# How to combine/pool binomial proportion confidence intervals after multiple imputation?

After I multiply imputed my dataset m times I wanted to calculate a binomial proportion confidence interval. I did that formerly using the Hmisc::binconf() function in R, but pooling seemed to be impossible using the mice package. How do I combine these confidence intervals into one considering within and in-between imputed dataset variance? Can I use Rubin's rules?

Update:

After reading the article mentioned in the comments, I may have found a way to pool binomial proportion confidence intervals. First I used the Wilson Score Interval formula:

Then I calculated the relocated centre estimate p' using the following formula for every complete dataset:

And after that I calculated the correct standard deviation s' for every complete dataset using a formula found in this article:

By calculating the standard deviation and relocated centre for every complete dataset I was able to pool the results using mice::pool.scalar(Q, U, n = 100), and calculate the final confidence interval using the qnorm(0.975, Q, sqrt(U) function. Can anybody confirm whether this is correct or not?

• For a general idea of Rubin's combining rules, you can read ncbi.nlm.nih.gov/pmc/articles/PMC2727536. The original book is eu.wiley.com/WileyCDA/WileyTitle/productCd-0471655740.html – Qaswed Jun 22 '16 at 18:29
• @Qaswed I rephrased the question as it might have been too broad. Thanks for the article, can you have a look at my updated question, I hope I managed to do it correctly now. – Roger Jun 23 '16 at 13:50
• @Roger Although the score interval formula is a good way of calculating confidence intervals around a proportion, you could also try using the easier to apply Agresti-Coull adjusted Wald confidence intervals (see link ) The problem however, lies in taking into account the variance between imputation datasets. Neither the Score or Agresti-Coull CIs take this into considerations. I have upvoted your question as I'd also like an answer to this, but as to whether or not using only Score CIs is correct, I'd say no. – IWS Jan 11 '17 at 13:16

This is indeed an interesting problem. The issue is that the standard errors that are based on the central limit theorem for proportions are often undesirable because proportions are a computed quantity and for that reason exhibit skewed uncertainty over sampling. The Wilson score, such as you mentioned, gets around the skewness by estimating a different quantity than the standard proportion $k/n$. What you need to use Rubin's rules is an estimate of the within-imputation variance of this transformed proportion, which is just the variance/standard error estimated on a single dataset, along with the transformed proportion itself for each dataset.
So for the Wilson score interval, you first need to calculate the transformed estimate $\hat{p} + \frac{1}{2n}z^2$ and then separately the variance, which from your formula is $(z\sqrt{\frac{1}{n} \hat{p}(1-\hat{p}) + \frac{1}{4n^2}z^2})^2$
That will give you estimates of the transformed parameter and the transformed parameter's variance for each of $m$ datasets.
You can then combine these estimates using some of the available R tools, such as mi.meld from Amelia or mice as you mentioned or the R package mitools. Then once you have the transformed parameters, you can compute the confidence interval based on the newly derived variance/parameter estimate.