One area that variable control charts would be have a big impact is in the preventive/predictive maintenance. I think the x-bar- r chart would be a good chart to use. Following the performance on the R chart would indicate if a machine is or will be in need of maintenance. This can also predict the probability of a future failure. The benefit of this is two-fold. First, it could help with the reduction of waste and cost of poor quality; and secondly, it will help lower repair costs by spending less on smaller maintenance and upkeep than large expensive full repairs that could in addition cost a lot of down time. Discuss this statement briefly. What are the challenges? What are the substituations and why?