External Validity of Medical Trials

Question 1

the validity of the study depends on what?

Answer 1

it depends on our ability to determine the result is a consequence of the treatment and nothing else= you have to prove that the other factors are not contributory

Question 2

what is the differnece between external and internal validity?

Answer 2

• Internal validity: the features of the trial which eliminate bias from its conduct and reporting
• External validity: the degree to which the trial is relevant to clinical practice- requires more clinical knowledge

Question 3

what factors affect external validity?

Answer 3

1. Health care system factors- trials done in one country may not apply to another (especially true when applying results of developed world studies to developing world studies)
2. Service resources available (may have higher staff - to patient ratios and more intensive clinical monitoring tha occurs in real life) b/c trials are often run at centres of excellence
3. Eligibility criteria for participants- may make trial unrepresentative of the patient population - trials based at tertiary referral centers will see a biased subset of the patient population - trials often exclude comorbidity despite the frequency with which it occurs in real patient populations
4. Appropriateness of controls- Failure of using ‘treatment as usual’ control group makes experimental treatment hard to interpret
5. Choice of outcome measures- trials should have clinically relevant endpoints -

Question 4

what is the difference between explanatory trials and pragmatic trials?

Answer 4

explanatory trials = generally measure efficacy

pragmatic trials = generally measure effectiveness