it is used to show how to sets of data relate to one another
if you make a scatter plot, you can draw a line that shows the pattern of the two sets of data.
If the line has a positive slope (/) the correllation is positive
if the line has a negative slope (\) the correllation is negative (a regression)
a positive correlation means
as X increases, y increases (or as y increases x increases)
a negative correlation means
as X increases, y decreases (or as y increases x increases)
if the two sets from random patterns (not positive or negative correlation) we say there is no correlation
