Types of Processes (by Don Reinertsen)


Note: This was originally published on the Net Objectives blog on October 6, 2009.


In May 2009, there were some discussions on the Kanban Dev group about types of processes. The dialog that resulted is an important part of the Lean thought process. It is helpful to reproduce it here. The discussion on the thread about quality and pressure helped me to thinking more deeply on what I consider an important issue: Can effective software development be defined.

I had troubles formulating answers to people’s questions, but I knew there was more there. I decided to talk about this with Don Reinertsen, the author of two critical books that are foundations for Lean-Thinking.


The latter book is astounding book but read the former book first.

A conversation with Don

Here is my conversation with Don: my question and his answers. It is used by permission). Yes, this this is a long post but it is well worth the time to read it.

My question for Don

In The Scrum Papers, Jeff Sutherland and Ken Schwaber say, “In this paper we introduce a development process, Scrum, that treats major portions of systems development as a controlled black box.” The basis for this is “Concepts from industrial process control are applied to the field of systems development in this paper. Industrial process control defines processes as either `theoretical’ (fully defined) or `empirical’ (black box). When a black box process is treated as a fully defined process, unpredictable results occur.”

I had always interpreted this to mean that you need feedback in order to control a software development process. I agree with this. However, it actually says the process is a black box process – meaning one learns about it through observation.

We believe there are two different orthogonal issues. The first is the one Jeff and Ken mention – defined or empirical. The second, distinct issue is whether a process is predictable or requires feedback to control. These are not the same thing. Defined or empirical relates to whether there is a model underneath the system that one can learn. Predictable Vs Requiring Feedback means can you predict the outcome of the system. …..

Don’s answer

Hi Alan,

By coincidence I have some experience with very advanced industrial process control systems, having spent many years operating nuclear reactors in the Navy. In my experience, such systems do not tidily divide into the categories of fully-determined and empirical. Nevertheless, it is not surprising that software gurus, who have “either/or” hard-coded into their DNA, would perceive them in a binary way.

Let’s start by cleaning up the terminology. We can view the output of a process as deterministic or stochastic. In a deterministic process the outputs are 100 percent determined by the inputs. In a stochastic process the output is a random variable—it has different values that occur with different probabilities.

Fully-determined systems do not exist, except in academia and thought experiments. All industrial process control systems have stochastic outputs. They are partially determined. We use imperfect models to determine how to vary process inputs to control certain important measures of output within a useful control range. We embed these models in control strategies; for example, we might use a PID controller to weight the current level of parameters, their time derivatives, and their integrals to generate a control signal. We optimize our control strategy to balance the cost of control with the benefits of control. The tightness of the control band is simply an economic choice. We do not control for the sake of control. We do not believe that the lowest possible variance is the most desirable operating point.

It is useful to make a distinction between whether a process is fully-determined and whether its output is fully-determined. Although many people have a tendency to assume a defined process will produce a deterministic output, this is not always true—a precisely defined process can still produce an output that is random. For example, the process for obtaining and summing the results of flips of a fair coin may be precisely defined, while its output is a random variable.

Well-defined systems can produce outputs that range on a continuum from deterministic to purely stochastic. Just as we can structure a financial portfolio to change the variance in its future value—by ranging from all cash to all equity—we can make design choices that affect amount of the variance in a systems output.

I believe that thinking of system output as a random variable may be more useful than labeling it as either unpredictable or predictable. We could think of the output of a system as completely unpredictable, macroscopically predictable, or microscopically predictable. It is unclear if anything falls into the first category—even a random number generator will produce uniformly distributed random numbers. It is the zone of what I would call “macroscopic” and “microscopic” predictability is most interesting.

I can describe the distinction using the coin-tossing analogy. When I toss a fair coin 1000 times, I cannot predict whether the outcome of the next coin toss will be a head or tail—I would call these individual outcomes “microscopically unpredictable.” There may be other microscopic outcomes that are fully determined since I have a fully-defined process. For example, I could define this process such that there is a zero percent chance that the coin will land on its edge and remain upright. (If coin land on its edge, then re-toss the coin.)

Even when the outcome of an individual trial is “microscopically unpredictable,” it is still a random variable. As such, it may have “macroscopic” or bulk properties that are highly predictable. For example, we can forecast the mean number of heads and its variance with great precision. Thus, just because the output of a process is stochastic, and described by a random variable, does not mean that it is “unpredictable.” This is a rather important point because the derived random variables describing the “bulk properties” of a system are typically the most practical way to control a stochastic process.

If we conclude it is economically advantageous to make a system output variable more deterministic, then we can do this with or without feedback. For example, I can achieve good frequency response and low distortion on an audio amplifier either by selecting robust components, or by using feedback. We only use feedback when this is the most cost-effective way to achieve our goal. Using feedback is a technical choice, it is not “required” to deal with variation.

Thus, I agree with you that in that I believe that some of these apparent clusters can obstruct clear thinking. I do not believe that there are two dyads: fully-determined, use no feedback; empirical, use feedback.

Furthermore, I do not believe that there are two triads: defined process, fully-determined output, use no feedback; and undefined process, unpredictable output, use feedback. I consider it more useful to treat these attributes as three distinct dimensions:

  1.  The degree of process definition.
  2.  The randomness of its output.
  3.  The amount of feedback that the process uses.

Of course, if we try to describe the space spanned by these three dimensions we would find certain zone with very low occupancy. For example, we wouldn’t find people use lots of feedback in well-defined systems that are already producing highly predictable output without feedback.

I hope this reinforces your intuition on this issue.

Best regards,

Don Reinertsen

I totally agree with this answer. Don beautifully described what I was intuiting but not able to write myself. I believe this is at the heart of the difference between Scrum and Lean. Lean attends to the theory of flow and therefore provides the ability to create hypotheses about what will happen in the future. While we still, of course, have to validate these hypotheses with experiments, this can have us be more forward looking to anticipate challenges and not have to totally rely on past looking retrospectives.


Read the discussion between Al and Don on the Net Objectives Portal. Go to https://netobjectivesthoughts.com/types-of-processes-by-don-reinertsen.

This is Al Shalloway. Visit us at www.netobjectives.com.


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