To change together in a related way, usually used in statistics or science when two things move up or down in connection with each other.
From co- (together) + vary (to change), both from Latin. The prefix 'co-' indicates joint action, while 'vary' comes from varius (different). Modern usage primarily mathematical.
When variables covary, it doesn't mean one causes the other—a common mistake! Ice cream sales and drowning deaths covary (both increase in summer), but ice cream doesn't cause drowning.
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