Robert McNamara was the principle architect of the Viet Nam War effort for the United States, and was principally responsible for the wrong decisions made during the course of that conflict. They would count the number of enemy dead bodies found and other statistics that were over relied on….. ignoring other data that was less quantifiable. In this particular case thinking the number of dead and associating it with the resolve of the Vietnamese fighters, leadership, and the will of the people to support their efforts. This over reliance on quantifiable data is now known throughout the world.
The McNamara Fallacy - Making the Measurable Important rather than Measuring what IS Important
In engineering there is a term known as engineering judgement. Notice first of all that this is engineering judgment, not expert judgement, salesman judgement, homeowner judgement or anyone else. As mentioned before, engineers complete 4-5 years of formal education, culminating in preparation to take the Fundamentals of Engineering exam. Once that exam is passed, a 3 internship under the tutelage of a Licensed Professional Engineer, where all the understanding and book knowledge is then put into practice where that Engineering Judgment is developed before the final PE exam can be administered.
In our previous blogs, we have mentioned how important it is to be objective, fair, and data driven. Once it a while that data does not present clear cut and dried evidence that support a particular conclusion. In those cases engineering judgment is required.
A good example is overreliance on the tilt and deflection as defined by the FPA and PTI. While a good tool, overreliance on it may cause us to miss the bigger picture. A good example is when a deflection occurs near an edge. When this happens the horizontal part of the calculation is overstated, giving a overly low result that could hide a problem that might go un noticed if we had an overreliance on the calculation. See below.
This is an exaggerated example of how the current calculation leads to an overly conservative calculation that under reports the significance of the problem. The horizontal distance is not the real distance that the deflection occurs over, but rather the short distance.
I should add that the FPA/PTI calc is more widely accepted by engineers and most of the time gives a good more consensus adopted gage to the severity of problem. However over reliance on it without engineering judgement could lead to a McNamara kind of mistake.
When this condition occurs, it is appropriate for an engineer to use his/her judgement. There are other cases just like this where engineers must use their judgement to make the best recommendations on behalf of the Homeowner.
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