Part 2
Attrition: Bias and Loss of Power
Relevant Papers

Graham,

Collins,
(2009).
Missing data analysis: making it
work in the real world. Annual Review of Psychology, 60, 549-576.
J.W.,
L. M.,
Schafer,
J. L.,
& Kam,
C. M.
(2001).
A
comparison of inclusive and restrictive strategies in modern
missing data procedures. Psychological Methods, 6, 330_351.

Hedeker, D., & Gibbons, R.D. (1997).

Graham, J.W., & Collins, L.M. (2010, forthcoming).
Application of
random-effects pattern-mixture models for missing data in
longitudinal studies, Psychological Methods, 2, 64-78.
Using
Modern Missing Data Methods with Auxiliary Variables to Mitigate
the Effects of Attrition on Statistical Power. Chapter 10 in Graham
(2010, forthcoming), Missing Data: Analysis and Design. New
York: Springer.
Relevant Papers


Graham, J.W., Palen, L.A., et al. (2008).
Attrition: MAR & MNAR
missingness, and estimation bias. Annual Meetings of the Society
for Prevention Research, San Francisco, CA. (available upon request)
also see: Graham, J.W., (2010, forthcoming). Simulations with
Missing Data. Chapter 9 in Graham (2010, forthcoming), Missing
Data: Analysis and Design. New York: Springer.
What if the
cause of missingness is MNAR?
Problems with this statement



MAR & MNAR are widely misunderstood
concepts
I argue that the cause of missingness is
never purely MNAR
The cause of missingness is virtually
never purely MAR either.
MAR vs MNAR


"Pure" MCAR, MAR, MNAR never occur
in field research
Each requires untenable assumptions

e.g., that all possible correlations and
partial correlations are r = 0
MAR vs MNAR


Better to think of MAR and MNAR as
forming a continuum
MAR vs MNAR NOT even the dimension of
interest
MAR vs MNAR: What IS the
Dimension of Interest?

How much estimation bias?

when cause of missingness cannot be
included in the model
Bottom Line ...


All missing data situations are partly
MAR and partly MNAR
Sometimes it matters ...


bias affects statistical conclusions
Often it does not matter

bias has tolerably little effect on statistical
conclusions
(Collins, Schafer, & Kam, Psych Methods, 2001)
Methods:
"Old" vs MAR vs MNAR

MAR methods (MI and ML)



are ALWAYS at least as good as,
usually better than "old" methods
(e.g., listwise deletion)
Methods designed to handle MNAR
missingness are
NOT always better than MAR methods
Yardstick for Measuring Bias
Standardized Bias =
(average parameter est) – (population value)
-------------------------------------------------------- X 100
Standard Error (SE)

|bias| < 40 considered small enough to be
tolerable

t-value off by 0.4
A little background for Collins,
Schafer, & Kam (2001; CSK)

Example model of interest: X  Y
X = Program (prog vs control)
Y = Cigarette Smoking
Z = Cause of missingness: say,
Rebelliousness (or smoking itself)

Factors to be considered:



% Missing (e.g., % attrition)
rYZ
rZR
rYZ

Correlation between

cause of missingness (Z)


e.g., rebelliousness (or smoking itself)
and the variable of interest (Y)

e.g., Cigarette Smoking
rZR

Correlation between

cause of missingness (Z)


and missingness on variable of interest


e.g., rebelliousness (or smoking itself)
e.g., Missingness on the Smoking variable
Missingness on Smoking (often
designated: R or RY)
 Dichotomous variable:
R = 1: Smoking variable not missing
R = 0: Smoking variable missing
CSK Study Design

(partial)
Simulations manipulated



amount of missingness (25% vs 50%)
rZY (r = .40, r = .90)
rZR held constant


r = .45 with 50% missing
(applies to "MNAR-Linear" missingness)
CSK Results
(partial)
(MNAR Missingness)




25%
25%
50%
50%
missing,
missing,
missing,
missing,
rYZ
rYZ
rYZ
rYZ
=
=
=
=
.40
.90
.40
.90
...
...
...
...
no problem
no problem
no problem
problem
* "no problem" = bias does not interfere
with inference
These Results apply to the regression coefficient for X  Y
with "MNAR-Linear" missingness (see CSK, 2001, Table 2)
But Even CSK Results
Too Conservative

Not considered by CSK: rZR
In their simulation rZR = .45

Even with 50% missing and rYZ = .90



bias can be acceptably small
Graham et al. (2008):

Bias acceptably small
(standardized bias < 40)
as long as rZR < .24
rZR < .24 Very Plausible
Study
_________
HealthWise
(Caldwell, Smith, et al., 2004)
AAPT (Hansen & Graham, 1991)
Botvin1
Botvin2
Botvin3

rZR (estimated)
_____
.106
.093
.044
.078
.104
All of these yield standardized bias < 10
CSK and Follow-up Simulations



Results very promising
Suggest that even MNAR biases are
often tolerably small
But these simulations still too narrow
Beginnings of
a Taxonomy of Attrition
Causes of Attrition on Y (main DV)




Case 1: not Program (P), not Y, not PY
interaction
Case 2: P only
Case 3: Y only . . . (CSK scenario)
Case 4: P and Y only
Graham, J. W. (2009). Annual Review of Psychology.
Beginnings of
a Taxonomy of Attrition
Causes of Attrition on Y (main DV)




Case
Case
Case
Case
5:
6:
7:
8:
PY interaction only
P + PY interaction
Y + PY interaction
P, Y, and PY interaction
Taxonomy of Attrition

Cases 1-4


often little or no problem
Cases 5-8



Jury still out (more research needed)
Very likely not as much of a problem as
previously though
Use diagnostics to shed light
Use of Missing Data
Diagnostics

Diagnostics based on pretest data not
much help


Hard to predict missing distal outcomes
from differences on pretest scores
Longitudinal Diagnostics can be much
more helpful
Hedeker & Gibbons (1997)

Plot main DV over time for four groups:



for Program and Control
for those with and without last wave of
data
Much can be learned
Empirical Examples

Hedeker & Gibbons (1997)


Drug treatment of psychiatric patients
Hansen & Graham (1991)


Adolescent Alcohol Prevention Trial (AAPT)
Alcohol, smoking, other drug prevention
among normal adolescents (7th – 11th grade)
Empirical Example Used by
Hedeker & Gibbons (1997)


IV: Drug Treatment vs. Placebo Control
DV: Inpatient Multidimensional Psychiatric
Scale (IMPS)







1
2
3
4
5
6
7
=
=
=
=
=
=
=
normal
borderline mentally ill
mildly ill
moderately ill
markedly ill
severely ill
among the most extremely ill
From Hedeker & Gibbons (1997)
5.5
Placebo
Control
5
IMPS
4.5
low = better
outcomes
4
3.5
Drug
Treatment
3
2.5
0
1
3
Weeks of Treatment
6
Longitudinal Diagnostics
Hedeker & Gibbons Example




Treatment
 droppers do BETTER than stayers
Control
 droppers do WORSE than stayers
Example of Program X DV interaction
But in this case, pattern would lead to
suppression bias

Not as bad for internal validity in presence
of significant program effect
AAPT (Hansen & Graham, 1991)


IV: Normative Education Program vs
Information Only Control
DV: Cigarette Smoking (3-item scale)


Measured at one-year intervals
7th grade – 11th grade
AAPT
Control
Program
Control
Cigarette
Smoking
(high =
more
smoking;
arbitrary
scale)
Program
th
th
th
th
th
Longitudinal Diagnostics
AAPT Example

Treatment
 droppers do WORSE than stayers


Control
 droppers do WORSE than stayers



little steeper increase
little steeper increase
Little evidence for Prog X DV interaction
Very likely MAR methods allow good
conclusions (CSK scenario holds)
Use of Auxiliary Variables


Reduces attrition bias
Restores some power lost due to
attrition
What Is an Auxiliary Variable?

A variable correlated with the variables
in your model




but not part of the model
not necessarily related to missingness
used to "help" with missing data estimation
Best auxiliary variables:

same variable as main DV, but measured at
waves not used in analysis model
Model of Interest
X
Y
1
res 1
Benefit of Auxiliary Variables

Example from Graham & Collins (2010)
X Y Z
1 1 1
1 0 1


500 complete cases
500 cases missing Y
X, Y variables in the model (Y sometimes missing)
Z is auxiliary variable
Benefit of Auxiliary Variables

Effective sample size (N')


Analysis involving N cases,
with auxiliary variable(s)
gives statistical power equivalent to N'
complete cases without auxiliary variables
Benefit of Auxiliary Variables


It matters how highly Y and Z (the auxiliary
variable) are correlated
For example




rYZ
rYZ
rYZ
rYZ
=
=
=
=
.40
.60
.80
.90
increase
N
N
N
N
=
=
=
=
500
500
500
500
gives
gives
gives
gives
power
power
power
power
of
of
of
of
N'
N'
N'
N'
=
=
=
=
542
608
733
839
( 8%)
(22%)
(47%)
(68%)
Effective Sample Size by rYZ
1000
900
Effective
Sample
Size
800
700
600
500
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
rYZ
Conclusions






Attrition CAN be bad for internal validity
But often it's NOT nearly as bad as often
feared
Don't rush to conclusions, even with rather
substantial attrition
Examine evidence (especially longitudinal
diagnostics) before drawing conclusions
Use MI and ML missing data procedures!
Use good auxiliary variables to minimize
impact of attrition
Part 3:
Illustration of Missing Data
Analysis: Multiple Imputation
with NORM and Proc MI
Multiple Imputation:
Basic Steps

Impute

Analyze

Combine results
Imputation and Analysis

Impute 40 datasets


Analyze each data set
with USUAL procedures


a missing value gets a different imputed
value in each dataset
e.g., SAS, SPSS, LISREL, EQS, STATA, HLM
Save parameter estimates and SE’s
Combine the Results
Parameter Estimates to Report

Average of estimate (b-weight)
over 40 imputed datasets
Combine the Results
Standard Errors to Report
Weighted sum of:
 “within imputation” variance
average squared standard error
 usual kind of variability

“between imputation” variance
sample variance of parameter estimates over
40 datasets
 variability due to missing data
Materials for SPSS Regression
Starting place
http://methodology.psu.edu

downloads (you will need to get a free user ID to
download all our free software)
missing data software
Joe Schafer's Missing Data Programs
John Graham's Additional NORM Utilities
http://mcgee.hhdev.psu.edu/missing/index.html

(this mcgee website is currently down, but I hope to have it
up again in the Fall). Please email me with any questions.
exit for sample analysis