case control study - 2009
TRANSCRIPT
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In the early 1940s, Alton Ochsner, a surgeon inNew Orleans, observed that virtually all of thepatients on whom he was operating for lung cancer gave a history of cigarette smoking. Although this
relationship is accepted and well recognized today,it was new and controversial at the time thatOchsner made his observation.He hypothesized that cigarette smoking was linkedto lung cancer . Based only on his observations incases of lung cancer,
was this conclusion valid ?
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Again in the 1940s, Sir Norman Gregg, an Australianophthalmologist, observed a number of infants and youngchildren in his ophthalmology practice who presented withan unusual form of cataracts.Gregg noted that these
children had been in utero during the time of a rubella(German measles) outbreak.He suggested that there was an association betweenprenatal rubella exposure and the development of theunusual cataracts.Keep in mind that at that time there was no knowledge thata virus could be teratogenic. Thus, he proposed hishypothesis solely on the basis of observational data, theequivalent of data from ambulatory or bedside practice
today.
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Disease
No Disease
Exposed
NotExposed
Exposed
NotExposed
Design of a case-control study
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CASES(with disease)
aa + c
CONTROLS(without disease)
bb + d
Were exposed a b
Were not exposed c d
TOTAL a + c b + d
Proportionsexposed
First, select :
Then, measurepast exposure :
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CHD CONTROLS
Smoke cigarettes 112 176
Do not smoke cigarettes 88 224
Total 200 400
% Smoking cigarettes 56.0 44.0
Hypothetical example of a Case-Control Studyof Coronary Heart Disease (CHD) and cigarette smoking
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artificialsweetener use CASES CONTROLS
Ever 1,293 2,455
Never 1,707 3,321
Total 3,000 7,776
Leon Gordis, p.126
History of use of artificial sweetenersin bladder cancer, cases and controls
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Average daily
cigarettesCASES CONTROLS
01-45-1415-24
25-4950+
Total
755
489475
29338
1,357
61129570431
15412
1,357
From Doll R, Hill AB - Leon Gordis , p.126
Distribution of 1,357 male lung cancer patients and a male controlgroup according to average number of cigarettes smoked daily overthe 10 years preceeding onset of the present illness
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Selection of cases
Sources :- hospital patients
- physicians practice patients
- clinic patients- registries of patients with certain diseases
in the community
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Selection of controls
Sources :
- Non-hospitalized persons living in thecommunity
- Hospitalized patients
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2. Hospitalized patients
- advantages* Captive population
* more economical to carry out a study- disadvantages
they represent a sample of an ill-health defineddiffer from people in the community
- from the same hospital?- all other patients admitted?
(other than those with the cases diagnosis)
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Population-based CCS Hospital-based CCS
Source population is better defined
Easier to make certain that casesand controls derive from the samesource population
Exposure histories of controls morelikely to reflect those of personswithout the disease of interest
Subjects are more accessible
Subjects tend to be more cooperative
Background characteristics of cases andcontrols may be balanced
Easier to collect exposure informationfrom medical records and biologicalspecimens
Relative strength of population-based and hospital-based
case-control study
Greenberg, p. 132
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Without disease
With disease
Source population
CASES
Study sample
Sampling
CONTROLS
Sampling
exposed
unexposed
THE ORIGIN OF SELECTION BIAS
Greenberg, p.132
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Is the process of selecting the controls so that they aresimilar to the cases in certain characteristics, such as age,race, sex, socioeconomic status, and occupation.
2 TYPES OF MATCHING
1. GROUP MATCHING2. INDIVIDUAL MACHING
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MATCHING1. GROUP MATCHING
(FREQUENCY MATCHING)
Selecting the controls in such a manner that the proportionof controls with a certain characteristic is identical to theproportion of cases with the same characteristic
e.g. :
If 25 % of the cases 25% of the controlsare married are married
ALL OF THE CASES BE SELECTED FIRST !!!
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2. INDIVIDUAL MATCHING(MATCHED PAIRS)
For each case selected for the study, a control isselected who is similar to the case in terms of thespecific variable or variables concern
using hospital controls
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THE PROBLEMS WITH MATCHING
PRACTICAL PROBLEMS
TOO MANY CHARACTERISTICS TO BE MATCHEDDIFFICULT OR IMPOSSIBLE TO IDENTIFY
AN APPROPRIATE CONTROL
CONCEPTUAL PROBLEMSONCE WE HAVE MATCHED CONTROLS TO CASES ACCORDINGTO A GIVEN CHARACTERISTIC, WE CANNOT STUDY THATCHARACTERISTIC
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ADVANTAGES DISADVANTAGES
May increase the precision of case-control comparisons and thus allowa smaller study
The sampling process is easy to
understand and explain
May be time-consuming and expensiveto perform
Some potential cases and controls maybe excluded because matches cannot
be made
The matched variables cannot beevaluated as risk factors in the studypopulation
Greenberg R.S, p 133
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Limitations of recall Recall bias
- Collecting data from subjects by interviews
- Human beings are limited to varying degrees in their ability to recall information
A more serious potential problem in case-control studies
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1. CONTROLS OF DIFFERENT TYPES
2. CONTROLS OF THE SAME TYPE
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USE OF MULTIPLE CONTROLS
1. CONTROLS OF THE SAME TYPE
2 OR 3 CONTROLS FOR EACH CASE
ARE USED TO INCREASE THE POWER OF THE STUDY
UP TO A RATIO OF ABOUT 1 CASE TO 4 CONTROLS
Why not keep the ratio of controls to cases at 1 : 1, and just increase the number of cases ?
FOR MANY OF THE RELATIVELY INFREQUENT DISEASES,THERE MAY BE A LIMIT TO THE NUMBER OF POTENTIAL CASESAVAILABLE FOR STUDY
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2. MULTIPLE CONTROLS OF DIFFERENT TYPES
e.g.
Brain tumor CASES
Other cancer CONTROLS
NormalCONTROLS
Prenatal history of radiation exposure
No history
CHILDREN
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CASE CONTROL STUDY
CASES CONTROLS Total
Exposed A B A + B
Unexposed C D C + D
Total A + C B + D A+B+C+D
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Exposed cases ACase exposure probability = =
All cases A + B
Odds of Exposed cases Unexposed casescase exposure =
All cases All cases
A C= A + C A + C
= A
C
Odds of Bcontrol exposure = D
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A B A x DODDS RATIO = =
C D B x C
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