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A beginner’s guide to probability
  1. Carl Thompson
  1. Department of Health Studies; University of York; York, UK

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Uncertainty is both “irreducible”1 and inescapable in health care: no intervention ever leads with complete certainty to a given clinical outcome, no diagnosis is ever completely established, and no prognosis is ever completely accurate. A nurse will never have all of the reliable and valid clinical information needed to make choices with 100% certainty. Because of this uncertainty, nurses know that when we make judgement calls and decisions, we can only think in terms of the balance of probabilities.

Despite recognising the probabilistic nature of judgement and decision making in health care, most nurses (as well as doctors and patients) prefer to discuss chance events using words rather than numbers. For example, if you listen to surgical nurses talking with patients about their chances of passing flatus in the 24 hours after abdominal surgery, you will hear expressions such as, “It is rare that patients will start to pass wind in the first 24 hours after major abdominal surgery; it is more likely you will begin passing wind again 24–48 hours after surgery.” Likewise, most community nurses are unlikely to be heard telling their patients that “There is a 45–50% chance of your venous leg ulcer healing in 6 months with this compression bandage.” These qualitative expressions of uncertainty, although relatively easy to use, are prone to misunderstanding. When asked to quantify their uncertainty in response to hearing words such as “rare” or “likely,” patients and professionals—even when given access to the same information—often give widely variable estimates of what the phrase means to them.2

Fortunately, for patients and professionals alike, there is an alternative: the language of probability. As well as being the language of uncertainty, it is also the language used in evidence-based decision making. Determining the statistical significance of a research result means knowing what …

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  • A complete version of this Notebook appears as a chapter in the forthcoming text: Thompson CA, Dowding D. Essential clinical decision making for nurses. London: Elsevier Science. To be published summer 2009.