Saturday, September 4, 2010

Standing waves

Is everyone familiar with the "standing wave ratio", SWR?

If you are a little vague on this, first think about the incident [or driving] energy that is directed toward an objective. If the objective absorbs all the effort directed toward it, there is no reflected or unused effort. But when there is unused effort then interference is felt between the effort directed toward the objective and effort reflected from it or not used.

Interference causes waves form; in the project context interference waves cause periodic delays alternating with progress. The degree, or amplitude of the interference is felt by the duration of the delays and progress.  These waves are called 'standing waves', and the SWR is simply the ratio of the energy directed to the objective to the energy unused and reflected back.

The traffic model
We've all experienced standing waves. Just drive in traffic. Inexplicably the traffic stops or slows, then speeds up, only to slow or stop again.  Why?!

These are standing waves of progress and delay. The objective [the road ahead] can't absorb all the incident effort [traffic], so some effort is reflected that interferes with oncoming traffic.

Typically, traffic waves occur--and flow is disturbed--when the road changes character, usually because of a discontinuity: a traffic accident, a slow driver, a change in the number of lanes, or a traffic light. Remove the discontinuity, and the standing waves decrease, flow improves, and effort is used more effectively. Friction [non value add effort, in this case standing in traffic burning fuel and consuming time without result] is reduced. Thus, during rush hour, you may notice the traffic lights tend to be retimed to reduce the SWR of the traffic and improve flow.

The project effects
Now, what about projects? Don't project discontinuities produce a project SWR? They do indeed. Project processes are like the highway, conveying effort to the objective. But processes are embedded in the project network schedule. If there are sequencing, resource, or task discontinuities then there will be waves of delay and progress.

So, what are we talking about when we say "discontinuities"?  Examples:
  • Successor tasks mismatched to predecessor tasks; the successor resources can't absorb all the predecessor results, thereby creating a constraint and changing flow
  • Incorrect sequencing that disturbs timing
  • Parallel paths joining; two or more teams now feed into one or more successor teams, disturbing flow
  • Work-flow decision loops that are out of sync or not paced to maintain flow in project processes
  • Batch schedules rather than real-time
  • Lead and lag effects in schedules that change the coupling between the project clock and the wall clock
  • Idle staff...marching army effects...who distract engaged staff
So, in one sense, the project SWR is the ratio of the applied effort to the effective effort. Or, in more familar words: the ratio of the total effort to the value add effort.

Think about this the next time you are in traffic.

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  1. John,
    As a physics guy - I actually practiced this profession at one time - the "standing waves" for traffic are called "backward Waves." The the phase velocity is opposite to the energy velocity. The cars are still moving forward, but the phase of the collection of cars is moving backwards.

    Water surface waves exhibit this as well, when the depth of the water is much less than the wave length, the phase velocity is then v=√(gh) and is independent of the wave length and the propagation is non-dispersive. This non-dispersive nature would be very troubling in projects, since it means the disruption propagates all the way to the end - you can never make it up.

    Reference from my book shelf (always keep your grad school books, they may come in handy someday, like today).

    S. Ramo, J. R. Whinnery and T. Van Duzer, Fields and Waves in Communication Electronics (New York: John Wiley & Sons, 1965). pp. 51-53.

    The Ramo and Whinnery are Simon Ramo and Thompson of Thompson, Ramo, and Whinnery of TRW!

    Never thought of the project delays in that way. In water waves that's a good thing, surfers like non-dispersive waves, they hold their shape all the way to the beach where they crash.

  2. As a sailor, I am always aware of the the standing wave along the hull. When the period of the wave is equal to the water line length, the boat is at hull speed and more energy will not move it faster in displacement mode. In the case of sail boat, the boat heels more as energy is applied, thereby changing the waterline length, but after a point sailing on your side is not faster.


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