The Gallup Poll has decided to increase the size of its random sample of voters from about 1500 people to about 4000 people right before an election. The poll is designed to estimate the proportion of voters who favor a new law banning smoking in public buildings. The effect of this increase is to

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Using the margin of error, it is found that the effect of this increase is to reduce the variability.

Confidence interval of proportions and margin of error

  • In a sample with a number n of people surveyed with a probability of a success of [tex]\pi[/tex], and a confidence level of [tex]\alpha[/tex], we have the following confidence interval of proportions.

[tex]\pi \pm z\sqrt{\frac{\pi(1-\pi)}{n}}[/tex]

  • In which z is the z-score that has a p-value of [tex]\frac{1+\alpha}{2}[/tex].

The margin of error of the interval is given by:

[tex]M = z\sqrt{\frac{\pi(1-\pi)}{n}}[/tex]

  • For it, it can be seen that M and n are inversely proportional, that is, a higher sample size leads to a smaller margin of error and consequently, less variability.
  • Hence, the effect of this increase is to reduce the variability.

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