214 (2) lecture 1 Flashcards
(22 cards)
What do correlations tell you?
How 2 variables are related
What do scatterplots do?
Plot the values for each case on each variable using a single point
They tell you if the relationship is linear and in which direction
What does the gradient of the scatterplot show you?
The strength of the relationship, the stronger the relationship the more likely it will be signficant .
What do correlations range between?
0-1
1= strong
0= weak
correlation coefficients range from -1.00 and +1.00
How do you run a scatterplot in SPSS?
Graphs Legacy Dialogues Scatter/Dot Simple scatter Define Enter variables into X and Y Ok
What are the two common types of correlation coefficients?
Spearman (non-parametric)
Pearson (parametric)
What is Spearman correlation?
Non-parametric
Computed on ranks
Depicts a monotonic relationship
What is a monotonic relationship?
X moves only in one direction (up or down) as y increases)
May/may not be linear
What is Pearson Product-Moment correlation?
Parametric Computed on raw values Very strict monotonicity Depicts linear relationships most commonly used
What does Pearson’s look at ?
The average of the sum of the cross-product of z scores .
When can z scores be used?
When variables have different metrics
- age (yrs) and problem solving (no. correct)
What do we need to remember about z scores?
they have the same mean (0) and SD (1) so we can compare non standard scores
How do you work out Z score
(original score- mean)/ SD
What is the pearson’s formula?
r xy = S(zx zy)/(N-1)
*means little
How do you use the pearson’s formula?
- Convert scores into z scores
- Find product of z-scores by multiplying each pair of z scores, Z (of X) X Z (of Y)
- Sum the products of Z
- Divide the sum of the products by the number of scores -1
What is coefficient of determination? r2
provides information on how much variance in one variable can be explained by variation in the other variable
The square of the correlation predicts the variance
What is the coefficient of non-determination?
The amount of variation in one variable that is NOT explained by the variation in another variable .
How is the coefficient of non-determination calculated?
I= r2
How do you test the significance of r?
The r value needs to be larger than the critical value for n-2
How do you run a correlation in SPSS?
- Variables in columns
Analyze
Correlate
Bivariate
How do you report the results of a pearsons correlation?
r (df-2) = correlation, p> 0.01
What factors distort correlations?
Outliers
Combining subgroups
