The method that was used to arrive at the approximate population probability is called; a reasonable statistical generalization
Statistical generalization is a concept that involves inferring the results from a sample and applying it to a population. Carrying out this means that the sample must be selected randomly and be representative of the population.
Now, in this case, we see that the sample gave a probability of 68% to represent those were in favor of legalizing recreational marijuana use.
Now, since the conclusion was approximated to 70% to reflect the probability of the population then we can say that a reasonable statistical generalization was utilized.
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