Correlation Flashcards
Correlation is a form of…
Bivariate Analysis
What does correlation quantify
A linear relationship between 2 variables in terms of direction and degree
Two variables: Can it be non-linear
Yes. An association can be linear or non-linear
So what if correlation is used if the variables are non-linear
The correlation value is deceptive
What is a perfect linear relationship
Every change in X variable is accompanied by a corresponding change in Y variable
What is small, medium, large correlation
S: .1-.3
M: .3-.5
L: >.5
Just a rule of thumb. Context matters.
What is the difference between variability and covariability
Variability: How much a given variable varies from observation to observation
Co-variability: How much TWO variable varies together
What is the Sum of Squares
The difference between each observation and the mean summed together. Hence, more variable = larger SS
What is the Sum of Products
The difference between each observation and the mean for EACH VARIABLE multiplied together.
If X=Y, SP=SS
What is the pearson r
r = (Co-variability of X & Y)/(Variability X)(Variability Y)
If SP=SS, R=1 and there’s a perfect correlation
What is H0 for correlation
The correlation in the population is 0
What does the p-value depends on in correlations
p value will depend on size of sample and tthus degrees of freedom
df = n-2 (2 groups)
What is Spearman correlation and how is it different
Coverted data to ranks
Why do we use Spearman correlation
- ) Non-linear data
2. ) Outliers
How do we write up this correlation
df = 22
r = -.68
p < .05
r = .68, df = 22, p
