88.2 % of all statistics are made up on the spot
– Victor Reeves
There’s a growing movement in medicine in general and imaging in particular which wishes to attach a number to everything.
It no longer suffices to say: “you’re at moderate risk for pulmonary embolism (PE).”
We must quantify our qualification.
Either by an interval. “Your chances of PE are between 15 and 45 %.”
Or, preferably, a point estimate. “You have a 15 % chance of PE.”
If we can throw a decimal point, even better. “You have a 15.2 % chance of PE.”
The rationale is that numbers empower patients to make a more informed choice, optimizing patient-centered medicine and improving outcomes.
Sounds reasonable enough. Although I find it difficult to believe that patients will have this conversation with their physicians.
“Thank god doctor my risk of PE is 15.1 % not 15.2 %. Otherwise I’d be in real trouble.”
What’s the allure of precision? Let’s understand certain terms: risk and uncertainty; prediction and prophesy.
By certainty I mean one hundred percent certainty. Opposite of certainty is uncertainty. Frank Knight, the economist, divided uncertainty to Knightian risk and Knightian uncertainty (1).
What’s Knightian risk?
If you toss a double-headed coin you’re certain of heads. If you toss a coin with head on one and tail on the other side, chance of a head is 50 %, assuming it’s a fair coin toss. Although you don’t know for certain that the toss will yield head or tail, you do know for certain that the chance of a head is 50 %. This can be verified by multiple tosses.
