System Dynamics of Diaper Delivery

The impetus for my first personal, practical application of pure system dynamics modeling came from my sleep-deprived thoughts about Baby Diaper Service, which works as follows:

On your first and second deliveries you receive the full number of diapers you order. Thereafter, you will be on a rotation system. The number of diapers you turn in one week will be returned to you the next, up to your maximum order.

Simple? Sure, but complex enough to have two hallmarks of SDM: feedback and delay.

  • Feedback: The number of diapers delivered depends on the number I turn in.
  • Delay: The number of diapers I get this week is the number I turned in last week.


SDM, as developed in the early days of computer research, focused on “what if?” analysis. Not predicting the future that will be, but generating some range of ideas of the futures that could be.

This led to a very different sort of science that the contemporaneously developing field of econometrics. This divide is captured in a dialog I enjoyed in The Electronic Oracle (if I recall correctly, I don’t have it nearby).

However, the futures that could be are just what I was concerned about in those sleepless early months. I kept worrying about a dirty diaper catastrophe, what if we forget to put dirty diapers out this week?

SDM can answer:

If I can make it through one tough week, I’ll earn a free pass for when I forget again.

Needless to say, I was greatly relieved to have the results of this simulation in my back pocket. Thank you, System Dynamics Modeling!

Epilogue: eventually I did miss one week and failed to turn in diapers to the delivery service. I was all ready to tough it out for my reward. But it turns out that you just call up the service and get them to come by for a special pickup for a small fee. Thank you, Baby Diaper Service!

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