Epidemiological study designs:analytical studies Flashcards
Epidemiological study designs are divided into two name them
Descriptive Studies
•B. Analytic Studies
What kind of studies are done under descriptive and analytical studies (observational and experimental as well and give examples under each)
Types of studies under cohort studies as well
Descriptive Studies
- Case Reports
- Ecologic or Correlational Studies
- Cross-sectional studies
Analytic Studies
1. Observational •Cross-sectional studies(there’s analytical work done to see the relationship between determinants and diseases •Case control studies •Cohort studies •Prospective •Retrospective •Nested Case Control Studies
- Experimental
•Clinical Trials
•includes community based interventions
What are case reports
What three things do they show?
What are they used for?
Name two advantages and disadvantages of case reports
An article that describes and interprets an individual case, often written in the form of a detailed story.
•
Cases that show an important variation of a disease or condition
•Cases that show unexpected events that may yield new or useful information
•Cases in which one patient has two or more unexpected diseases or disorders
•
Primary use is for initial description of a “new” disease or phenomenon
•May be hypothesis generating
•
Advantages
•May be quick and inexpensive to perform
•“Easy” publications for trainees
Disadvantages
•Measures only the frequency of occurrence of associated symptoms, signs, exposures, etc.
•There is no comparison group and no possible measure of association
•without comparison, you have no basis to estimate whether the
occurrence of a particular finding is exceptionally common, etc.
- Frequently data based on chart review;
- data collected are not standardized
- missing information may be considered as not present, rather than not evaluated
What are ecological or correlational study
What is the unit of analysis in this study
What two things differentiates ecological study from other epidemiological studies
Give an example
Examines the rates of disease in relation to a factor described on a population level
•The unit of analysis are populations or groups of people rather than individuals
•Two key things that distinguish a traditional ecologic study from other types of epidemiologic studies are:
•1. the population unit of analysis
•2. an exposure status that is the property of the population
•Eg researchers conducted an ecologic study with groups identified by place to determine the association between air pollution and mortality rates
•Study to examine the association between drinking chlorinated water and birth defects
Hypothesis is generated in ecological studies
State the advantages and disadvantages of these kind of studies
What is an ecological fallacy?
Ecological or correlational Studies •Hypothesis generating •Advantages •Typically uses information that is readily available (census data, mortality data, etc.) •Inexpensive •
Disadvantages
•Lack of linkage between individuals and exposures/outcomes
•Ecologic Fallacy
= an association observed between variables on an aggregate level does not necessarily represent the association that exists at the individual level eg. Study to examine the association between drinking chlorinated water and birth defects or
tendency to draw conclusions on the individual level based on the observations from the aggregate level.
What is the measure of frequency and measure of association in cross sectional studies
Principal measurements: •Exposure(contact with something or anything that can lead to a disease or protect you From a disease)and outcome at the same time (snap shot)(using questionnaire) •Measure of frequency: Prevalence •Measures of association: •Prevalence odds ratio •Prevalence ratio (relative prevalence)
How is a cross sectional study designed
Define your population
Gather data on exposure and disease
Group the data into four groups
Exposed and have the disease or exposed and do not have the disease
Not exposed but have the disease or Not exposed and do not have the disease
Disease or no disease is vertical
Exposed or not exposed is horizontal
How is a cross sectional study analyzed( by stating the prevalence of exposure in people with the disease and people who don’t have the disease and stating the prevalence of the disease in exposed compared to not exposed )
Exposed = people with th disease plus people without the disease(a+b)
Not exposed= people w disease plus people without disease(c+d)
People w disease= people exposed plus people not exposed (a+c)
People without disease= people exposed plus people not exposed(b+d)
Before all this draw the 2by 2 table and Input the values so you don’t confuse yourself
Please
Prevalence of exposed in people with disease(a), then you find value for people with disease (a+c)
Is a divided by a+c
Vs.
Prevalence of exposed in people without the disease(b) , then you find value of people without the disease
b divided by b+d
Prevalence of disease in exposed is prevalence of the disease(a) then you find the value of those exposed(a+b)
a divided by a+b
Vs.
Prevalence of people having the disease but we’re not exposed is (c) then you find the value of those not exposed(c+d)
Name five advantages and disadvantages
of epidemiological study designs
Advantages:
•Relatively short time period
•Relatively inexpensive
•Estimates the disease burden
•May simultaneously investigate several exposure/outcome relationships
•If disease is persistent, the prevalence ratio may estimate the risk ratio.
Disadvantages:
•Difficult to estimate the timing of the exposure-outcome relationship, unless the exposure is permanent
- Bias of exposure measurements possible, especially if historical (recalled)
- !” “survivor” bias - persons with severe exposure may die more quickly, would not be included in assessment of prevalent cases of disease
- Typically cannot estimate risk ratio
Name five uses of cross sectional studies
Used primarily to determine the prevalence of outcomes of interest
- Used to determine the burden of disease
- Assesses associations between exposures and outcomes at the same time
- Helps to provide useful questions for further studies
- REVIEW OF SELF-REPORTED ADHERENCE TO DIET ARTICLE
Name three examples of case control studies ,define cases and controls,the traditional view,the modern view,state which view is used in case control studies
What is control group and state it’s purpose
In case control, Investigators obtain and compare the exposure histories of cases and controls true or false
Case-control studies:
•case-referent, case-comparison, TROHOC
- Traditional view:
- Subjects in case control studies are selected on the basis of whether they have or do not have the disease.
•Those that have the disease are termed cases and those who don not have the disease are termed controls
Cases and controls are picked from different groups
Modern View:
•It is a method of sampling a population in which cases of disease are identified and enrolled and a sample of the source population that gave rise to the cases is also identified and enrolled
Cases and controls are taken from same group
Modern view is used
The sample of the source population is known as the control group
- Its purpose is to provide information on the exposure distribution in the population that produced the cases, so that the disease rates in exposed and non-exposed can be compared
- This answers the question: if this control subjects would have become a case in this study, would they have been included in the case population?
What is trohoc?
Opposite of cohort
We know the disease but go back to find the contact
Which type of study does the investigator determine the outcome but not exposure?
Which study does the investigator determine the exposure not the outcome
In cross sectional study it is difficult estimating the timing of the exposure and outcome cuz the person doesn’t have the disease anymore but had it sometime ago and was exposed sometime ago true or false
Case control
Cohort
True
What is the principal measurement of case control and the measure of association
Principal measurements:
•Exposure(s)
•Measures of association:
•Odds ratio
How do you define a case
Name three sources of cases
CASES
•Case definition
•Part of the definition is classic epidemiology
•Person, Place, and Time
•Also include diagnostic tests, clinical and pathological exams etc.
•Use as much information as is available
•Source of cases
•Hospitals, population based, disease registry
How are cases selected
Define our cases
•Workers in factories who develop leukemia (or developed leukemia if retrospective) over the 20-year period
•Take a sample of cases (or enroll all cases if possible)
How are controls selected ,state the difference between hospitalized controls and population controls , in a selecting a control from the hospital what are you supposed to look out for? In selecting a control from the population what are you to look out for?
state the difference between cohort study and case control study with respect to control
A sample of the source population that gave rise to the cases
The would criterion
•The source population is a group whose members would end up as a case in the study if they developed the disease
Hospitalized controls
•Must make sure that the condition for which they are in the hospital has no relationship with the disease or exposure of interest
•The illness in the control group should have a similar referral pattern to the disease you are studying example- if Ure doing a study on hypertension don’t pick controls with diabetes cuz hypertension and diabetes have similar risk factors
•
Population Controls
•More generalizable
•Likely the same source population that gave rise to cases
•Time consuming, less interest to participate, recall bias
In a cohort study you are following this source population until they develop disease
•In a case-control study, you sample this population to assess the exposure distribution in the population
What is the most important criteria in selecting a control and what does this criterion ensure
The most important criteria: if these non-cases would have become diseased would we have identified them as cases?
•This ensures that the following goals for control selection are met:
•Controls come from the same source population as cases
•The exposure distribution in the controls should match the exposure distribution of the entire source population
- Most studies have one control per case true or false
- In instances where cases are very difficult to get, the number of controls per case could be increased to?
- what is a way of increasing the power of the study
- No value in having more than 4 controls per case. Example- if you have 100 cases and you’re using 1 case to 4 controls then you’ll get 400 controls true or false
True
4
Increasing number of controls per case
True
How is a case control study designed
2 by 2 table
Case and controls instead of disease and no disease
Exposed and not exposed are still there horizontally
Everything else is the same as cross sectional study
Create a case control study Using the following values 5exposed cases 6unexposed cases 3exposed non cases 18 unexposed non cases
Using this,find the odds ratio
Find the percentage of exposed people with the disease
Find the percentage of exposed people without the disease
Cases=5+6
Controls=3+18
Exposed=5+3
Not exposed=6+18
Odds ratio:odds that the case was exposed(exposed divided by not exposed cases) divided by odds that the controls was exposed(exposed divided by not exposed among controls)
Odds ratio=6/5 divided by 3/18
=6/5 multiplied by 18/3
=7.2
Define odds ratio
Odds ratio
•OR = odds of exposure among cases divided by odds of exposure among controls
Define odds
Ratio of the probability that the event will occur to the probability that the event won’t occur
Example:in 100 births the probability of the delivery being a boy is 51percent and delivery being a girl is 49percent
Odds of delivery being a boy is 51/49=1.04
How is odds ratio interpreted
An OR = 1 means?
An OR>1 means
An OR<1 means?
=1:
Exposure is not related to disease
No association, exposure is independent of disease
> 1:
Exposure is positively related to disease
Positive association;causal?
<1:
Exposure is negatively related to disease
Negative association;protective?
The probability of getting the disease due to the exposure is low
What is a cohort
Cohort is also used to describe a group of individuals with a common characteristic or experience eg birth cohort consists of individuals who are born during a particular year or period
Which study compares exposed and unexposed populations and compares their disease incidence? Or which study finds out the outcome of an exposure (people have come into contact w something and they want to know the result of them coming into contact with that thing) example: vaccinated people monitored for a while to see what happens to them after the vaccine
Cohort studies
Which study chooses the exposed and unexposed populations (these populations are monitored for the outcome under study )but doesn’t assign the exposure (observational studies)
Cohort
Which study is used to study rare exposures to find the outcomes and which study is used to study rare outcomes to find the exposure
Cohort study
Case control study
Cohort isn’t used to study rare outcomes
Name three outcomes of interest in cohort study
More than one outcome may be investigated true or false
Outcome of interest may include first occurrence of disease, disease recurrence, or death
True
What are the types of cohort studies and explain them
Prospective cohort study: Investigator identifies the original population at the beginning of the study and accompanies the subjects concurrently through calendar time until the point at which the disease develops or does not develop. Investigator doesn’t know the outcome but knows the group had been exposed so investigator follows the group to find the outcome
- Retrospective cohort study (use of historical data): Here exposure is ascertained from past records and outcome is ascertained at the time the study has begun eg.
- Using 2014 data to determine who developed lung cancer and who has not from a population whose smoking habits were surveyed in 1990.
The information on the exposure is gotten from the past or existing records
You already know the outcome but you go to the past to find the exposure to see if there’s a link example:in 2000 a study was done on
people who smoke
In 2015 we know there’s something called lung cancer so we go back to the study in 2000 to see what caused people to get lung cancer
What is the difference between retrospective and case control studies
Retrospective cohort study the info on exposure is taken from the last records
Case control the info on exposure is now being taken from interviews done
What are the two Main types of experimental study
True experiments
Quasi experiments
Wha us the role of the researcher in observational studies and experimental studies
To either observe for a disease or an exposure
Experimental:researcher determines the exposure
Why are ecological fallacies not usually true
the conclusions drawn may not be true cuz if a group does something that doesn’t mean every individual in the group does that
Example:certain ethnic groups are perceived to have a certain kind of behavior but you may meet an individual from that ethnic group whose behavior is different from the perceived behavior of the ethnic group
Exposure can be positive or negative true of false
Which study checks people who aren’t sick but have been exposed to something before
Which study measures exposure and people who are sick or have the disease and questions are asked about the things they’ve come into contact w?
Which study measures the outcome?
People who either come into contact w something and get the disease or come into contact w the thing and don’t get the disease are still exposed true or false
True
Cross sectional
Case control
Cohort
True
How is a cohort study started
The investigator can start with people after the exposure has occurred
• HIV infection
•Exposure to chemicals
- The investigator can define the population and then wait for exposure to occur
- Birth cohort
- Graduating class
State three differences between retro and prospective
Prospective
•Long wait time
•More expensive
•Data collected specifically for the study
•New data can be collected if necessary (e.g. blood samples for specific tests)
•Less prone to bias
Retrospective
•Little or no wait time
•Less expensive
•Measures of effect can be calculated as in prospective study
•Data were not originally collected for study purposes and thus may be incomplete
•Prone to information bias
How are incidence rates calculated for exposed and unexposed for cohort studies
What is relative risk for cohort studies using the formula
Exposed:
a/(a+b)
Unexposed c/(c+d)
Exposed divided by unexposed (use formulas)
Define relative risk
If it’s =1,>1,<1 what does it mean
Relative risk is a measure of the strength of an association between an exposure or attribute and a disease.
- If the relative risk is 1, then the incidence in the two groups is the same.
- If it is greater than 1, then the attribute or exposure is associated with an increased incidence of the disease, and
- If less than 1, with a decreased incidence of the disease
Name four advantages and disadvantages of cohort studies
They are rated very highly in terms of level of evidence support for an intervention
- Temporal (time) relationships can be clearly established especially for prospective study
- Can be used to study multiple outcomes
- Best type of study for rare exposures
Disadvantages:
Cannot study multiple exposures
- Very expensive
- Risk of loss to follow-up
- Take a long time to obtain results (prospective study)
When are cohort studies warranted
Longitudinal data are needed to determine the sequence of exposure and outcome in proper time
- Exposure is rare but incidence of disease among the exposed is high
- When loss to follow up can be minimized
- When ample funds are available
Name four potential bias in cohort
Selection bias
•Determined by whether the exposed and unexposed groups have the same background characteristics
- Information bias
- More information sometimes available on the diseased in retrospective cohort studies
•Bias in assessment of outcome
•More diagnostics applied to the exposed in measuring outcomes
•
Non-response bias
•People lost to follow up not having the same disease experience as those remaining in study
Name four differences in the advantages of case control and cohort
And four differences in the disadvantages of case control and cohort
Case control Good for rare outcomes/diseases •Multiple exposures can be assessed •cheaper, quicker and easier to conduct •Appropriate for diseases with long latency periods • Cohort Prospective •Can estimate incidence and risk •Multiple outcomes/disease can be assessed •Appropriate for rare exposures
Disadvantages:
Case control
Retrospective design limits causal inference
•Potential bias: recall bias, reverse causation (outcome changes exposure)
•Not suitable for calculating incidence/risk
•Cannot establish prevalence
•
Cohort
Expensive, longer and harder to conduct
•Not suitable for rare outcomes/diseases
•Not good for diseases with long latency periods
How are randomized trials done
Get a study population
Randomly assign new treatments and current treatments
Under each type of treatments Check the people who improve and those who don’t
