A cohort study helps in establishing causation
With
exposure
Without
exposure
A cohort group
followed
Methodology
A population of exposed and non-exposeddetermined & followed prospectively for a certain period of time “e.g. one year”
Incidence rate of an event for both exposed & non-exposed calculated & relative risk is measured
Statistical analysis (by a test of significance )to assess that the observed difference is real not due to chance
-RR= A/(A+B) / C/(C+D) it is a risk measurement ,It measures the association between incidence of disease and certain exposure.
It answer the question:
How many times an exposed person is at risk of developing disease compared to non- exposed?
RR= incidence among exposed / incidenceamong non-exposed.
RR=1 : no association between exposure and the disease. Incidence rate is the same in both groups
RR = › 1 : positive association or increased risk ; exposed group has higher incidence than non exposed.
RR = ‹ 1: negative association or protective effect; non exposed group has higher incidence.
AR= incidence among exposed – incidence among non exposed.x100%
It study the rate of occurrence of the disease in exposed individuals that can be attributed to the exposure.(a real exposure and not by a chance)
Strengths Limitation
Valuable in rareexposure
Less bias in recall & observation
Allows for calculation
of Incidence rate” & Relative risk
Inefficient in evaluating rare diseases.Costly & time consuming Loss of follow up may affect validity of results.
Historical cohort study:
(Better than case control (incidence rate can be
calculated besides causal association)
Ex. Cancer in certain occupations
Nested case control study:
ExampleTo determine if H. pylori (HP) infection was associated with the development of gastric cancer,Parsonnet et al (N Engl J Med. 1991 Oct 17;325(16):1127-31) first identified a cohort of 128,992 persons who had been followed since the mid-1960’s. Of the original cohort, 189 patients developed gastric cancer. The investigators carried out a nested case-control study by selecting all of the 189 gastric cancer patients as cases and another 189 cancer-free individuals from the same cohort as controls. HP infection status was determined using serum obtained in the beginning of the follow-up. All total of 84% of the confirmed gastric cancer cases had been infected previously with HP, while only 61% of the controls had been infected; this indicated a positive association between HP infection and gastric cancer risk.
In cross-sectional study both the risk factor and outcome are present at time of study:
For example, a study finding an association between low CD4 counts and HIV infection does not demonstrate whether HIV infection lowers CD4 levels or low CD4 levels predispose to HIV infection.
For example cases of DM (outcome at present) are matched with a control group and the
History of risk factors are traced retrospectively (Exposure)
An experiment is a
A procedure carried out with the goal of verifying, or establishing the validity of a hypothesis. Experiments provide insight into cause-and-effect.
The investigator can control both the population & factors exposed
The experimenter control the independentvariable (ex. Treatment) Then he measure the dependent variable (ex., health status).
It is the best epidemiological study design to prove causation.
A true experiment is characterized by:
It Randomly allocates the subjects bias.
Controling all Confounding factors.
A comparison group :Assessing the efficacy and safety of the intervention before
being a standard practice through a Comparison with a standard intervention or therapy.
According to randomization; experimental studies are divided into:
Experimental studies
•(Randomized studies
Quasi experimental studies
•Non randomized studies
A- Before and after (Pretest-post test) experimental studies (with no control group):
B- Randomized controled trial (RCT) ;also called (Pretest-posttest control group design)
Post-test only control group design) (rarely)
C- Clinical trials
A-Before and after (Pretest-post test) experimental studies (with no control group):
The study subjects are randomly allocated
The researcher take the pretest measurements or variables
Then intervene with the drug or vaccine and follow up
The researcher take the posttest measurements or variables and then analyze the results
B- Randomized controlled trial (RCT) ;also called (Pretest-posttest control group design)
N.B In a few cases, a pre-test is impossible
because the participants have already been exposed to the treatment, or it would be too expensive or too time-consuming.
And it is called ( post-test only control group design)
1. Identify the target population. ( inclusion & exclusion criteria)
2. Consent (all participants)
3. Random Allocation
Each subject has an equal chance of being included in the control or experimental group.
1. The experimental group Intervention or drug
2. Control group Placebo or traditional therapy
3. Follow up for a specified period
4. The outcome (preventive or curative) is measured (often before and after intervention
5. The outcome is compared (experiment & control)
Examples of the outcomes in clinical trials:
Systolic blood pressure in a hypertension trial
Caesarean section rate in an obstetric trial
Survival time in a cancer trial
Clinical TrialsResearch activities in which: Administration of therapeutic or preventive intervention to evaluate its safety and efficacy.
Clinical trials are useful in evaluating new:
Drugs in treatment of diseases.
Medical technology.
Methods of prevention.
Program for screening and diagnosis.
A-Pre-clinical trials in animals and lab (in vitro)
B-Testing in human through 4 phases”Pharmacokinetics includes the study of the mechanisms of absorption and distribution of an administered drug, the chemical changes of the substance in the body (e.g. by metabolic enzymes such as CYP or UGT enzymes), and the effects and routes of excretion of the metabolites of the drug.[1] Pharmacokinetics is often studied in conjunction with pharmacodynamics , the study of a drug's pharmacological effect on the body.
Volunteers Small
doses
Short
period
Close
supervision
Small doses taken for short period
on severely ill or healthy volunteers
Patient
VolunteersSmall
doses
longer
period
Close
supervision
sometimes this phase is associated with a control group
Patient’s
consent
Usual
doses
longer
periodControl Gp
A classical phase,Mostly this phase is associated
with randomization, double blind and a controlgroup.
1. Single blind : studied persons don’t know whether they belong to the treatment or control group
2. Double blind : studied persons & experimenter not known
3. Triple blind : all the three studied persons, experiment & statistician not known
1.(S )Allows for control of other variableswhich affect outcomes under Investigation.
2.It is the best study design to provecausation and prove validity
3.(L)Insufficient number of subjects lead to
failure to detect true differences
Design a community trial
1. Single community design (before and after intervention).
2. Two Community design, one intervention community and control.
3. One-to-many, the intervention community has several control community.
4. In a many-to-many design there are study with multiple intervention communities and multiple control communities.
S It is useful in natural field circumstances.
LSelection biasPeople in the control community may receive the intervention under study on their own becauseThere is no strict control as occurs in laboratory experimental or animal studies is not possible with humans.
The Salk vaccine trial carried out in 1954 had 200,000 experimental group and 200,000 control groupThe aspirin-myocardial infarction study (as therapeutic) The intervention group received 1.0 gram of aspirin daily whereas the reference group received a placebo.
Two major types of quasi- experimental designs:
1. Pre test – post test, Nonequivalent control group designs with-no random assignment to groups
2. Before and after design- has pretest and posttest with no comparison group with non randomization to the study group
Definition:
It is a : Statistical technique that combine the findings of a set of studies addressingcommon research hypotheses to derive definite conclusion
It Combines contradictory results of several clinical studies on the same subject to derive definite conclusion.
Study question: used to
Describe the best clinical approach to a problem.
Identify new directions for research.
Qualitative aspects
Developing research question to be analyzed
Reviewing literature for relevant clinical studies
Selecting variable common to the studies
Evaluating studies to identify similarities or explain differences Quantitative aspects : statistical analysis of results
Common designObjective
Cross sectionalPrevalence
CohortIncidence
Controlled trial, Cohort, case-control, cross sectional
Cause (in order ofreliability)
CohortPrognosis (future or recovery
Controlled trialTreatment effect
Protection of human rights
Any person who is requested to consent toparticipate as a subject in a research study has the right to be
Informed of nature & purpose of experiment
Given description of any risks to be expected from experiment
Given explanation of expected benefitsfrom the experiment
Informed of medical treatment to subject after experiment if complicated
Given an opportunity to ask any questionsconcerning experiment or procedures involved