From "Diffusion of Laparoscopic Cholecystectomy in the Veterans Affairs Health Care System, 1991-1995" Effective Clinical Practice, March/April 1999. 2:49-55.
Figure 1. Rates of open and laparoscopic cholecystectomy by year in the Veterans Affairs system. The increase in the rate for all cholecystectomies is significant over time (P<0.001 by the chi-square test for linear trend)>