AnnMaria De Mars PhD The Julia Group amp 7 Generation Games WHY Imagine this What exactly were you planning on doing with that Lets say you have a massive pile of data You Could ID: 279113
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Slide1
Factor Analysis is Your Friend
AnnMaria De Mars, PhD.The Julia Group & 7 Generation Games Slide2
WHY?
Imagine thisSlide3
What exactly were you planning on doing with that?
Let’s say you have a massive pile of data … Slide4
You Could:
“9% of adolescents reported blah blah blah”
“23% of adults said blah blah blah” Slide5
The Problem:
Boring! No one is going to read each one.Cannot conduct relational analysis of each variable --- statistical sin
Individual items are notoriously unreliable. Slide6
Subscales ?
Do you own guinea pigs?
Do you have any stuffed animals?Slide7
Conceptually, it’s pretty simple
Don’t Be Scared of Factor Analysis!
Image Source:
scottyoungpsalm37.blogspot.com
Slide8
Factor analysis is for …
1. Revealing patterns of interrelationships among variables2. Detecting clusters of variables3. Reducing a large number of variables to a smaller number of variables, the factors of factor analysis.Slide9
You can factor analyze anything
test scores, individual items on a test
measurements
of various dimensions
(i.e. height or
weight)
agricultural
measures like yield of a rice
field
Socioeconomic
measresSlide10
Is this a fair test?
You have studied for a final exam in Biology 101. There is one question, “What is the relationship between respiration and photosynthesis?”Your child is in fifth grade. Her weekly spelling test consists of one word.Slide11
You already understand this
True Variance vs. Error Variance
Variability
& ValiditySlide12
Questions …
Image Source:
www.edudemic.comSlide13
How do I …
Decide on the number of factorsInterpret factorsWith SAS Enterprise GuideSlide14
How to do Factor Analysis using SAS Enterprise GuideSlide15
A Brief Overview of the Process
Open a data set, run a factor analysis, and
observe the data’s fit.
2. If necessary, run a correlation analysis to create a dataset to analyze
3. If necessary , make modifications and run your model once or twice moreSlide16
A complete projectSlide17
Our data
From the 500 Family StudyHundreds of questions answeredExample uses 42 items asked adolescents regarding parent communication, rules, decision-makingSlide18
FILE > OPEN > DATASlide19
Hold down shift key to select more than one at a time
Select the variablesSlide20
TASKS
> MULTIVARIATE > FACTOR ANALYSISSlide21
Look at your log first!
When you get your results, do NOT look at your results first. Be smarter than most people and look at your log. To do that you click on the tab that says LOGSlide22
What if you see this?
WARNING: 123 OF 465 OBSERVATIONS IN DATA SET WORK.SORTTEMTABLESORTED OMITTED DUE TO MISSING VALUES.If we didn’t have a lot of people missing data, we could skip the next few steps, but hey, that’s life. Slide23
Tasks > Describe > Summary statistics
Drag and drop to select variables to analyzeSlide24
The default for SAS is to delete a record if it is missing ANY of the variables.
Why factor analyze the correlation matrix?
My first analysis was missing 120 records but no single item was missing
for more than 49 peopleSlide25
by clicking on them and pressing the
blue arrows in between the panes
Select the variables you wantSlide26
DATA SET > TASKS > MULTIVARIATE > CORRELATIONS.
Step Two:
Create a Correlation Matrix DatasetSlide27
Output matrix as SAS dataset of type=CORR:
OUTPUT DATA > SAVE OUTPUT > DATA > RUNSlide28Slide29
TASKS
> MULTIVARIATE > FACTOR ANALYSISSlide30
Hold down shift key to select more than one at a time
Select the variablesSlide31Slide32
So Now What does all of this Factor Analysis crap mean, anyway? Slide33
What exactly is a
factor?Recall, a factor is some underlying trait that is measured indirectly by the items you measured directly
.
AKA
‘dimension
reduction
technique
’
Slide34
How many factors?
3 possibilitiesSlide35
Eigenvalue
The amount of variance in the individual measures explained by the factor.
Square the loadings in the factor pattern and add them up. The total is the
eigenvalue
.
Prediction: At least one person who reads this will do exactly that
and be surprised that I am right. Contrary to appearances, I do not make this
s
*** up. Slide36
Eigenvalue
Common criterion for deciding the number of factors is “Minimum eigenvalue greater than 1.”
Makes intuitive sense but …Slide37Slide38
Method 3
Parallel analysis criterion (macros available – Google it)Slide39
Now do it againSlide40
What do the factors mean?Slide41
Important Point O
neThe correlation of a variable with a factor is called the loading.loadings can be positive or negative. Slide42
Important point two
To ease interpretation we’d really like to have “simple structure”variables load close to 1.0 on one factor and close to zero on the others. Slide43Slide44
Next step: iterate
No, it is not party in the pool, sorry.Slide45
Secret to factor analysis
Finding a solution that is defensible BOTH statistically and theoreticallyNice first step to structural equation modeling. In fact, it IS the first step.