Statistics 101: Key Concepts

Mean

Sum of all values divided by the number of values; arithmetic average

Median

Middle value when data is ordered; less affected by outliers than mean

Mode

The value that appears most frequently in a data set

Standard Deviation

Measure of spread; square root of variance; σ or s

Variance

Average of squared deviations from the mean; σ² or s²

Normal Distribution

Bell-shaped, symmetric curve; mean = median = mode; ~68-95-99.7 rule

68-95-99.7 Rule

In normal distribution: 68% within 1σ, 95% within 2σ, 99.7% within 3σ of mean

Z-Score

z = (x - μ)/σ; number of standard deviations from the mean

Probability

Measure of likelihood; P(A) = favorable outcomes / total outcomes; 0 ≤ P ≤ 1

Independent Events

Events where the occurrence of one doesn't affect the probability of the other