External Validity (Generalizability)

Internal Validity asks: "Was the study done correctly?" External Validity asks: "Does the result apply to me?"

The Perfect Patient

Clinical trials often exclude:

• Old people.
• Pregnant women.
• People with other diseases.
• People taking other drugs.

They want a "clean" sample.

But real patients are messy. They are old. They have diabetes. They take five other pills.

The Lab vs. The Street

A drug might work perfectly in a controlled hospital setting where nurses ensure every pill is taken at the exact right time.

But does it work when a busy mom has to remember to take it three times a day with food?

The Diagnosis

A study with high internal validity (perfect RCT) might have low external validity (useless for real patients).

Always check the Inclusion/Exclusion Criteria.

If the study excluded everyone who looks like your patient, the results do not apply to your patient.