The flip side of this - is that service techs are by their nature biased to failures. They don't come out to the machines that are working. This is an issue for

The flip side of this - is that service techs are by their nature biased to failures. They don't come out to the machines that are working. This is an issue for top-selling items where they may see a lot of service calls but the overall rate is lower than something they see less frequently. In my experience I have known few technicians that have the statistical smarts to account for this.