Quick & Dirty Confidence Intervels with Excel
The Descriptive Statistics tool in Excel's Data Analysis add-in provides a very useful number with a very misleading label. What Excel calls "confidence level" is in fact the margin of error for a two sided confidence interval for the mean of the sample's parent population. If the specified confidence level is 90%,
The margin of error given by Excel's Descriptive Statistics tool (under the misnomer "confidence level") is only strictly accurate if the parent population is normally distributed. Statistics textbooks detail specific ways to construct confidence intervals in many specific circumstances, but using the margin of error from Excel's Descriptive Statistics tool will very often be good enough for business decision making.