What are some of the problems with stepwise regression?
Title


Problems with stepwise regression

Author

Bill Sribney, StataCorp

Note:
All of this material is quoted from emails that originally appeared on
STATL/SCI.STAT.CONSULT in 1996. Thanks go to Richard Ulrich, who originally
compiled these comments, and to Frank Ivis, who did minor editing and posted them
to Statalist.

Frank Harrell’s comments:
Here are some of the problems with stepwise variable selection.
 It yields Rsquared values that are badly biased to be high.
 The F and chisquared tests quoted next to each variable on the
printout do not have the claimed distribution.
 The method yields confidence intervals for effects and predicted values
that are falsely narrow; see Altman and Andersen (1989).
 It yields pvalues that do not have the proper meaning, and the proper
correction for them is a difficult problem.
 It gives biased regression coefficients that need shrinkage (the
coefficients for remaining variables are too large; see Tibshirani [1996]).
 It has severe problems in the presence of collinearity.
 It is based on methods (e.g., F tests for nested models) that were
intended to be used to test prespecified hypotheses.
 Increasing the sample size does not help very much; see Derksen and
Keselman (1992).
 It allows us to not think about the problem.
 It uses a lot of paper.
“All possible subsets” regression solves none of these
problems.
Conclusions
“The degree of correlation between the predictor variables affected the
frequency with which authentic predictor variables found their way into
the final model.”
“The number of candidate predictor variables affected the number of
noise variables that gained entry to the model.”
“The size of the sample was of little practical importance in
determining the number of authentic variables contained in the final
model.”
“The population multiple coefficient of determination could be
faithfully estimated by adopting a statistic that is adjusted by the
total number of candidate predictor variables rather than the number of
variables in the final model.”
References
 Altman, D. G. and P. K. Andersen. 1989.
 Bootstrap investigation of the stability of a Cox regression model.
Statistics in Medicine 8: 771–783.
 Copas, J. B. 1983.
 Regression, prediction and shrinkage (with discussion).
Journal of the Royal Statistical Society, Series B 45: 311–354.
 Shows why the number of CANDIDATE variables and not the number in the
final model is the number of degrees of freedom to consider.
 Derksen, S. and H. J. Keselman. 1992.
 Backward, forward and stepwise
automated subset selection algorithms: frequency of obtaining authentic
and noise variables. British Journal of Mathematical and Statistical
Psychology 45: 265–282.
 Hurvich, C. M. and C. L. Tsai. 1990.
 The impact of model selection on
inference in linear regression. American Statistician 44: 214–217.
 Mantel, Nathan. 1970.
 Why stepdown procedures in variable selection.
Technometrics 12: 621–625.
 Roecker, Ellen B. 1991.
 Prediction error and its estimation for subset—selected models.
Technometrics 33: 459–468.
 Shows that allpossible regression can yield models that are too
small.
 Tibshirani, Robert. 1996.
 Regression shrinkage and selection via the lasso.
Journal of the Royal Statistical Society, Series B 58: 267–288.
Ronan Conroy’s comments:
I am struck by the fact that Judd and McClelland in their excellent book
Data Analysis: A Model Comparison Approach (Harcourt Brace
Jovanovich, ISBN 0155167650) devote less than two pages to stepwise
methods. What they do say, however, is worth repeating:
 Stepwise methods will not necessarily produce the best model if there
are redundant predictors (common problem).
 Allpossiblesubset methods produce the best model for each possible
number of terms, but larger models need not necessarily be subsets of
smaller ones, causing serious conceptual problems about the underlying logic
of the investigation.
 Models identified by stepwise methods have an inflated risk of
capitalizing on chance features of the data. They often fail when
applied to new datasets. They are rarely tested in this way.
 Since the interpretation of coefficients in a model depends on the other
terms included, “it seems unwise,” to quote J and McC, “to let an automatic
algorithm determine the questions we do and do not ask about our data”.
 I quote this last point directly, as it is sane and succinct:
“It is our experience and strong belief that better models and a better
understanding of one’s data result from focussed data analysis, guided
by substantive theory,” (p. 204).
They end with a quote from Henderson and Velleman's paper “Building
multiple regression models interactively” (1981, Biometrics 37:
391–411): “The data analyst knows more than the
computer,” and they add “failure to use that knowledge produces
inadequate data analysis”.
Personally, I would no more let an automatic routine select my model than I
would let some bestfit procedure pack my suitcase.