correlations

strength

absolute value of the number (closer to 1 = stronger).

example of positive 

More study hours → higher grades.

direction

sign of the number (+ = same direction, – = opposite direction).

example of negative

More time watching TV → lower grades.

limitations Directionality problem

which variable comes first?

limitations Third variable problem

a hidden factor may affect both.

limitations non-linear correlation

when a relationship exists between variables, but is not exclusively positive or exclusively negative

example of non-linear

people may enjoy a song more at first, then like it less the more they listen

example of directionality 

We don’t know which causes which, like low Vitamin D and depression

examples third variable problem

A hidden factor affects both, like income affecting both children and appliances.