Key measures of central tendency and spread
The mean (average) is the sum of all values divided by the count. The median is the middle value when data is sorted — for an even count, it is the average of the two middle values. The mode is the most frequently occurring value; a data set can have no mode, one mode, or multiple modes.
Standard deviation measures how spread out values are from the mean. Population standard deviation (σ) divides by N and is appropriate when your data includes every member of the population. Sample standard deviation (s) divides by N−1, applying Bessel's correction to provide an unbiased estimate when working with a sample from a larger population.
Mean = Σx / n
Sum of all values divided by count.
σ = √(Σ(x−μ)² / N)
Population standard deviation.
s = √(Σ(x−x̄)² / (N−1))
Sample standard deviation with Bessel's correction.