Are chance and probability the same thing?

No!

It is common to use probability and odds as synonyms mistakenly. These terms are widely used in scientific articles, and we must not confuse them!

Probability refers to the occurrence of an event out of all the possibilities of its occurrence. For example, the probability of getting “heads” in a “heads or tails” game is ½ (50%). The probability ranges from 0 to 1 and can be expressed as a percentage.

Risk and probability are equivalent. Some terms used in surgical articles are risk measures, such as postoperative mortality, rate of postoperative complications, and risk of anastomotic dehiscence.

Thus, we can say that "The postoperative mortality of esophagectomy is 7%" (risk of death: 0.07).

Odds refer to the occurrence of an event versus the non-occurrence of the event. For example, the chance of "heads" falling is "1 to 1" (one for "heads", one for "tails"). Thus, the chance is 1:1 or simply “1”.

It is a measure widely used in case-control articles and is usually expressed as an odds ratio (OR).

Thus, we can say that "For every seven deaths in the postoperative period of esophagectomy, 93 remain alive" (odd: 7/93 = 0.075)

As a curiosity, for rare events, odd and probability are very close, and we can take them as equivalent.

Ex.:

Risk of death in cholecystectomy: 1/1000 = 0.001

Chance of death in cholecystectomy: 1/999 ≈ 0.001

This text was written by:

Francisco Tustumi,

FMUSP Digestive System Surgeon

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