Well, for normally distributed data, we could apply the 68-95-99.7% for 1, 2 and 3 standard deviations away from the mean. For 90-95-99% confidence level, the number of standard deviations from the mean where illustrated below.
For data that are not normally distributed, the corresponding percentages are much lower than those presented above. For this course, the confidence intervals formula are for normally distributed data.
Hope this helps.
The 365 Team