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Under what conditions is the 'plus four' method recommended for a proportion confidence interval?
AConfidence level at most 50% and sample size at least 100
BOnly when the population standard deviation is known
CConfidence level at least 90% and sample size at least ten
DOnly when np' and nq' both already exceed 30
Answer & Solution
Correct answer: C. Confidence level at least 90% and sample size at least ten
1. The plus-four method targets situations where the ordinary formula's small-sample error is worst.
2. Two conditions apply together: the desired confidence level must be at least 90%, and the sample size must be at least ten.
3. Both conditions must hold for this adjustment to be recommended.
4. Option B describes a condition relevant to means, not to this proportion adjustment.
_Source: OpenStax Introductory Statistics (CC BY 4.0), Ch 8 "Confidence Intervals", section 8.3 A Population Proportion_
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