What’s the difference between proportional and proportionate?
Proportional and proportionate are often used interchangeably and the difference between them is subtle.
Proportional involves fairness or equity or proportionality. It’s proportional if it’s in a proper or accepted relationship to something else. An act of sabotage, for example: if hacking Sony was the act, then retaliating with some form of reciprocal hacking or a measured sanction would be proportional. Ordering a bombing campaign would be grossly disproportional.
Proportionate, on the other hand, would involve the partitioning of a whole, or the allocation of its parts. In the present example, calling for a proportionate response might indicate that each of the parties involved would be punished according to their determined level of blame or involvement — their share of the act. Perhaps this would mean that the North Korean government would receive a sanction, while the Chinese servers which hosted the operation would be electronically attacked, at the same time that the responsible hackers themselves would be targeted for prosecution. The response would be portioned out (“pro-portioned” or apportioned) to those involved. (Note that I’m just making the details up: how this attack was done and who was behind it isn’t something I’ve spent a lot of time worrying about.)
Proportional also has a specific mathematical meaning that proportionate doesn’t. For instance, you could say:
The field strength is proportional to the transmitted power.
The concentration of the final solution is proportional to the amount of solute.
The force on the pinion is proportional to the length of the lever and the applied force.
You could not use proportionate in any of these examples.
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