You can’t precisely compare projects before and after
One of the characteristics of projects is that they are all unique. Therefore, you cannot make a direct apples-to-apples comparison of what projects looked like before the use of project management processes and after the fact. What you would like to say is that it took us X hours to do a project before, and now it takes Y. However, two projects are never exactly the same to make this comparison.
You don't have the baseline metrics
You may be able to make some general statements by comparing projects with similar characteristics. However, most companies don’t keep any historical records of project characteristics and costs to use in this type of comparison. You can track the costs and durations of current projects, but how do you compare to the past when you weren't capturing any information.
There is a lot of other stuff going on
The rollout of project management processes in a large company requires a fair amount of time to be successful. In fact, it may take a few years in a large company before everyone is trained and using the new methodology. Of course, no organization can stand still while a long culture-change initiative is going on. The problem is that it is hard to tell how much impact the project management initiative has on the organization versus the other factors that are coming into play at the same time. For instance, over a couple years, you find that new tools are being introduced, human resource changes are occurring, reorganizations happen and other culture change initiatives are vying for focus. If it appears that projects are being managed more successfully, it is hard to isolate what factors are coming into play to drive that outcome.
You do not have a basic and consistent unit of work
The fact that projects are unique would not be a problem if we could factor them down into some basic units of work. In other words, if we could say that one project without project management required 25 units of work, and a second project with project management required 40 units of work, we could easily determine the effort and cost per one unit of work and then compare these numbers. However, projects, unlike products, don't tend to have these common defining characteristics.
Things may be a little worse before they get better
Lastly, the introduction of structured processes in an organization that did not have them before might well result in some short-term incremental costs before the long-term value comes in. For instance, you may need to invest in training and perhaps some ongoing coaching. In addition, when a project team uses an unfamiliar process for the first time, there will probably be a learning curve. If the project is long enough, the long-term savings could outweigh the learning curve. However, if the project is short, the participants may tell you that it took longer than it would have if the methodology were not in place. This is not surprising. The overall value of the project management processes must be measured over time, since much of the value will kick in through the reuse of the common processes.
All that being said, there are some ways to show the value of project management. Look for these ideas next week. (Same bat-time, same bat-channel...)