Conveying Probability

Equivocal Language

If I were to say to you “It might rain this afternoon”, how likely do you think it is that it would rain in percentage terms? 10%? 20%? 70%? In truth, if I were to ask this same question to a group of 20 people, it is likely I would receive 20 different answers.

It is highly likely that your customer's interpretation of the level of probability that "might" represents is different to yours - for instance, they may thing it represents a 20% likelihood, while you may think it refers to a 40% likelihood. If this is the case, the accuracy of any assessments they receive from you that use the term "might" will reduce considerably.

The Probability Yardstick

To avoid the potential confusion that can be caused when using equivocal language, we can use The Probability Yardstick. This is a scale which applies relative value to probabilistic terms.

You will notice there are deliberate buffers of 5% between the percentages of probability assigned to each term. These are there to reduce ambiguity and confusion in the mind of the analyst and customer.

There are no terms which convey absolute certainty. This is deliberate - when judging probability the most concrete assessment you should offer is that something is "almost certain" or "highly unlikely".

If terms from the Probability Yardstick are used in a product, it should contain a tabular representation of it similar to the example above. Neither the terms you use or the relative values you apply to them have to be the same as in this example, but whichever terms and values you use must be clearly indicated in your tabular representation.

Examples of assessments using terms from the Probability Yardstick include:

• ”It is unlikely the incumbent will win the election”;

• ”There is a realistic possibility the enemy will deploy fighter jets”; and

• “It is almost certain the business will make a profit this year”

Avoid Reintroducing Ambiguity!

When conveying probability using specific or judged probability, ensure you use an unequivocally certain modal verb after the term used to convey probability, for example:

“It is highly likely that Boko Haram will attempt to enter the village”

Using an equivocal modal verb after the term used to convey probability risks making the assessment just as ambiguous as it would have been had you not used a specific term or term from the probability yardstick in the first place, for example:

“It is highly likely that Boko Haram might attempt to enter the village”

Further examples include writing:

• “It is likely the business is overvalued” instead of “It is likely the business may be overvalued”;

• “There is a 50% chance the enemy will attempt to cross the river using Bridge X” instead of “There is a 50% chance the enemy might attempt to cross the river using Bridge X”;

• “It is almost certain Suspect X has left the country” instead of “It is almost certain Suspect X could have left the country”.