Purchasing parts is a useful strategy to reduce the dependence on internal resources while allowing a manufacturer to focus on their core business strengths. But any part purchased outside, or produced internally carries some element of risk. The ranking system below provides a quick way to apply a score to each part in the design. Obviously the scoring should focus on special parts in the design. It would be wise to ignore commodity parts such as surface mount discrete components, nuts/bolts/washers, generic raw materials, etc.
These rules are often learned from horror stories about part identification and purchasing. Many students have ordered a standard item such as an integrated circuit, motor, or gear set, only to find that these are out of stock or back ordered. Even items that are available can be lost in shipping. Rare items are a terrible part to include in any design.