The trailcam photo below shows an underweight horse on the Virginia Range. A WHB specialist might give it a BCS of 3. A wild horse critic (ahem, cough-cough, hater) might argue the horses are overpopulated and need to be removed from the range.
Would that be a fair statement?
A ‘common cause’ belongs to the system and affects all horses in an area. The illness affecting the Waipio Valley horses would be an example. It will probably be attributed to common causes, something in their environment. Ditto for the Sand Wash horses, who are threatened by fencing and drought.
A ‘special cause’ is outside the system and affects only individual herd members. An example would be the horse recently removed from Big Summit WHT.
If you saw many horses with low body condition scores, you’d be thinking common causes, such as excess animals / not enough land / insufficient forage.
In this example you only see one such horse, so you’re looking for a special cause, like illness, injury or loss of teeth.
Two mistakes are possible: (1) attributing a problem to a special cause when it was produced by common causes and (2) attributing a problem to common causes when it was produced by a special cause.
Calling a horse ‘stupid’ because he wandered onto a road and was hit by a car is an example of Mistake #1. Any of the horses in his band could have done that, because a new subdivision is chewing up their land and now they must find food elsewhere.
The critic above is wrong. He has made Mistake #2.