expressions - Why is "a 100% increase" the same amount as "a two-fold increase"?


and is such interpretation the norm?


When something went from 4 units to 8 units, most authoritative sources seem to agree with the use of "a two-fold increase", even though what was actually increased is more like "one-fold", i.e. the original quantity.


But if the "two-fold increase" is the correct usage, why most people seem to interpret "a 100% increase" the same thing?



Answer



Yes, the correct usage is that 100% increase is the same as a two-fold increase. The reason is that when using percentages we are referring to the difference between the final amount and the initial amount as a fraction (or percent) of the original amount. So, if something gets multiplied by two, it experiences a positive increase equal to 100% of the original amount. The confusion arises because the word "increase" is used differently in each case. In the first case we mean the change between initial and final value; while in the second situation we interpret the change as a multiple of the original quantity.


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