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Introduction:

Logical reasoning is a form of thinking in which premises and relations between premises are used to infer conclusions that are caused (or implied) by the premises. Fallacies are mistakes in logical reasoning that can lead to flawed conclusions. One such example is the ‘affirming the consequent’ fallacy.  

Author: David Brown, Quantum Expert, UK

The ‘affirming the consequent’ fallacy can occur within expert testimony when a subject matter expert erroneously concludes that a statement is true simply because its consequence is true. I explore the particulars of this logical error with a specific focus on the implications in expert opinion. I also draw upon my experience of proffering opinion evidence in formal dispute resolution proceedings and cite examples of fallacies in logical thinking that I have encountered.

Understanding affirming the consequent fallacy:

The ‘affirming the consequent’ fallacy is a common misconception that is categorised as a deductive reasoning error. The reasoning is flawed, which often leads to invalid conclusions, for example:

Example one:

1. If it rains, my hair gets wet (the premise).
2. Now, my hair is wet.
3. Therefore, it must be raining.

This fallacious reasoning becomes apparent when we realise that there could be multiple reasons why my hair is wet, and asserting that it must be raining is an oversimplification.

Examples of fallacious reasoning in opinion evidence

Another example of this fallacy relevant to construction and engineering might be:

Example two:

1. If there is inadequate project management, then cost overruns will occur.
2. Cost overruns have occurred.
3. Therefore, inadequate project management was in place.

In the above example, concluding that inadequate project management occurred based solely on the observation of a cost overrun is an oversimplification of the situation. Cost overruns on construction projects can occur, and often do, for a myriad of reasons.

Affirming the consequence in this way, by asserting that inadequate project management must have caused cost overruns, overlooks alternative explanations and results in an inaccurate opinion.

Consider a further example of an expert witness proffering opinion related to cost overruns:

Example three:

1. If the Quantity Surveyor provides an under-assessed cost plan, then cost overruns will occur.
2. Cost overruns have occurred.
3. Therefore, an under-assessed cost plan was provided.

Where a construction project has suffered cost overruns, several contributory causes can be to blame; examples may include unforeseen ground conditions, changes to project specifications and design, or external macroeconomic factors. A subject matter expert witness must avoid the temptation to affirm the consequent in this manner and must consider alternative explanations.

Conclusion:

Recognising and understanding the ‘affirming the consequent’ fallacy is crucial for any subject matter expert offering opinion evidence, particularly in formal resolution proceedings. Experts providing opinions on cost-related  matters must be cautious not to succumb to this fallacy, as it will likely result in invalid conclusions and compromise the integrity of the expert opinion evidence.

More broadly, by understanding its structure and recognising instances where this fallacy may occur, construction professionals can improve critical thinking skills and become more open to consideration of alternative reasons why projects have become constrained. As the construction and engineering industry grapples with the complexities of disputes, being vigilant against logical fallacies can help avoid incorrect and often costly conclusions.

This article was originally written for issue 29 of the Diales Digest. You can view the publication here: https://www.diales.com/diales-digest-issue-29