Correlation

Statistics — Third Secondary

Common mistakes

Comparison aspect	Main idea	Keywords
Mixing up direction and strength	the sign shows only the direction. r = −0.9 is a strong negative correlation while r = 0.2 is a weak positive correlation	signshowsstrongnegative
Wrong denominator	(Σx)² means the square of the sum of the x-values not the sum of their squares. This is a very common error	meanssquarevaluessquares
A value greater than 1 or less than −1	this is impossible and it shows an error in the addition or the substitution	impossibleshowserroraddition
Believing that r = 0 means no relationship at all	r = 0 means only that there is no linear correlation	linearcorrelation
Concluding a cause from a correlation	a strong correlation does not prove that one variable causes the other	strongdoesprovevariable

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