I recently had a discussion with a friend, and he was using a phrase repeatedly which said "Conversion Rate vs Time". I pointed out to him that Rate already has the time factor, so you don't have to say Rate vs Time, but he then pointed me to the Wikipedia page for Rate
The most common type of rate is "per unit time", such as speed, heart rate and flux. Ratios that have a non-time denominator include exchange rates, literacy rates and electric flux.
Which is fair enough, but it did leave me confused (for the first time so many years) by the term Literacy Rate. I never realized it till now, but why do we append Rate to Literacy?
Is Literacy alone not enough to define the metric which we now call Literacy Rate? Why do we use Rate? What does Rate signify in this scenario?