ABSTRACT
In a normal population, referred to as normal bell-shaped curve, the proportion of data within intervals around the mean is known. The proportion of sample data within k standard deviations around the mean for k=1,2, and 3 are as follows:
INTERVAL DATA PERCENT
LOWER LIMIT UPPER LIMIT IN THE INTERVAL
If a large random sample is selected from a normal population, these lower and upper intervals will approximate the percent of data within the intervals of a normal bell-shaped curve. The proportions of sample data should be good approximations if the population is approximately normal, symmetrical, and unimodal. If the population is non-normal, the bell-shaped curve may not provide results close to these proportions of sample data.
