Science · Biology
Chapter 1: The Nature of Life
Controls are what make a result mean anything.
In the 1954 polio vaccine trial, unvaccinated children whose parents refused the vaccine got polio at 44 per 100,000. Unvaccinated children in the comparison grades got it at 54 per 100,000.
Neither group was vaccinated, so why were their polio rates different?
Make your guess to turn the page.
Still to come: 9 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students take an improved treatment group as proof that the treatment worked. The book confronts that with the 1954 polio trial, where children of refusing families differed from the comparison grades with no vaccine at all, and with a lab comparison that changed two settings and pointed the wrong way. Ask: what else differed between these groups? The idea to reach is that only a comparison differing in one variable can be attributed to it.
The 1954 numbers — The rates on the page are the rounded figures in Freedman, Pisani and Purves, Statistics, from the Francis report of 1955; they count all reported polio. The explanation that higher-income children were more susceptible, meeting the virus later when it more often paralyses, is the standard account. The randomized design gives the fair estimate; the observed design understated the vaccine.
Biological variability demands replication — Two organisms treated identically respond differently. Sample sizes in biology have to be large enough that individual variation does not masquerade as an effect, which is why n = 1 proves nothing.