1. Lecture 21: Non-experimental intervention studies (cont)
Describe the following sources of bias in the evaluation of screening programs:
- Length-biased sampling
- Lead time bias
Compare experimental vs non-experimental intervention study designs, taking into account the following considerations:
- Ethical issues
- Feasibility and practicality
- Validity (internal and external), precision, and efficiency

2. Evaluation of screening Example: evaluation of the effectiveness of breast cancer screening (HIP study)
1st RCT of breast cancer screening
Study population: Members of HMO
Intervention: Invitation to receive annual mammography and clinical exam (3 years)
Possible outcomes:
survival rate (1 year, 5 year)
case-fatality rate
mortality rate
Which would you use?

4. Bias in RCTs of screening
Definition of time zero?
Date of first symptoms?
Date of detection?
Date of diagnosis?
Bias if difference in “time zero”between study groups:
screening/early detection intervention shifts time zero
intervention appears to lengthen time to outcome without real change in prognosis
“lead time” bias
“length” bias

5. Other types of bias in studies of interventions
Hawthorne effect:
Non-specific effect of being in a study
Prevention?
Contamination bias:
Control group receives some component(s) of intervention
Confounding variables
Variables associated with intervention group and outcome, not in causal chain

6. Internal vs external validity
Internal validity
Lack of bias in study
External validity
Generalizability
Representativeness of study sample

7. Selection of study design for evaluation of interventions
Observational study design
Cohort
Case-control
Quasi-experimental study design
Non-equivalent control group
Time-series: single or multiple (controlled)
Experimental
RCT

8. Considerations in selection of a study design
Cost
Feasibility
Ethical issues
Internal validity
External validity
Credibility