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Equation 1 · Part 4 · Attribution: Tying One Event to a Changing Climate

fraction

PR=p1p0,FAR=1−p0p1=1−1PR.\mathrm{PR} = \frac{p_1}{p_0}, \qquad \mathrm{FAR} = 1 - \frac{p_0}{p_1} = 1 - \frac{1}{\mathrm{PR}}.
fraction

What this part means

Divide the expression above the line by the one below it.

Its job in the formula

The expression above the fraction bar is divided by the complete expression below it. The denominator must not be zero.

The passage around this formula

The standard output is a probability ratio, also called a risk ratio, comparing the annual probability of the event in the factual climate with its probability in the counterfactual one [ 3 ] . Writing those probabilities as the factual and counterfactual annual exceedance probabilities, PR=p1p0,FAR=1−p0p1=1−1PR\mathrm{PR} = \frac{p_1}{p_0}, \qquad \mathrm{FAR} = 1 - \frac{p_0}{p_1} = 1 - \frac{1}{\mathrm{PR}}. The fraction of attributable risk is a monotone transform of the ratio and contains no additional information. Its name invites a misreading that the field has warned about for two decades: it is not the probability that this event was caused by forcing, and it does not license the statement that a given fraction of the damage was anthropogenic in any individual case. It is a…

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Learn the underlying idea

A fraction a/b means a divided by b. The top number is the numerator; the bottom number is the denominator, and it cannot be zero.

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Sources cited in the surrounding passage

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