statistics

Statistics deals with the collection, organisation, analysis, interpretation, and presentation of data.

types

Descriptive statistics summarise data
Inferential statistics make predictions

data types
type	examples
binary	yes or no
categorical	film genre
count	number of pushups
number	42
ordinal	low, medium, high

terms

mean

The arithmetic mean is the sum of all values in a given set divided by the number of values in that set:

x = \frac{1}{N} \sum_{i = 1}^{N} xi = \frac{x_{1} + x_{2} + \dots + x_{n}}{N}

Other types of means:

The geometric mean is the root of the product of all values in a given set
The harmonic mean is the reciprocal of the arithmetic mean of the reciprocals of a given set. Useful for finding the average rate
The root mean square is the square root of the arithmetic mean of the square in a given set

standard deviation

The standard deviation (σ) of a set determines the amount of variance in its values.

σ = \sqrt{\frac{1}{N - 1} \sum_{i = 1}^{N} {(x_{i} - x)}^{2}}

other

The median is the middle number in a sorted set
The mode is the most frequent item of data

correlation

Covariances measures how one variable relates to another variable. Postive covariances indicate that the two variables move in the same direction; conversely, negative covariances indicate they move in opposite directions.

Correlation is basically just covariance but normalised. You’ll only get correlation values that range from -1 to 1. A correlation of 1 is a positive correlation, -1 is a negative correlation, and 0 means no correlation.