Design Levels and Required Artifacts
Decision Levels and Required Artifacts
Artifacts are a fundamental part of the design process. They play a key role in documenting decision decisions. Kelly Johnson of the Lockheed Skunk Works (which was responsible for creating the U2, the SR-71, and F117) had 14 rules to allow flexibility and rapid progress in the Skunk Works (Skunk Works Rules). Two of these rules that are related to artifacts are “A very simple drawing and drawing release system with great flexibility for making changes must be provided”, and “There must be a minimum number of reports required, but important work must be documented thoroughly.” As you consider what artifacts are necessary, we believe it is helpful to think of the decision you are making and how it relates to the needed artifacts.
| Decision levels and their required artifacts. | |||
|---|---|---|---|
| All decisions must be recorded in design artifacts, but critical decisions should also have justification and analysis artifacts. | |||
| Decision Type | Capture Decision | Justify Decision | Predict Effects of Decision |
| Critical (must be optimized) | X | X | X |
| Major (dramatically affects performance, but optimization not required) | X | X | |
| Mundane | X |
Critical Decisions
The first level is critical decisions. These decisions greatly affect the performance of the product and require optimization. This means that the decision needs to be just right – there can neither be too much nor too little. An example of a critical decision might be the wing spar for an airliner. If the spar is too weak, the airliner will be unsafe. If the spar is too strong, the airliner will be too heavy and have poor performance. So the spar must be just right, which is decided by careful analysis to find the optimum. Likewise, choosing the hardware platform for an embedded IoT device or system is a critical decision. It will significantly affect the cost, reliability, possible features, capabilities, and performance of the completed product.Major Decisions
The second level of decisions is major decisions. A major decision greatly affects the performance of the product, but optimization is generally not required to make an appropriate decision. We might consider the foam in an airliner's seat to be a major decision. The comfort of the seat will greatly affect the passenger experience, but most people would not consider optimization required.Mundane Decisions
The third level of decisions is mundane decisions. Mundane decisions don't really have to be right; they just have to not be wrong. As long as they are not wrong, they will have relatively little effect on the final product. An example of a mundane decision might be the rivets on an airliner. The rivets will be selected according to a standard that defines acceptable rivets for a given condition. Specific stresses expected for each rivet on the skin of the airplane will not be calculated, and an optimal rivet used for each hole. Instead, the standard rivet will be used everywhere, meaning that some rivets will be larger than necessary (although none will be smaller than necessary).Artifacts Required for Each Design Decision Level
Depending on the level of decision, different artifacts will be necessary. All decisions need artifacts recording what decisions have been made. The artifacts recording what the decisions are will form the core of the design package that is released to the production system. Typical artifacts that capture what the decisions are include a bill of materials, engineering drawings, component specifications, and software release notes, although other artifacts may also be used.For critical and major decisions, artifacts that justify the decision that was made are generally required. These artifacts often include test reports, engineering analysis reports, and decision reports that show how the decision meets the design requirements.
For critical decisions, a detailed mathematical analysis is often required that shows how the given decision is optimal. Significant resources are often used in making optimal decisions, because the decision is critical.
Most of the decisions in any given design are mundane. A rule of thumb is that approximately 80% of the decisions will be mundane. Roughly 15% will be major decisions, and about 5% will be critical decisions. It is important to make sure that the artifacts match the level of the decision. It is wrong to make an optimization report on a mundane decision; it is also wrong to avoid an optimization report on a critical decision.