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From December 12 – 14th of 2008, Gallup in conjunction with USA Today, surveyed 1,008 U.S. adults using random digit – dialing about their Christmas practices. 937 of them stated that they celebrate Christmas. You will create a 95% confidence interval for the proportion of all U.S. adults that celebrate Christmas.

I don't know the sample mean, or the standard deviation so I can't create the confidence interval, Can someone please tell me how I can find those? Or how can I calculate the confidence interval ?

Emy
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  • Asked and answered (in abstract terms) at https://stats.stackexchange.com/questions/4756/confidence-interval-for-bernoulli-sampling/6184#6184. – whuber Mar 16 '21 at 16:05

2 Answers2

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You are constructing a confidence interval on a proportion. Your sample proportion is $\hat p = \frac{937}{1008} \approx .93$.

The confidence interval for a proportion is: $\hat p \pm z \sqrt{\frac{\hat p(1 - \hat p)}{n}}$

$n = 1008$ and for a 95% confidence interval $z = 1.96$

So $\text{CI}_{95} \approx [.922, .938]$

Jeff
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There are various styles of confidence intervals (CIs). The 95% Wald CI discussed in @Jeff's Answer (+1) sometimes have 95% coverage of the true population proportion for large $n.$ But for small $n$ the estimated standard error $\sqrt{\frac{\hat p(1-\hat p)}{n}}$ may not be sufficiently close to the true standard error $\sqrt{\frac{p(1-p)}{n}}$ to give good results. [Also for observed proportions near $0$ or $1,$ endpoints of Wald intervals may extend outside of the interval $(0,1)$ or the intervals may be of length $0.]$

You can read Wikipedia on binomial confidence intervals to see a discussion of various styles of CIs, some of which give good results for small $n.$

The Jeffreys interval estimate is based on a Bayesian argument starting with a non-information Jeffreys prior $\mathsf{Beta}(.5. .5).$ It also has good frequentist properties, covering the true proportion $p$ with nearly the 'promised' long-run frequency--for small and large sample sizes $n.$

For your situation with an observed proportion of 937 out of 1008, a 95% Jeffreys CI consists of quantiles 0.025 and 0.975 of $\mathsf{Beta}(.5+937, .5+1008-937)$ $\equiv \mathsf{Beta}(937.5, 71.5),$ which is easily computed in R as $(0.913,0.944).$

qbeta(c(.025,.975), 937.5, 71.5)
[1] 0.9125267 0.9441387

Perhaps no style of confidence interval may be ideal for all purposes, but it is worthwhile knowing the advantages and disadvantages of several styles CIs.

Ref: For graphs of coverage probabilities of Jeffreys CIs.

Nick Cox
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BruceET
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