A little background: I work for a large corporation and was tasked to perform a process improvement study on an often overlooked department. This department gets little attention and performs mostly business processes. They are, however, very consistent and repeatable. I mapped out their process and delegation of tasks, gathered readily available data on occurances of these tasks, and performed time studies on the tasks as well as incorporating down-time in my model.
The buildup: The findings of my study show that one group within this department is grossly under-utilized (~40% utilization including downtime). I brought this up to the manager and they dismiss it and said that their workers perform other tasks outside the scope of my study. When I asked for a list of other duties they may perform, they were unable to come up with any and suggested that interruptions play a large part and tout the fact that these employees are consistently working overtime. So I re-studied their interruptions and came to the same conclusion I did before, I even was very conservative in my estimates and they still seem very under-utilized. It seems that their workload is very reactionary in the sense that they do not have a backlog, they cannot work ahead, and they work things as they arise. During my studies I often had to leave after an hour or two because they would run out of things to work on. I also had my colleagues pay attention and they observed excessive idle-time as well.
The problem: I have to report out on this to the manager, their manager, and upper management. My concern is that even though I have done my due-diligence, since the manager is the SME my study will lack credibility if they begin disagreeing with me.
The question: What path is there to minimize the conflict with this manager and retain my credibility in the face of this disagreement? I do not want to openly disagree with the manager if at all possible, but I do not see any way around it.