Sometimes it's easy to make a mistake when picking a photo of an animal — presenting an image of a rat instead of a mouse, for example. Or mistaking a crane fly for a mosquito. But whoever made this missing cat poster has taken the crown for animal mis-classification.

In any case, there is now a way to express just how much the possum featured in the missing poster is not a cat, because biologist and blogger Alex Wild has come up with a brilliant method of quantifying the extent of taxonomic misidentification. He calls it the Taxonomic Fail Index (TFI) and it's actually a pretty straightforward little equation:

Taxonomy Fail Index (TFI) = T/H

Where T = the number of million years since the two species (the correct species and the species an organism is misidentified as) shared a common ancestor; and H = the number of million years since humans and our closest relatives, the chimps, shared a common ancestor (about 6.4).

"In other words," writes Wild, "the Taxonomy Fail Index scales the amount of error in absolute time against the error of misidentifying a human with a chimp." He continues:

If I were to run a story about Sarah Palin, but accidentally illustrate it with a photo of a bonobo, that would be a taxonomy fail of magnitude 1.

The classic Taxonomy Fail of possum/cat has a TFI of 24.6.

The yellow jacket/honey bee fail...has a TFI of 25.2, or slightly stupider than mistaking an opossum for a cat.

Let's try putting the equation to work, shall we? This image is from a post I wrote back in August about how rats were not actually responsible for the Black Death.

The image features what I thought was a black rat (Rattus rattus) — but commenter mikeiko seemed to think that I had failed by "putting a pet fancy rat on the picture, which is a Brown rat [Rattus norvegicus], rather than...[a] Black Rat [Rattus rattus]."

Rattus rattus and Rattus norvegicus last shared a common ancestor roughly 3.0 million years ago. According to Wild's handy formula, that would give my black rat/brown rat fail a TFI of 3.0MYA/6.4MYA, or approximately 0.469. It's a far cry from mistaking a cat for a possum, but the point is that the next time one of you feels like publicly shaming us for using the wrong image of a species in a post, you can do it with numbers and figures! (You can use a resource like TimeTree to calculate time of divergence.) [MYRMECOS]