To the Editor: Although we do not necessarily
disagree with the conclusion of Dr Himmelstein and colleagues1
that investor-owned HMOs deliver lower quality of care than not-for-profit
plans, we think that their use of linear regression may not be appropriate
for analyzing their data. One of the assumptions of linear regression is homoscedasticity,
ie, the variance of the dependent variable is the same for any fixed combination
of explanatory variables.2 For data used
in this study, the dependent variable is a percentage or, equivalently, a
proportion (p). The variance of a proportion is p(1 − p)/n, where n is the sample size. Because variance
is a function of p, the use of linear regression
to model a percentage or p response variable violates
the homoscedasticity assumption. The appropriate technique for modeling a
percentage or proportion is logistic regression.
Register and get free email Table of Contents alerts, saved searches, PowerPoint downloads, CME quizzes, and more
Subscribe for full-text access to content from 1998 forward and a host of useful features
Activate your current subscription (AMA members and current subscribers)
Purchase Online Access to this article for 24 hours
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
Some tools below are only available to our subscribers or users with an online account.
Download citation file:
Web of Science® Times Cited: 1
Customize your page view by dragging & repositioning the boxes below.
More Listings atJAMACareerCenter.com >
and access these and other features:
Enter your username and email address. We'll send you a link to reset your password.
Enter your username and email address. We'll send instructions on how to reset your password to the email address we have on record.
Athens and Shibboleth are access management services that provide single sign-on to protected resources. They replace the multiple user names and passwords necessary to access subscription-based content with a single user name and password that can be entered once per session. It operates independently of a user's location or IP address. If your institution uses Athens or Shibboleth authentication, please contact your site administrator to receive your user name and password.