Het volume van chirurgische ingrepen en de impact ervan op ... - KCE

KCE Reports 113 Volume Outcome 35
3.7.3 Use of instrumental variables
Some authors have used the instrumental variables in a different context than
simultaneous equations modelling. Indeed, their interest lied in estimating consistently
the effect of volume on outcome in equation (I), taking into account the endogenous
character of that variable. An additional variable is needed, called instrumental variable,
assumed to be uncorrelated with the errors of the model but correlated with
endogenous variable. The real problem is that it is sometimes far from obvious which
variables could act as appropriate instruments, and the quality of the method depends
crucially of the quality of the instruments. The use of an instrumental variable, a
standard method in economics, is rather new in the medical literature. It has been used
for instance to assess the effect of different treatments for acute myocardial
infarction, 100 in the context of evaluating the mortality as a measure of quality of hospital
care in pneumonia diagnoses, 101 or else to assess the impact of health care acquired
infections on length of stay. 102
In the context of the volume outcome relationship, different instruments have been
used. The number of hospital beds 99 103 104 and the distance between the patient home
and the hospital 84 105 106 . Different procedures and conditions have been studied:
congestive heart failure 84 , heart attack 105 , Whipple procedure, CABG and abdominal
aortic aneurysm repair 106 . Instrumental variables are still an area of research in statistics.