Week 7 - SMR/HMR

Question 1

• List the two types of tests in multiple regression (MR) 


Answer 1

Standard

Hierarchical

Question 2

• Explain the different questions that are addressed in bivariate regression (x1)

Answer 2

o Does the predictor account for significant variance in the DV?

Question 3

• Explain the different questions that are addressed in multiple 
regression 
(x5)

Answer 3

Extending on bivariate regression
Do predictors jointly account for significant variance in the DV?
• F test of Model R2
For each IV: does it uniquely account for variance in the DV?
• t-test of β for each IV

Question 4

• Explain (in words) what R2 represents in each of the following cases: 

o Bivariate regression

o Multiple regression with uncorrelated predictors
o Multiple regression with correlated predictors
(x2)

Answer 4

• Strength of overall relationship between criterion and predictor/s
Variance accounted for by the IV/set of IVs

Question 5

What is r2 in bivariate regression? (x3)

Answer 5

coefficient of determination -
Proportion of variance in one variable that is explained by variance in another
(like eta2 in ANOVA - SSeffect/SStotal)

Question 6

What is error variance in bivariate regression? (x2)

Answer 6

1 -r2

SSresidual/SSy

Question 7

• Conceptually speaking (i.e. in words), how is shared variance determined in:
o Multiple regression with uncorrelated predictors
 (x1)

Answer 7

There isn’t any,

So R2 is just r2 for each IV/predictor added together

Question 8

• Conceptually speaking (i.e. in words), how is shared variance determined in:
o Multiple regression with correlated predictors (x3)

Answer 8

It’s the overlap of the predictors with each other
So you have to account for it - so it doesn’t get used twice in calculating R2 (which measures non-redundant variance)
(can’t just add, as you do for uncorrelated)

Question 9

• Explain the difference between a zero-order (Pearson’s) correlation, a partial correlation and a semi-partial correlation 
(x1, x1, x1)

Answer 9

Zero-order - only 1 source of variance in DV, so none shared

Partial: relationship between predictor 1 and the criterion, with the variance shared with predictor 2 partialled out of BOTH dv and iv

Semi-partial: relationship between predictor 1 and criterion, after partialling out of predictor 1 variance shared between predictor 1 and 2

Question 10

• In a Venn diagram representing one criterion and three predictors, indicate how shared variance between predictors 1 and 2 would be represented, and how the unique variance of predictor 3 would be represented. 
(x2)

Answer 10

1 and 2 would overlap with each other, and the DV

While 3 would only overlap the DV

Question 11

• List the 4 key differences between the structure of ANOVA tests and MR tests 


Answer 11

No test of overall model in ANOVA - auto in MR
Main effect of IV in ANOVA disregards effect of other variables - MR tests unique variance in each (controls for other variables)
Interactions auto in ANOVA - hard in MR (need MMR)
ANOVA = Fs, effect sizes for IVs/interaction, plus follow-ups
*MR = Rs F-test, beta t-tests, plus follow-ups

Question 12

What is semi-partial correlation squared (sr2)? (x3)

Answer 12

proportion of variance in DV uniquely accounted for by IV1, out of total

• A/ A + B + C + D
• (where C and D are shared IV2/DV variance)

Question 13

What is partial correlations squared (pr2)? (x3)

Answer 13

Proportion of residual variability in DV accounted for by IV1, after IV2 variance removed

• A/A + B
• (where a is unique, b is unaccounted for DV variance)

Question 14

• Explain the difference between the semi-partial correlation squared (spr2/sr2) 
and the partial correlation squared (pr2) 


Answer 14

Partial is like partial eta2 - leftover variability in DV that IV accounts for
Semi-Partial is like eta2 - bit of total DV variability that uniquely due to IV
*the go to effect size for regression

Question 15

• Identify the linear model for a multiple regression analysis with 2 predictors (x2),
And explain what b1, b2, and a represent 


Answer 15

Ŷ = b1X1 + b2X2 + a
Ŷ - predicted score is still a function of slopes, X scores and constant
b1 - slope of plane relative to x1-axis
b2 - slope of plane relative to x2-axis
a - the constant (y-intercept, when x1 and x2 = 0)

Question 16

• Explain why means and standard deviations are not as critical for interpreting MR results as they are for t-tests and ANOVAs 


Answer 16

Because you’re not interpreting/comparing means, but the direction relationship between variables
Although SD still tells us change in DV for every SD change in it’s IV

Question 17

• Explain what Cronbach’s is (x2), and what values of this index we would ideally like to have 
(x2)

Answer 17

index of internal consistency (reliability) for a continuous scale
*how well items “hang together”
Use scales with high reliability ( > .70) if available – less error variance

Question 18

• Define validities (x1), and identify their ideal levels (i.e. high or low) 
(x1)

Answer 18

Relationship between IV and DV

High - show a strong association, yay!

Question 19

• Define collinearities (x1), and identify their ideal levels (i.e. high or low) 
(x1)

Answer 19

Relationships between IVs

Low, because the higher they are, more chance of redundant IV

Question 20

• Explain the principle of parsimony (x2)

Answer 20

The simplest explanation for the data is good

ie predictors explain different variance in the model

Question 21

How does parsimony relate to validities and collinearities 
(x2)

Answer 21

More parsimonious when high validity (predictor/criterion relationships)
And low collinearity (predictor relationships)

Question 22

• Define R (x1) R2, (x1) Radjusted, and R2adjusted (x1) in the context of multiple regression

Answer 22

Multiple correlation coefficient (R) is bivariate correlation between criterion (Y) and best linear combination of the predictors (Ŷ)
R2 - square R to find variance in Y accounted for by composite (Ŷ)
The adjusted versions attempt to correct positive sample bias by correcting for sample size (so no point for 30+)

Question 23

• Identify the test used to assess the overall variance explained (x1, plus explain x1), and explain what a 
significant result means 
(x1)

Answer 23

F-test: MSregression/MSerror
*variance accounted for (R2/p) divided by that not accounted for (error/N - p - 1)
The relationship between predictors (as a group) and criterion is different from zero

Question 24

• If the sr values for the predictors are known, explain how to work out the unique 
variance (x1) explained by each predictor and the variance shared between all predictors 
(x1)

Answer 24

Square the semi-partial correlation to get the coefficient

Shared variance = R2 - sum of all sr2

Question 25

• Explain the difference between a zero-order coefficient, a first-order coefficient, 
and a second-order coefficient 
(x3)

Answer 25

Zero-order - doesn't take other predictors into account First-order - 1 other taken into account Second-order - 2 accounted for

Question 26

• Identify the type of coefficient that SPSS provides in its output 
(x2)

Answer 26

Highest order possible - | Auto control of all other variables

Question 27

• Identify the test used to assess predictor importance, and explain what a significant 
result means 


Answer 27

Use a t-test - beta divided by its standard error | That the variable contributes significantly to prediction of DV, independently of other predictors

Question 28

• Define a partial regression coefficient and it's symbol/notation (x1, x1) But... (x2)

Answer 28

Importance of the predictor, after correlation with other predictors has been adjusted for b Can't use relative magnitude of b - scale-bound  *Importance of given b depends on unit/variability of measure

Question 29

• Define a standardised regression coefficient and it's symbol (x2, x1)

Answer 29

Rough estimate of relative contribution of predictors *b divided by SD of the DV beta

Question 30

• Explain why standardised coefficients can be more useful than unstandardised coefficients (x1, but remember... x1) 


Answer 30

Can compare samples within the study | but not across studies - different sample Sds

Question 31

• Explain the key difference between standard multiple regression (SMR) and hierarchical multiple regression (HMR) 
(x2)

Answer 31

SMR - simultaneous entry of predictors, shared variance never 'owned' HMR - sequential, shared variance goes to previous block

Question 32

• Explain how model R2 and b’s are assessed in SMR (x2), and at each step of HMR 
(x2)

Answer 32

SMR - b's are evaluated for contribution beyond that of all others, R2 assessed in 1 step HMR - b evaluated for unique contribution, controlling for other IVs in current and earlier steps, but not later ones *R2 takes more than 1 step

Question 33

• In HMR, explain the difference (if any) between R2 and R2change: 
 o at Step 1 of the analysis (i.e. Model 1) o at Step 2 of the analysis (i.e. Model 2) (x2)

Answer 33

R2 and R2change are same in step 1 - block and total accounting for same variance At step 2, R2 becomes total of previous step's R2, plus the R2 change between step 1 and 2

Question 34

• Identify the two rationales for the order in which predictors may be entered in HMR 
(plus explain/eg)

Answer 34

.Partial out effect of control variable • Like ANCOVA: predictor(s) at step 1 like covariate Build sequential model according to theory • e.g., personality measure entered at step 1, attitudinal measure entered at step 2

Question 35

• Explain the link between results at Step 2 of HMR (e.g. where predictor A was 
entered at Step 1, and predictors B and C were entered at Step 2), and the results of an SMR (e.g. which includes predictors A, B, and C in the model) 
(x1)

Answer 35

R and R2 are the same as our full SMR

Question 36

• Explain the similarities and differences in the structure of SMR and HMR (with 2 steps) tests 


Answer 36

R2: *At step 1, model fit of IVs added so far *At step 2, same F/p-value as SMR Coefficients: *Step 1, beta/p-value of unique contribution at that step *Step 2: same as you'd get in SMR (so step 1 beta/p-value drops)

Question 37

• Explain what statistics need to be reported in SMR and HMR tests 
(x4, x5)

Answer 37

``` SMR: o Table of M, SD, r o Model R2 with F test o Each IV’s β with p values o Any relevant follow-ups ``` HMR: o Table of M, SD, r o Each block R2 change with F change test o IVs’ βs with p-values from each block as entered o Final model R2 with F test o Any relevant follow-ups

Question 38

• Explain what multicollinearity and singularity are, and why they are a problem 
(x1, x1, x2)

Answer 38

Multicollinearity when predictors are highly correlated Singularity when their explaining the same thing Instability of regression coefficients: *More type 1 AND 2, and changeable by moving single scores around

Question 39

• List the main assumptions re distribution of residuals in multiple regression analyses 
(x1, x2, x2, x2)

Answer 39

Conditional Y (y-hat) values normally distributed around regression line Homoscedasticity: variance of Y values are constant across different values of Ŷ (homogeneity of variance) • Similar DV variance at all levels of continuous IVs No linear relationship between Ŷ and errors of prediction • Error the same across different y-hat values Independence of errors • Can't tell anything about error at one point from error on another – different people, different scores

Question 40

• List the main assumptions re predictor/criterion scales/scores in multiple regression analyses 
(x1, x1, x2, x1)

Answer 40

Variables are normally distributed Linear relationship between predictors and criterion Predictors are not singular (extremely highly correlated) • If predictors are highly related, form a singularity – Ivs collapsing in on each other Measured using a continuous scale (interval or ratio)

Question 41

What is the effect of correlated IVs on betas in MR?

Answer 41

bs and ßs (coefficients) measure unique contribution of each IV to variance left in DV after controlling for other IVs (i.e., they test the partial IV : DV relationship) When IVs correlated, effects of each IV depend heavily on what other predictors are in model. Variables can pop in/out of significance, even change directions (+ vs – ßs) when you add other variables. *betas can be a lot stronger than the r, weaker, or even in opposite direction

Question 42

Why do we have to plan carefully (based on theory) the order of entry in MR? (think effect of correlated IVs... ) (x2)

Answer 42

If you just mess around, the instability associated with correlated IVs can produce both Type 1 and Type 2 error And you will not know which.

Question 43

What is homoscedasticity? (x2) | What is the effect of violating this assumption of regression? (x1)

Answer 43

Homogeneity of variance of residuals *residuals at each level of predictors should have same variance Invalidates significance tests