They rank 53rd in Stanley Coren's The Intelligence of Dogs, being of average working/obedience intelligence.
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They rank a low 42nd in Stanley Coren's The Intelligence of Dogs, but keep in mind that Coren's criteria focus on how fast a dog can learn obedience "tricks"—a limited test of intelligence.
The Pembroke Welsh Corgi has been ranked at #11 in Stanley Coren's The Intelligence of Dogs, and is thus considered an excellent working dog.