"the crude marriage rate" v "the annual marriage rate"

[Odessa Dawn]

"What the above fails to mention, of course, is that the number of births outside marriage is rising along with the age of the mothers (interestingly their median age is around the same that for first-time brides), and the crude marriage rate is declining ..."
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I have no problem with the annual marriage rate, but the crude marriage rate is completely over my head. Would you please paraphrase what has been stated here? Does the crude marriage rate mean that we have to stick to a given year?

[Rover_KE]

It's meaning #4 here:

Concise Oxford English Dictionary © 2008 Oxford University Press:
crude
▶adjective

• 1 in a natural or raw state; not yet processed or refined: crude oil.

• 2 made or done in a rudimentary or makeshift way: a relatively crude nuclear weapon.■ (of an estimate) approximate.

• 3 offensively coarse or rude, especially in relation to sexual matters.

• 4 Statistics (of figures) not adjusted or corrected.

Rover