A conversation on the LinkedIn ’Decision Model and Notation (DMN) Group: Sub-decisions, are they really?

Mark Norton  19 March 2015 01:15:10 PM

Paul Konnersman Diagnostician and Designer, organizational work processes

[Opening statement]

I see repeated references to "sub-decisions" which strike me as misleading. Doesn't, "sub-x" imply a subordinate echelon of a hierarchy in which "X" is superior? While I understand the attractiveness of hierarchy for modeling and understanding, I don't think it is an accurate characterization of the relationship between decisions that are being termed "sub-decisions." It is deceptive because while the simplest examples that we use could be seen as hierarchal, a decision which is a sub-decison of two or more other decisions would be participating in two or more hierarchies. While I suppose this is possible It seems more appropriate to see what have been called "sub-decisions" as decisions that are simply logically, and therefore temporally, prior to the decision to which they are alleged subordinate. Therefore, I would refer to them as prior, or earlier decisions rather than sub-decisions

[other parties debate the meaning of decision, sub decision, and context]

Mark Norton CEO and Founder, Idiom Ltd

This thread appears to be raising questions about what a decision is or is not (what is a sub-decision?), and about the relationship between decisions (is the relationship governed by context or by sequence or by both?). Is this correct? If so, what does this thread imply for the DMN standard itself?

Paul Konnersman Diagnostician and Designer, organizational work processes

Mark,
I speak only for myself, but I think it is clear that a decision is simply a choice between two or more alternatives. I hope that is not in dispute. I raised a minor concern about the use of the term “sub-decision” to refer to a decision (say “A”) that was required by another decision (say “B”). By extension I also dislike using the term “decision decomposition” to describe the process of eliciting so-called “sub-decisions.”

My reasons in both cases are that this terminology suggests a compositional hierarchy in which decision “B” is an aggregation of one or more other decisions. It seems to me that this is a misleading and unhelpful representations because the relationship between the various decisions is one of logical priority rather than aggregation. They don’t decompose, they are arrayed in time; they are sequenced, like it or not. I do understand that this is very much disputed and that I may be a minority of one on this point. Paul V. has sent me off to learn “goal-directed reasoning and the like,” which are alleged to be outside of any domain of logic with which I am familiar. I’m working on it and will report back. Any suggestions of tutorials will be appreciated.

I don’t think I understand the use of the term “context” as it’s being used here. Of course the a decision is being called a “sub-decision” in the context of a related decision which requires that “sub-decision.” But that just restates the usage that I’m questioning. I would say that the relationship is not governed by either the context nor the sequence, but rather that the sequence is governed by the relationship which is one of logical priority—If the task is to determine the quantity of carpeting required, we cannot make the area decision, without first making both the length and width decisions; and we can’t make either the length or the width decision without first making the rooms to be carpeted decision.

As for what this thread implies for the DMN standard itself, I’m not sure that it implies anything, but I’m not the person to ask. I believe that the descriptions we use, including notations, are of the greatest importance. But I’m afraid OMG went off into the deep woods a long time ago. How many pages of notations do we need? …how many decorative icons?

I offer as food for thought, the following words of Nobel laureate and polymath Herb Simon:

“How complex or simple a structure is depends critically upon the way in which we describe it. Most of the complex structures found in the world are enormously redundant and we can use this redundancy to simplify their description. But to use it, to achieve the simplification, we must find the right representation.”– H. A. Simon, “The Sciences of the Artificial,” 2nd ed. (Cambridge, MA: MIT Press:, 1981), p. 215.

Mark Norton CEO and Founder, Idiom Ltd

Hi Paul,
Your reference to Herb Simon is very appropriate. And it begs the question: Do we (and perhaps does DMN) have the right representation?

Your definition of decision as ‘simply a choice’ might not be in dispute within DMN, but it is quite limiting nonetheless. This definition presumes that a finite set of alternatives is already known to the decision making process, so that determination of a price or any other continuous (i.e. infinite) variable would seem to be outside of the scope of DMN. Similarly so with transformations, where there is no choice as such. Both calculations and transformations are important to any decisioning process and in our view are peers with ‘choice’ or ‘constraints’ as decisioning concepts. It is our experience that most real-world decisioning problems require transformations, calculations, and choices to achieve the ultimate decision outcome. The need for transformation as a core part of the decisioning repertoire is discussed in this paper on Modern Analyst [http://bit.ly/1B1XyrU] (and this on the Business Rules Journal [http://bit.ly/1BWSREo])

Regarding your second para, perhaps the concepts of sequence and context are not as much in conflict as you seem to suggest. If, in order to determine decision A, I need to derive decisions B, C, and D, and if I only need B, C, and D to derive question A, then the sequence issue is clear. However, this is also a decomposition: B, C, and D are all inherent and indivisible parts of A, as in ‘what decisions do I need to make in order to decide A’. So a hierarchical structure of decisions could be defined by its dependency on both context and sequence simultaneously.

Context is an interesting issue, which has been recently discussed in this article on Modern Analyst [http://bit.ly/19ACJ0W], and this on the ‘Business Rules Journal’ [http://bit.ly/1AGXxdD]. Context frames the data and the decisions, and we believe is a necessary part of any decision definition. My definition of ‘context’ is that it is all those characteristics of the actual ‘instance transaction’ that define the boundaries and execution pathways for both the data acquisition, and for the decision-making that uses that data, for that transaction execution.

For instance, in our example above, decision B might only be required when determining A if the customer is in Europe, because B implements an EU policy. Note that in this case, context is not simply the key of the data; the location of the customer is not part of its key, but it is part of the context for determining decision B, and by extension, A.
Another almost universal example of context is effective dating. If the effective date of the transaction is 25th December 2014, then the data must be selected according to this constraint, and all decisions must execute as at that date, even if the transaction is executing today.

Thanks for bringing up the issue of context in particular. I am sure there is a lot more to come on this topic – we appreciate the conversation, no matter where it leads.

Paul Konnersman Diagnostician and Designer, organizational work processes

Mark N.,

Thanks for the detailed critique and references.

I intended to raise the the question with my Simon quote rather than begging it. I do indeed think that BPMN has adopted a very poor representation which is why it keeps expanding notations seemingly without limit and that DMN is problematic in having attached itself to BPMN.

My definition of “decision” does not presume a finite set of alternatives.” The phrase “two or more” encompasses infinite sets, both denumerable and non-denumerable. Actually, I see now that I can make it more elegant by saying simply that a decision is a choice from among alternatives,” since zero and one do not provide alternatives.

I’ve not yet had time to read the four papers you cite, but aren’t transformations and calculations ways of automating a decision — that is, of specifying how a choice is to be made from among alternatives?

I don’t see “context” as being in conflict with sequence. I simply don’t see why the reference to context is necessary or what it implies beyond what is already there.

I would say that to the contrary, B, C and D are patently divisible. Every so-called sub-decision can be separately identified and named. Each can be separately considered and made without reference to, or even knowledge of decision A. I acknowledge that some simple decisions including most of those used as examples can be seen a hierarchal without problem. However, more complex networks (known mathematically as directed acyclic graphs) that cannot be viewed as hierarchies and as decompositions. They are better regarded as be associated by relations of logical precedence than by aggregation. I am saying that so-called sub-decisions are not components of the decisions that depend upon them, they are logical (and therefore temporal) predecessors of the decisions that depend upon them.

I have no problem with dependency relationships being contingent if that is what you mean by “context,” but I think that it is far less clear to invoke “context” than to simply indicating the contingency.

Finis

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