Epidemiology Topic 3: Case-Control Studies Quiz 3

Case-Control Definition

Comparison of exposure frequencies between persons with a specified illness or injury (cases) and other persons (controls)

General population controls

Controls selected from a random sample of the general population e.g. random dialing *provides highly representative controls *costly method, refusal rates, lack of phone coverage

Selection of Cases: Eligibility Criteria

Establish a set of inclusion & exclusion criteria for cases Some reasons may exist to exclude cases (existence of chronic diseases other than the disease under study)

Selection of Cases: Diagnostic criteria

Establish a set of objective diagnostic criteria for case selection

What is the unit of analysis for ecologic studies?

Groups

Example of Case-control study

Levin 1950 cancer research & treatment center Identified 236 ppl with lung cancer cases identified 481 ppl similar to cases with other non-cancerous conditions 156/236 cases (66%) smoked 212/481 non-cases (44%) smoked Because smoking was more common in cases, we can infer that there was a positive association between smoking and lung cancer

Odds Ratio Interpretation

OR=1 implies no association OR=2 cases were twice as likely as controls to be exposed OR<1 suggests a protective factor *provides a good approximation of risk when: controls are representative of a target population, cases are representative of all cases, the disease is rare.

Ecologic Study Correlations

Obtained between exposure rates and disease rates among different groups or populations

Provide an example of a cross sectional study correlates?

Overweight and diabetes

Cross-secrional study

Used to provide Snapshot of population at a point in time.

Limitations of cross sectional studies

1. doesnt provide incidence data 2. difficult to study low prevalence or rare diseases 3. cannot determine temporality of exposure and disease

Disadvantages of hospital controls

1. hospital based controls are ill, they may not accurately represent the exposure history in the population that produced the cases 2. hospital catchment areas may be different for different diseases e.g. cardiology ward vs. ER

Sources of controls

1. population-based controls 2. patients from the same hospital as the cases 3. relatives of cases 4. friends of cases -- SES control

Advantages of hospital controls

1. same selection factors that led cases to hospital led controls to hospital 2. Easily identifiable and accessible 3. Accuracy of exposure recall comparable to that of cases since controls are also sick 4. more willing to participate than population-based controls

when is it desirable to conduct a case-control study

1. when exposure data are expensive or difficult to obtain 2. when the disease has long induction/latent pd. (cancer) 3. when the disease is 4. when little is known about the disease (early studies of AIDS)

Selecting a control3 qualities needed in controls

1.) comparability: more important than representativeness 2.) the control must be at risk of getting the disease 3.) the control should resemble the case in all respects except for the presence of disease

Case-Control sampling

1.) det. source population through recruiting (hospital, geographic area) 2. select a random sample of cases from source population 3. select a sample of non-cases using the same criteria for cases. EXCEPT disease status (these people do not have the disease 4. Compare exposure status (determine % exposed) in cases and controls

Types of Matching

1.) individual (paired) matching: select one or more controls for each individual case 2.) Group (frequency) Matching: control group is selected so that its distribution with respect to variables such as age, race and gender, is similar to cases

Selection of Cases: Selection Criteria

1.) representativeness 2.) Incidence cases 3.) prevalence cases

Ecologic Study Design

A study in which the units of analysis are populations or groups of people rather than individuals Usually takes advantage of pre-existing data collected for other purposes - an efficient and economical study design No time element - a "snapshot"of populations - think cross sectional studies of populations not individuals

Disadvantages

Ecologic fallacy Imprecise measurement of exposure and disease Does not demonstrate that a casual relationship exists

Strengths of case control studies

Efficient for rare disease and diseases with long induction and latent period. Can evaluate many risk factors for the same disease so good for diseases about which little is known Good for outbreak investigations

True/False? A cohort study is best when little is known about a rare disease

False In a cohort study you start by looking at specific exposure/outcome. It's going to be harder to find people (Use case control study instead - bc you find people that have the disease and you can compare them with similar individuals who don't have it.

True/False? Loss to follow up can be a problem in a cohort stud but not in an experimental study

False You still have to follow people for a long amount of time. Think about what you can do to keep people around in your study

Sources of cases

Hospital-based cases are the most commonly used Random sample of the general population -highly representative -very time intensive and labor intensive -cases can be identified from other sources such as cancer registries, ambulatory care facilities, medical insurance companies, retirement groups, etc..

Advantages of an ecologic study

Hypothesis building - especially for a disease with unknown etiology data is easy to obtain: no followup or individual contact is needed - thus, quick and inexpensive can suggest avenues of research that may cast light on an etiologic relationship between exposure and disease. evaluating new policy

Weaknesses of case-control studies

Inefficient for rare exposures Vulnerable to bias because of retrospective nature of study May have poor information on exposure because retrospective Difficult to infer temporal relationship between exposure and disease

What is the hallmark of Case-control studies?

It begins with people with the disease (cases) and compares them to people without the disease (controls)

Matching of Cases & Controls

Matching refers to selecting controls that are very similar to cases in terms of sex, age race., SES, occupation

The ecologic fallacy

Observations made at the ___ may not represent the exposure - disease relationship at the individual level The ecologic fallacy occurs when incorrect inferences about the individual are made from group level data

Correlation Coefficients

Range of r is from -1.0 to 1.0 R evaluated in relation to difference from 0 0 = no correlation .7 = positively correlated -.7 = negatively correlated

Measurement of exposure: challenges

Recall bias cases remember exposures differently from controls

What is the goal of analytic epidemiologic studies?

To elucidate exposure - disease relationships

True or false? Case-control methods allow researchers to overcome the challenge of obtaining large sample sizes for cohort studies

True

True or false? In cohort studies require large sample sizes

True

True/False? The ideal comparison group for a cohort study would consist of exactly the same individuals in the exposed group had they not been exposed

True you want the 2 groups to be as similar as possible except 1 group has the exposure

Analysis of Case Control Studies:

You cannot calculate incidence or cumulative incidence from case control studies as you are dealing with people who already have the disease

Population census

a cross-sectional study of an entire population Provides denominator data for many purposes Huge effort target population - sample - gather data on exposure and disease -exposure disease ++ -exposure no disease +- -No exposure Disease -+ -No exposure No disease --

Measurement of exposure

can reduce bias in measurement by exposure by: BLINDING the interviewer to case/control status Train interviewers to elicit the same info from both groups Use memory aids (calendars, diaries)

Advantages of cross-sectional studies

hypothesis generation intervention planning estimation of magnitude and distribution of a health problem Minimal costs Usually completed in a short amt of time

Analysis of Case-control studies: Odds Ratio

x axis = disease status: yes (cases) No (controls) y axis = exposure status: yes No Odds= a/c b/d Odds Ratio = ad/bc