Correlations L4 Flashcards
For normally distributed data, what type of test is used?
Parametric tests will be used.
What tests are used in bivariate correlations
Pearsons product moment correlation coefficient for a parametric test (r)
Spearman’s rho (rs) for a non parametric test.
What does partial correlation look at?
It looks between two variables whilst controlling for the effect of another
What are the assumptions of Pearsons test?
X and Y should be measured using an interval or ratio scale
X and Y should follow a normal distribution
For every value of X, the distribution of Y scores must have approximately equal variability
What does range restriction refer to?
The r value depends on the degree of range restriction, if X or Y are very homogenous the r value will underestimate their relationship
When would a one or two tailed test be used?
One tailed is used when the direction is known, but generally two tailed is preferred as it is more robust.
How are we made aware of whether the r value found is significant or not significant?
The obtained r value (robt) is compared with the minimum r value of the region of rejection (rcrit)
Robt > rcrit then null is rejected (r is significant)
Robt < rcrit then null is accepted (r is not significant)
What does the value of Rcrit depend upon?
Sample size (calculate DF (n-2)) Whether test is one tailed or two tailed The alpha level selected (cut off for significant results)
What did Cohen (1998) believe were small medium and large effect sizes?
Small r=0.1
Medium r = 0.3
Large r= 0.5
What is the coefficient of determination?
The r value squared, it indicates the proportion of variance in X that is associated in the variance in Y. (Partial correlation)
What are the considerations of a partial correlation?
Data must follow a normal distribution
