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A lower-case r, reporting how closely two variables move together.
Also known as: Pearson r, Correlation r
r runs from −1 to +1 and measures how well a straight line describes the relationship between two variables. It says nothing about cause, and nothing at all about relationships that are not straight.
Denotes the Pearson correlation coefficient: a measure of linear association between two variables, bounded between −1 and +1.
Contexts: Mathematics
Measures linear association only. A strong curved relationship can produce an r near zero, so a low value does not mean the variables are unrelated.
Its square, written r², is the proportion of variance in one variable accounted for by the other — a different quantity, frequently quoted as though it were the same one.
A lower-case italic letter r, usually followed by an equals sign and a decimal figure between minus one and one.
A correlation is almost always reported with a p-value assessing whether it differs from zero.