Crafting Great Business Definitions: Definitions vs. Rules

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Click to learn more about author Ronald G. Ross.

In this fourth and final post of a four-part series about crafting great business definitions, I will explain the difference between definitions and rules, and how each best serves its own particular purpose. Read Part 1, Part 2, and Part 3 here.

Definitional Rules

Definitions are fundamentally about the proper classification of things. Rules can also have a fundamental bearing on the classification of things. Especially, something is an instance of a concept only if it conforms both to the concept’s definition and to all relevant definitional rules.

Let’s take an example, cost.

  • Cost: a price to be paid

Suppose a car rental company has a special kind of cost designated as a drop-off fee.

  • Drop-off fee: the cost for returning a rental car to a location other than where the rental car was picked up

In addition, suppose the company has a rule that every drop-off fee must be greater than $9.99. You can express that criterion in either of the following ways. No matter which you choose, a cost less than or equal to $9.99 (e.g., $5) cannot be considered (classified as) a drop-off fee.

1. Include the criterion in the definition.

  • Drop-off fee: The cost not less than $9.99 for returning a rental car to a location other than where the rental car was picked up.

2. Specify a separate definitional rule.

  • Definitional rule: Every drop-off fee is greater than $9.99.

In the latter case, the definitional rule supplements the definition, providing additional criteria for classification. For that reason, definitions and definitional rules are both considered definitional criteria.

Specifying the $9.99 criterion as a separate rule is the better solution for the following reasons:

  • It results in a shorter definition, which is easier for humans to absorb.
  • The criterion is clearly secondary to the essence of the concept. The threshold $9.99 is nothing, for example, like a tricycle having to have three wheels.
  • The amount charged as a drop-off fee could actually involve many rules. For example, the drop-off fee might depend on whether the rental is local to an area, occurs at an airport, etc. Will you include all these criteria in the definition of drop-off fee? Not practical.
  • The greater than $9.99 criterion could easily change. Tomorrow it might be $99.99 or $.99 — or the threshold might be discontinued altogether. Best not to include things in definitions that have a substantial chance of changing.

In general, definitions should be immutable. Core essence is not to be messed with! Aspects of things that might change should be externalized as definitional rules. You have two goals for your total set of definitional criteria:

1. At any given point in time: Support the proper classification of things.

2. Over time: Serve as a stable blueprint for business knowledge and formal business communication.

Determining when something actually is a something (classification) can often prove complex, moving communication of knowledge toward computational needs. For the sake of human consumption, don’t make your definitions do the heavy lifting.

Definitions for Concepts Whose Instances Are Derived or Computed

When instances of a concept are computed or derived based on some authoritative rule(s), do you still need a definition? Judgment call.

For example, drop-off fees for rentals might be computed or derived by one or more explicit rules. If the rule(s) is(are) rather simple and easily digested, perhaps you don’t need a definition. In this case, however, any such rule(s) probably wouldn’t be as clear about the essence of the concept as the definition above. So, the definition is useful. In other words, when in doubt, define!

Behavioral Rules Are Not Definitional Criteria

Consider this example of a rule pertaining to the concept umpire.

  • Rule: An umpire for a game must be attired in black.

Treated as a definitional criterion, this rule expresses that to be considered (classified as) an umpire for a game, you must wear black. If you do not wear black, you cannot be considered an umpire.

That’s just silly. An umpire would still be an umpire even if they didn’t wear black. They might be in violation of the rule and face stiff league penalties, but that wouldn’t make them not an umpire (except maybe after the fact, as a sanction!).

Such rules are called behavioral rules. They govern the behavior of people and organizations (including their automated systems) and play no part in defining things. Don’t include behavioral rules among your definitional criteria. If you can break some rules, it’s simply not about defining things!

Extracted from Business Knowledge Blueprints: Enabling Your Data to Speak the Language of the Business, by Ronald G. Ross, 2020.

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