Why the Probability of Hurricane Erin Striking the US East Coast is 100% (or zero)
We could all use a better understanding of forecasts, predictions, and probabilities.
Predictions of a devastating hurricane pounding the US East Coast are plentiful. As of today (Sunday) the National Hurricane Center puts their forecast as 5 to 10 percent. However, the final probability will be 100 percent or zero.
Predictions and Forecasts
Prediction and forecast are often used interchangeably, but they aren’t exactly the same.
A prediction is a statement about what will happen in the future, often based on intuition, expert judgment, or limited data.
Two examples are: “The Cincinnati Reds will win tonight’s game, or Hurricane Errin will strike the US East Coast.”
A forecast is a systematic, data-based estimate of what is likely to happen, often expressed with probabilities or ranges. It’s more analytical than a prediction and acknowledges uncertainty.
An example is: “There’s a 65% chance the Reds will win tonight’s game, based on pitching matchups and recent performance.”
Frequencies and Forecasts
A frequency is simply how often something happens within a set of observations. It’s backward-looking because it comes from recorded past data.
An example is: “Out of 100 baseball games, the Reds won 55. The frequency of wins is 55/100, or 55%.
Forecasts are forward-looking. In forecasting, you usually start with base rates (historical frequencies). Then, you adjust them with new information, trends, or context.
An example of how frequencies (or base rates) are used is forecasting is: The Reds historically win 55% of games at home. For tonight, considering they’re facing an ace pitcher, the forecast drops to 45% win probability.
Probability and Subjective Probability
Probability is a way to quantify uncertainty about future outcomes. Probability is the measure of how likely an event is to happen. It’s expressed as a number between 0 and 1 (or 0% and 100%).
Like forecasts, probability is forward-looking. It’s a statement about how likely an event is to occur in the future. In practice, probability is an estimate informed by past frequencies, current information, and assumptions.
Probability is subjective because it reflects a person’s degree of belief in an outcome, given their knowledge and evidence. So subjective probability is a rational belief, but not one just based on historical frequencies.
All or Nothing with Hurricane Erin
Remember, probability is forward-looking. That means our forecasts reflect the chance we believe an event to occur. However, once an event passes, it either happened, or it did not (it's either 0 or 100%). Now we’re talking frequency.
JD Solomon is the founder of JD Solomon, Inc., the creator of the FINESSE fishbone diagram®, and the co-creator of the SOAP criticality method©. He is the author of Communicating Reliability, Risk & Resiliency to Decision Makers: How to Get Your Boss’s Boss to Understand and Facilitating with FINESSE: A Guide to Successful Business Solutions.
