Please enable JavaScript.
Coggle requires JavaScript to display documents.
Case 5: Public Health (Case Control Studies) - Coggle Diagram
Case 5: Public Health (Case Control Studies)
Case Control Studies
Outline the characteristics of Case Control Studies
Case control is an Observational Study because there investigator does not control the exposure
Case control studies are Analytic studies because Case control studies contain a comparison group
Differentiating between "Cases" and "Controls"
Differentiate between "Cases" and "Controls"
Cases refers to people with health outcomes/disease of interest
Controls refers to people without the health outcome/disease of interest
In controls, in respect to the exposure and outcome of interest, they must resemble the cases that we are studying
So that the controls are people that are likely to have been cases if they had been exposed to the exposure of interest in the study
NOTE: This does not mean that controls cannot have a disease
It just that controls can NOT have the disease of interest
Cross-sectional study
Cross-sectional study provides a snapshot of the exposure and the outcome at a fixed point (fixed period in time)
Advantages: Cross-sectional study is a fairly quick and relatively affordable way of describing the burden of disease
Disadvantages: Cross-sectional study cannot form a temporal relationship between the exposure and the outcome
Cannot say if the exposure came before the outcome
Cohort Study
Cohort study provides a prospective view to determine if a disease or an outcome develops
Cohort study begins with a disease free population. This population may have other disease but it can NOT have the disease of interest
The selected population is then divided into an Exposed and an Unexposed group
Then all participants are followed-up over time to measure the development of an outcome in the 2 groups
Advantages:
Cohort study provides a temporal relationship between the Exposure and the Outcome
Allows for the measurement of risk of developing a disease based on specific exposure
Disadvantages:
Cohort Study is complex
It often requires long follow-up times
It is expensive
It is prone to high losses to follow-up
Case Control Study
Outline the Case Control Study
Case Control Study requires a retrospective view or running a backwards analysis from Outcome to Exposure
In a Case Control Study, from a population of interest, Cases and Controls are selected.
A history taking is done for each group in order to ascertain each person's exposure status
This results in 4 groups that are divided as follows:
Cases with the exposure of interest
Cases without the exposure of interest
Controls who are exposed
Controls who are not exposed
Key Features of Case Control Studies
Outline the key features of Case Control Studies
In Case Control Studies:
Participants are selected according to their disease/outcome
Researcher begins with the "cases", individuals who have the outcome of interest
Researcher then selects an appropriate "control group", individuals who do not have the outcome of interest
Once the "cases" and "controls" have been selected the researcher will have to collect information on the exposure(s) of interest in both groups
Researcher then compares the results from the exposure history between the Cases and Controls
Then the Researcher will then calculate a Measure of Association
Example of a Case Control Study
Research question: Is there an association between Smoking and Lung Cancer ?
Outcome: Lung Cancer
Exposure: Smoking
Process of Case-Control Study
An specific population is identified from which Cases and Control Group are selected. For example a Hospital
Cases are then selected, who are the Group of people who have Lung cancer
Control group is then selected, who are the Group of people who do not have Lung cancer
Researcher will then undertake a history-taking to look back in time and determine each person's smoking exposure
From the results, the frequency of smoking is compared between the Cases and the Controls
This will allow for the calculation of the Measure of Association
Considerations in the design of Case-Control: Selection of Cases
Outline the procedure involved in the Selection of Cases
To select Cases, the study must make sure that they use a Standard Case Definition
If the Case Definition is not Standardised, the may accidently include Non-Cases in the Case group
This can lead to invalid results
Case definition may include Clinical symptoms, Laboratory Results, X-Ray findings etc
Standardizing the Case definition means that you determine who will be included in the Case definition before the study begins
Study should preferably use Incident (new) cases rather than Prevalent (old) cases
This is because the Diagnostic Criteria change over time
People who have had the disease for a long time: their exposure to certain risk factors may have changed over time as a result of the disease
Sources of study population include: Patient Hospitals, Clinics, Disease Registries, Mass Screening and Cohort Studies
Considerations in the Design of the Case-Control: Selection of Controls
Outline the procedure involved in the Selection of Controls
Control group needs to be free of the disease of interest
Control group needs to be comparable to the Cases, meaning that they need to come from the same general/source population as the Cases
Controls should represent people who would have been selected as Cases IF they had the disease of interest
This is the population at risk of becoming Cases
Usually similar in age, region or workplace to Cases
Selection of Controls needs be independent of the exposure being investigated
Sources of Controls include Hospitals, Clinics and Neighbourhood
If the Controls are not selected properly, this will result in Selection Bias
Consideration in the Design of the Case-Control: Selection of Controls
Example of Bias introduced by Inappropriate Controls
Research question : Do NSAIDs prevent colo-rectal cancer?
Case- control study
Cases are selected from hospital oncology department
Controls are selected from rheumatology clinic based at hospital
All the participants are from the same hospital BUT different departments within the hospital
Could this choice of controls introduce any bias?
Possible Bias:
Individuals at rheum clinic are more likely to be using NSAIDS that an individuals from the general population from which the cases came
Exposure will be higher than that in the general population
This Selection bias will result in a reduced Association between the Exposure and the Outcome
Considerations in the Design of the Case-Control Studies: Determining the Exposure
Outline the procedure involved in determining the Exposure
Exposure data can be collected at Face-to-Face Interviews, Telephone, Postal and Laboratory Tests
Methods used must be the SAME for Cases and Controls
Misclassification of Exposure will introduce bias
Outline some of the difficulties faced when determining the Exposure in Case-Control Studies
Recall Bias
There is a potential for difference in recall between Cases and Controls
It is then important to try to reduce this by using Memory aids such as Photographs, Calendars with Major events
Bias in Methods used to Collect Data
There is a potential for data collectors to determine exposure status differently in Cases and Controls
If possible data collectors should be blinded to disease status, to hypothesis and trained to elicit exposure information in a standard and structured way
Measuring the association between Disease and Exposure
Outline how to measure the association between Disease and Exposure
We cannot directly estimate the Incidence of Disease in the Exposed and Unexposed
We can measure the Odds of Exposure among the Cases and Controls
Odds = Number Events/Number of Non-Events
Measure of Association in Case-Control Studies is the Odds Ratio (OR)
Odds Ratio = Odds of exposure among Cases/ Odds of Exposure among Controls
Interpretation of Odds Ratio (OR) is similar to that of Relative Risk (Risk Ratio)
Calculation of the Odds Ratio
Outline the Calculation of Odds Ratio
Odds Ratio (OR) is a ratio in which the numerator is not included in the denominator
D represents the Case and Controls
D+ represents Cases
D- represents Controls
E represents Exposure
E+ represents Having the Exposure of interest
E- represents Not having the Exposure of interest
calculation of Odds of Exposure
Odds of Exposure among Cases = Number of Cases exposed/ Number of cases not exposed
Odds of Exposure among Cases = a/c
Odds of Exposure among Controls = Number of Controls exposed/ Number of Controls unexposed
Odds of Exposure among Controls = b/d
Odds Ratio
= Odds of exposure in Cases/ Odds of exposure in Controls
= (a/c)/ (b/d)
= (a/c) x (d/b)
= (ad)/ (cb)
Interpreting the Odds Ratio
Explain the interpretation of the Odds Ratio
If the Odds Ratio (OR) = 1, then the Odds of Exposure is same in both the Cases and Controls
If the Odds Ratio (OR) > 1, then the Cases have Greater Odds of being exposed than the Controls
The exposure is likely to be a risk factor
If the Odds Ratio (OR) < 1, then the Cases have Lower Odds of being exposed than the Controls
The exposure is likely to be a Protective Factor
For Example: Interpret the following Odds ratio
OR = 2.5
The Odds Ratio (OR) is greater than 1, then the Cases are 2.5 times more likely than the Controls to have been exposed
Exposure is likely a risk factor
OR = O.5
The Odds Ratio (OR) is less then 1, then the Cases are half as likely to have been exposed compared to the Controls
Exposure is likely a protective factor
Example calculate the Odds Ratio
Exposure: Smoking
Outcome: Lung cancer
a = 127
b = 35
c = 27
d = 165
Odds of exposure among the Cases
= (a/c)
= (127/73)
= 1.74
Odds of exposure among the Controls
= (b/d)
= (35/165)
= 0.21
Odds Ratio
= (a/c)/ (b/d)
= 8.20
Odds of exposure among cases was 8 times the odds of exposure among controls
Therefore, the Odds ratio is greater then 1, The Lung cancer patients are 8.20 times more likely to have been exposed to smoking compared to the Non-Lung Cancer Patients
There is an association between the Smoking and Lung Cancer. Smoking is likely to be a risk factor for Lung Cancer
Example calculate the Odds Ratio
Exposure: Diet high in vegetables
Outcomes: Obesity
a = 121
b = 171
c = 129
d = 79
Odds of exposure among the Cases
= (a/c)
= (121/129)
= 0.938
Odds of exposure among the Controls
= (b/d)
= (171/79)
= 2.16
Odds Ratio
= Odds of Exposure among the Cases/ Odds of Exposure in Controls
= (121/79)/ (171/79)
= 0.43
Odds of exposure among the Cases was 0.43 times the Odds of Exposure among the Controls
The Odds Ratio is less than 1, the individuals with Obesity are 0.43 times more to have a diet high in vegetables than the individuals that are not obese.
A diet high in vegetables is most likely a protective factor against obesity
Precision
Outline the concept of Precision in Case-Control Studies
Odds Ratio is calculated from a Sample of the Population and no the entire Population
Therefore, in order to get the Precision on how well the Odds Ratio estimates the True Odds in the population, we need to interpret the Confidence Intervals (CI)
Explain the interpretation of Confidence Intervals (CI) in the Case-Control Study
Confidence Intervals (CI) tell us how confident we are that the results we have obtained are not due to chance
But represent the True Odds in the population
95% means that we are 95% confident that the Estimate (True Odds Ratio) falls within the specific range
For Example: OR = 8.3 (95% CI 6.0 – 9.5) means that
Case are 8.3 times more likely to have been exposed compared to the Controls
We are 95% confident that the True Odds Ratio lies between 6.0 and 9.5
For example: OR = 0.43 (95% CI 0.65 – 0.25) means that
Cases are 0.43 times more likely to have been exposed compared to Controls
We are 95% confident that the Tur Odds Ratio lies between 0.65 and 0.25
Confidence Intervals
If we were to repeat the study a 100 times and calculated a 95% confidence interval each time, the true estimate would lie in the Confidence Interval 95/100 times
The purpose of the is Confidence Interval is that the Confidence Intervals helps interpretation due to uncertainty that results from random variation due to sampling
Example of a Case-Control Study
Case-control study to determine risk factors for Salmonella Enteridis
Salmonella lives in intestinal tracts of humans and other animals and birds. Usually transmitted to humans by eating food contaminated with animal faeces
Causes gastrointestinal illness characterized by fever, sudden onset of headache, abdominal pain, diarrhoea and sometimes vomiting.
Typically, illness last for 5 to 7 days – but sometimes can cause severe dehydration and death unless treated promptly with antibiotics
Incubation period 12 – 36 hours – up to 1 week.
Cases – patients with laboratory confirmed S Enteridis infection. They were enrolled shortly after the diagnosis.
Controls - persons who lived in the same neighbourhood as the cases and were a similar age with no diarrhoeal illness in the past 4 weeks. 2 controls were enrolled for each case
Standard questionnaire used to collect data on foods and beverages consumed, recent travel and recent food handling practices. Face to face interview.
Which exposures appear to be risk factors for S Enteridis?
Eating dishes that contained Raw or Undercooked Eggs
This is because the Odds Ratio is greater than 1, and the individuals with Salmonella Enteridis are 18.9 to have eaten dishes that contained Raw or Undercooked Eggs compared to those who do not have S Enteridis
Which exposure are protective ?
Eating chicken
Buying refrigerated eggs
Refrigerating eggs at home
Eating ground beef, Exposed to live chickens
This is because the CI include 1 and therefore the findings are not significant
Case-Control Studies: Strengths
List the Strengths of Case-Control Studies
Case-Control Studies:
Relatively quick and inexpensive compared to Analytic Study Designs
Well-suited in the evaluation of disease with Long-Patent Periods
These are diseases where the time period between the Exposure and Disease development is extended such as HIV
Can assess multiple risk factors for a single disease
Efficient for studying Rare disease (Compared to Cohort Studies).
Case-Control Studies: Limitation
List the Limitations of Case-Control Studies
Case-Control Studies:
Inefficient for evaluating rare exposures
Particularly prone to bias such as Selection Bias, Recall Bias, and Measurement Bias
Cannot directly calculate Incidence Rates (compared to Cohort Studies and RCTs)
More difficult to establish temporal relationship (Compared to Cohort Studies and RCTs)
Eg: Exposure occurred before outcome
Cofounding
Explain the concept of Cofounding
Cofounding is a type of bias which occurs in Observational Studies when a 3rd Variable (other than the Outcome and Exposure) results in an apparent association between the Exposure and Outcome
A cofounder can either increase or decrease the association between the Exposure and Outcome
An Example of Cofounding
Study looks at whether Coffee drinking causes Lung Cancer
After studying the results that confirm the Association between Drinking Coffee and the development of Lung cancer disease
A 3rd Variable (Cofounding Variable) is considered, where this variable has an association between the Exposure(Coffee drinking) and the Outcome (Lung cancer)
Majority of Cigarette smokers drink coffee and Cigarette smoking causes Lung Cancer
Therefore, the association between Coffee drinking and Lung cancer looks high because there is also an association between Coffee drinking and smoking
Cofounding
Outline the features of a Cofounder Variable
For a variable to be a Cofounder it must:
be an independent determinant of the Disease/Outcome
be associated with the exposure under investigation
Cofounding can be controlled either in the design of the study or in the analysis
Study Design 1
A study was conducted to investigate the association between having asthma and owning a pet. The hypothesis is that owning a pet is protective. Participants were randomly selected from a phone book by residential address and asked whether they had asthma and owned a pet.
What is the exposure ?
What is the outcome ?
What study design is being used?
What measure of association will be generated?
To show a protective effect should the measure of association be < 1 or > 1
Study Design 2
A study was conducted to examine the relationship renal cancer and dry-cleaning solvents. Patients with renal cancer were selected from a cancer clinic and compared them with patients admitted to the orthopaedic ward with respect to exposure to dry-cleaning solvents.
What is the study design and why?
What measure of association will this study generate ?
