merit, efficacy, efficiency, benefit
効 centers on the idea of producing a desired result: something that works well, has merit, or brings a benefit. The meanings of efficacy and efficiency are natural extensions of this core idea of effectiveness.
効 combines 交 (exchange, mix) and 力 (power, effort), suggesting the result or effect that comes from applying effort. The exact historical development is uncertain, but the component combination fits the meaning of efficacy.
Imagine mixing 交 your effort 力 with a task: the result is merit and efficiency. When effort is applied effectively, you get a benefit.
For コウ, think of a company CEO (こう) who values efficiency and merit. The CEO's decisions bring benefit to the whole organization.
effect; effectiveness; efficacy; result
valid; effective
efficiency
to be effective; to take effect; to be good (for)
invalid; void; ineffective; unavailable
efficacy; benefit; efficiency; effect; result; success
effect; efficacy; validity; potency
immediate effect; instant effect
statute of limitations; lapse of rights after a period of time; prescription (including acquisitive and extinctive prescription)
practical effect; efficacy; efficiency
effect; effectiveness; efficacy; virtue; benefit
effect; virtue; efficacy; impression
lapse; abatement; invalidation; expiration; expiry; becoming void
coming into effect
use; utility; effect; benefit
medical efficacy; effect of a medicine
specific medicine; wonder drug; silver bullet
striking effect (e.g. of a remedy); dramatic results; special effect; particular efficacy
speedy effect
delayed effect