ch5 - 1 Ch 5 Controlled Experiments and Observational...

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Unformatted text preview: 1 Ch 5 Controlled Experiments and Observational Studies It is useful to distinguish between controlled experiments and observational studies . Usually an investigator wishes to know the effect of a TREATMENT (such as a university education, aid, a vaccine, etc.) on a RESPONSE (like earning higher income, economic growth, getting a disease, etc.) To find this out, researchers compare responses of a TREATMENT GROUP with a CONTROL GROUP . A key idea here is COMPARISON . The Salk Vaccine Field Trial is a very good illustration of what a controlled experiment might look like. The first polio epidemic hit the US in 1916. By the 1950s, several vaccines were discovered. The most promising one was developed by Jonas Salk . The Public Health Service and the National Foundation for Infantile Paralysis (NFIP) were ready to test the vaccine. 2 Suppose the NFIP decided to test the vaccine comparing cases of polio in 1953 vs 1954: This is unreliable , because polio is an epidemic disease whose incidence varied from year to year. The only way to find out whether the vaccine worked was to deliberately leave some children unvaccinated, and use them as controls . The NFIP did run a controlled experiment. Subjects were children in age groups most vulnerable to polio grades 1, 2, and 3. 2 million children were involved and million were vaccinated (a million were deliberately left unvaccinated; half a million refused vaccination). Q) Does comparing different sizes of the control and treatment matter? 3 Tr eat ment Tr eat ment Gr oup Gr oup ( Vacci nat ed) ( Vacci nat ed) Ref use Ref use Vacci nat i on Vacci nat i on Par ent s Per m i ssi on Par ent s Per m i ssi on 1 Gr ade 2 Gr ade 3 Gr ade Cont r ol Gr oup Cont r ol Gr oup The NFIP study has 2 defects: (1) Subjects are not RANDOMLY assigned to the treatment and control groups (wealthier parents tend to favor vaccination but, polio is a disease of hygiene; children...
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ch5 - 1 Ch 5 Controlled Experiments and Observational...

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