According to the New Yorker:
"By far the biggest theft problem faced by retailers, however, is employee theft, which accounts for nearly 47% of profit erosion."
(from a 2005 Florida Study - companies lost 1.61 percent of sales to theft or fraud)
I am naive about this subject, but I feel it would be more cost effective on preventing this rather than shoplifters (who make up 33 percent of this sample). You have to consider the negative consequences of attempting to prevent customers from shoplifting like false positives (angry customers leading to lost businness, lawsuits/punitive damges). An earlier version of the study states: "While the average shoplifting incident costs the retailer $212.68, an employee theft averages $1,058.20 per incident."
Investigate Employee Crime more
Replace employees with self-check out machines/robots in the future?