Many analysts begin with the top 5 or 10 commodities by spend volume. The thinking is that because these commodities have the most spend associated with them, they have the greatest potential for savings. While this is true in theory, it's also true that, because of their size and importance, these spend categories are the most closely managed.
Another avenue to take is to first look at commodities that have little differentiation and a lot of competition, such as office supplies or printing. These commodities are good candidates for consolidation, which results in higher volumes and bigger discounts. Often, a spend analysis initiative is triggered by an event, such as a merger or acquisition. The starting point in these situations will be the consolidation of the two different commodity structures, which should uncover numerous opportunities for consolidation of vendors and spend.