We're unable to sign you in at this time. Please try again in a few minutes.
We were able to sign you in, but your subscription(s) could not be found. Please try again in a few minutes.
There may be a problem with your account. Please contact the AMA Service Center to resolve this issue.
Contact the AMA Service Center:
Telephone: 1 (800) 262-2350 or 1 (312) 670-7827  *   Email: subscriptions@jamanetwork.com
Error Message ......
Editor's Correspondence |

Controlling for Patient Risk in Volume-Outcome Studies

Vivian Ho, PhD
Arch Intern Med. 2005;165(14):1664. doi:10.1001/archinte.165.14.1664-a.
Text Size: A A A
Published online


Nallamothu et al1 concluded that differences in in-hospital mortality rates between low- and high-volume centers rose as the expected risk of in-hospital death increased. However, the predicted risk for each patient was derived from a risk prediction model that excluded volume as an explanatory variable. The coefficient for any risk factor that is correlated with volume in this risk prediction model is subject to omitted variable bias.2 For example, the authors noted that patients undergoing coronary artery bypass grafting at low-volume centers were older on average. Therefore, the described risk prediction model will mistakenly assign some of the lower mortality associated with high volume to the age variable because patients at high-volume hospitals are younger on average, and the association between volume and mortality is negative. The risk prediction model therefore systematically underestimates the association between risk factors and mortality for characteristics more prevalent in low-volume hospitals and overestimates these effects for characteristics more prevalent in high-volume hospitals. In turn, a regression analysis including volume and this measure of predicted risk as explanatory variables will produce a biased estimate of the association between volume and mortality. Analyses of the relation between volume and mortality by surgical risk strata will also be confounded. Although it is plausible that differences in outcomes between low- and high-volume health care providers will be greater for high-risk patients, the reported analyses cannot be used to draw this conclusion.

Sign in

Purchase Options

• Buy this article
• Subscribe to the journal
• Rent this article ?

First Page Preview

View Large
First page PDF preview





Also Meets CME requirements for:
Browse CME for all U.S. States
Accreditation Information
The American Medical Association is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians. The AMA designates this journal-based CME activity for a maximum of 1 AMA PRA Category 1 CreditTM per course. Physicians should claim only the credit commensurate with the extent of their participation in the activity. Physicians who complete the CME course and score at least 80% correct on the quiz are eligible for AMA PRA Category 1 CreditTM.
Note: You must get at least of the answers correct to pass this quiz.
Please click the checkbox indicating that you have read the full article in order to submit your answers.
Your answers have been saved for later.
You have not filled in all the answers to complete this quiz
The following questions were not answered:
Sorry, you have unsuccessfully completed this CME quiz with a score of
The following questions were not answered correctly:
Commitment to Change (optional):
Indicate what change(s) you will implement in your practice, if any, based on this CME course.
Your quiz results:
The filled radio buttons indicate your responses. The preferred responses are highlighted
For CME Course: A Proposed Model for Initial Assessment and Management of Acute Heart Failure Syndromes
Indicate what changes(s) you will implement in your practice, if any, based on this CME course.


Some tools below are only available to our subscribers or users with an online account.

0 Citations

Sign in

Purchase Options

• Buy this article
• Subscribe to the journal
• Rent this article ?

Related Content

Customize your page view by dragging & repositioning the boxes below.

Articles Related By Topic
Related Collections

Users' Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice, 3rd ed