I have received a number of comments regarding the Simplified Excel Model for market adoption published a few months ago. Reader Vince asked how to extend the math behind it to comprehend effects like cross-segment interactions.
There is no simple answer, and this post is an attempt to point readers to ways to think about what they want to model, as well as giving helpful resources for further study
In my opinion, one of the best approaches to understand market adoption is through system dynamics. One of the advantages of the methodology is that it allows you to conceptually link business effects and relationships to the equations. I touched on this issue on on a previous entry, and here I will try to explain further.
The logistic equation (shown below) is a commonly used way to model market adoption.
From a System Dynamics perspective, the logistic model can be explained looking at the following model (click for full size): The boxes, called “stocks” in SD terminology, represent an accumulated quantity over time. One way to think of stocks is a bathtub. The amount of water in the tub is the accumulation over time of how much water you added through the faucets, less how much water you let out through the drain.
On the model, there are two stocks: how many potential adopters are out there (left side) and how many adopters are (right side). The pipe that connects the boxes is called a “flow”, and it shows a valve, whose value represents how fast potential adopters turn into actual adopters (thus we call it Adoption Rate). Again, in the bath tub analogy, we can think of the value of the flow as how open or closed the faucet is.
Adoption rate depends on how big the population is (the larger the population, the larger the adoption rate), how much the adopters interact with potential adopters (creating the “word of mouth” benefits), etc.
As stocks are accumulations of whatever flows in minus what flows out, from a mathematical perspective, the value of a stock is calculated integrating over time the values of the net flow. On the logistic model, the arrow that links the stock and the adoption rate flow means that the flow changes proportionally to the stock – i.e. if I have more potential adopters, there are more possibilities for contagion, when a user talks favorably to a potential user about the product. The net result is an exponential behavior, which, after some mathematical reduction, is represented by the formula above.
If I want to explain a business audience some market adoption dynamic, it possible to do it talking in terms of stocks and flows (once the audience is comfortable with these terms). It’s almost a guaranteed failure if I try to explain it by using a mathematical formula with exponentials and integrals 
The Bass model addresses one limitation of the simple logistic model, regarding how the system “gets started”: with no adopters, there is no chance for interactions, so there is no inflow to the adopters stock. It does it through the use of an external force, like advertising.
Below is a Systems Dynamics interpretation of the Bass model. As you can see, the only difference is that now the Adoption Rate is the addition of two elements, adoption rate from advertising and adoption rate from word of mouth. The latter is exactly the same as the AR in the logistic model.
Returning to Reader Vince’s specific question on how to extend the logistic or Bass models to comprehend effects like cross-segment interactions, I would frame it like this:
- Identify the most important cross-segment interactions – How much “cross-shopping” exists between the segments? (using data like second choice selection); are there characteristics of the upper segment that consumers will translate into the lower segment favorably/unfavorably? consumers replace their vehicles within segment or they try to go up segment? etc.
- Incorporate the key cross-segment interactions on the model – They will most likely affect the Adoption Rate. It also may be necessary to model another stock or stocks (Upper Segment Adopters and Lower Segment Adopters, for instance)
- Check sensitivity of cross-segment assumptions – Understand how different the results are when the cross-segment assumptions are considered versus when they are not. What are the assumptions that most impact the results? A tornado diagram, as discussed in a previous entry, may provide a good way to show the sensitivity to the assumptions
As more dynamic effects are considered for inclusion in a model, it is better to move from a tool like Excel to something like Vensim, or iThink. Chapter 9 of John Sterman’s excellent book “Business Dynamics” talks about both the logistic and Bass models as shown here, and expands on ideas on how to extend them.
Here are some other very good references on the topic
- Forrester, J. W. 1980. Information Sources for Modeling the National
Economy. Journal of the American Statistical Association 75 (371):
555-574.
Argues that modeling the dynamics of firms, industries, or the economy requires use of multiple data sources, not just numerical data and statistical techniques. Stresses the role of the mental and descriptive data base; emphasizes the need for first-hand field study of decision making.
- Legasto, A. A., Jr., J. W. Forrester & J. M. Lyneis, eds. 1980. System Dynamics. TIMS Studies in the Management Sciences. Vol. 14. Amsterdam:
North-Holland.
Collection of papers focused on methodology. Includes Forrester and Senge on Tests for Building Confidence in System Dynamics Models and Gardiner & Ford’s discussion on Which Policy Run is Best, and Who Says So?
- Randers, J., ed. 1980. Elements of the System Dynamics Method.
Cambridge MA: Productivity Press. Includes Mass on Stock and Flow Variables and the Dynamics of Supply and Demand; Mass & Senge on Alternative Tests for Selecting Model Variables; and Randers’ very useful Guidelines for Model Conceptualization.
- Richardson, G. P., and A. L. Pugh, III. 1981. Introduction to System Dynamics Modeling with DYNAMO. Cambridge MA: Productivity Press.
Introductory text with excellent treatment of conceptualization,
stocks and flows, formulation, and analysis. A good way to learn the
DYNAMO simulation language as well.
- Morecroft, J. D. W. 1982. A Critical Review of Diagramming Tools for
Conceptualizing Feedback System Models. Dynamica 8 (part 1): 20-29.
- Critiques causal-loop diagrams and proposes subsystem and policy
structure diagrams as superior tools for representing the structure of
decisions in feedback models.
- Roberts, N., D. F. Andersen, R. M. Deal, M. S. Grant, & W. A. Shaffer.
1983. Introduction to Computer Simulation: A System Dynamics Modeling
Approach. Reading MA: Addison-Wesley.
- Easy-to-understand introductory text, complete with exercises.
- Homer, J. B. 1983. Partial-Model Testing As A Validation Tool for
System Dynamics. In International System Dynamics Conference: 920-932
- How model validity can be improved through partial model testing when
data for the full model are lacking.
- Sterman, J. D. 1984. Appropriate Summary Statistics for Evaluating the
Historical Fit of System Dynamics Models. Dynamica 10 (2): 51-66.
- Describes the use of rigorous statistical tools for establishing model
validity. Shows how Theil statistics can be used to assess
goodness-of-fit in dynamic models.
- Forrester, J. W. 1985. ‘The’ Model Versus a Modeling ‘Process’. System
Dynamics Review 1 (1): 133-134.
- The value of a model lies not in its predictive ability alone but
primarily in the learning generated during the modeling process.
- Richardson, G. P. 1986. Problems with Causal-Loop Diagrams. System
Dynamics Review 2 (2 ): 158-170.
- Causal-loop diagrams cannot show stock-and-flow structure explicitly
and can obscure important dynamics. Offers guidelines for proper use
and interpretation of CLDs.
- Forrester, J. W. 1987. Fourteen ‘Obvious Truths’. System Dynamics
Review 3 (2): 156-159.
- The core of the system dynamics paradigm, as seen by the founder of the field.
- Forrester, J. W. 1987. Nonlinearity in High-Order Models of Social
Systems. European Journal of Operational Research 30 (2): 104-109.
- Nonlinearity is pervasive, unavoidable, and essential to the
functioning of natural and human systems. Modeling methods must
embrace nonlinearity to yield realistic and useful models. Linear and
nearly-linear methods are likely to obscure understanding or lead to
erroneous conclusions.
- Barlas, Y. 1989. Multiple Tests for Validation of System Dynamics Type
of Simulation Models. European Journal of Operational Research 42 (1):
59-87.
- Discusses a variety of tests to validate SD models, including
structural and statistical tests.
- Barlas, Y., & S. Carpenter. 1990. Philosophical Roots of Model
Validation: Two Paradigms. System Dynamics Review 6 (2): 148-166.
- Contrasts the system dynamics approach to validity with the
traditional, logical empiricist view of science. Finds that the
relativist philosophy is consistent with SD and discusses the
practical implications for modelers and their critics.
- Wolstenholme, E. F. 1990. System Enquiry – A System Dynamics Approach.
Chichester: John Wiley.
- Describes a research methodology for building a system dynamics
analysis. Emphasizes causal-loop diagramming, mapping of mental
models, and other tools for qualitative system dynamics.
- Mass, N. 1991. Diagnosing Surprise Model Behavior: A Tool For Evolving
Behavioral And Policy Insights (written in 1981). System Dynamics
Review 7 (1): 68-86.
