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The given investigation is:

In the 1940s and early 1950s, the public was greatly concerned about polio. In an attempt to prevent this disease, Jonas Salk of the University of Pittsburgh developed a polio vaccine. In a test of the vaccine's efficacy, involving nearly a million grade-school children, half of the children received the Salk vaccine; the other half received a placebo, in this case, an injection of salt dissolved in water. Neither the children nor the doctors performing the diagnoses knew which children belonged to which group, but an evaluation center did. The center found that the incidence of polio was far less among the children inoculated with the Salk vaccine. From that information, the researchers concluded that the vaccine would be effective in preventing polio for all U.S. school children; consequently, it was made available for general use.

The evaluation sought to test the vaccine center's efficiency, so this inquiry is a designed experiment.

To arrive at a conclusion, they tested two different vaccines on two distinct groups of children, then counted the number of children who had polio.