Final answer:
STDEV.S is used for a sample and includes a correction factor, while STDEV.P is used for an entire population without the correction. Both are measures of data spread, and the formulas x = x + 1s and x = x - 2s calculate values at one and two standard deviations from the mean, respectively.
Step-by-step explanation:
The key difference between STDEV.S and STDEV.P is that STDEV.S is the standard deviation formula used when working with a sample of a population, and STDEV.P is used when you have data for the entire population. The standard deviation indicates how spread out the numbers in a data set are. STDEV.S includes a correction factor (n-1 in the denominator) since it's estimating the standard deviation from just a sample. On the other hand, STDEV.P uses n in the denominator as it has access to the whole population's data.
To calculate the value at one standard deviation above the mean, you would use the formula x = x + 1s for a sample. To find the value two standard deviations below the mean, you would calculate x = x - 2s. These calculations are based on the assumption that the data is normally distributed and can help you understand the dispersion of the data around the mean.