PlanMaker uses the normal distribution in the confidence function. We will check if we switch to the t-distribution when we review the extended and improved functions in Excel2010. (Thanks for the hint
However, I wouldn't overestimate the effect, for reasonable sample sizes the results are not so different, this is also stated in the article. The complaint was about a sample size of 6, which is imho not enough to do serious statistics.
Thanks for the reply. If you followed the links in the article you would have seen that MS is thinking of revising their functions so it would be good if PM also made use of industry's best practices...
As for the small sample well spreadsheets do get used for serious jobs including stats, and they often get used by people who need some guidance (students, businessmen without stats training, etc), so it is best if the functions available really are correct.