The Wall Street Journal explains something I could never quite figure out:
Twofold, fourfold, ninefold. But use numerals and a hyphen for numbers 10 and over: 12-fold.
Remember that -fold means times. So an increase from 1 to 2 is a twofold increase (1 times 2), or a 100% increase. An increase from $5 to $25 is a fivefold increase (we view -fold increase as an idiom that includes the base amount), but a 400% increase.
The use of -fold means your percentage amount and your fold amount will be off by one degree.
When in doubt, try to avoid the -fold construction entirely.
What Does the "Fold" Suffix Actually Mean?
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If WSJ is correct, I've been on the wrong side of this battle for my entire 40-year career.
ReplyDeleteSupporting WSJ's side:
1. Wikipedia ( https://en.wikipedia.org/wiki/Fold_change )
2.Numerous apologists, who explain, in logic worthy of Rudolph Giuliani, that an increase isn't really increase: http://mathcentral.uregina.ca/QQ/database/QQ.09.07/h/fred3.html
3. common usage--though I would argue, common misusage.
The most literate discussion I've read is found on Stack Exchange: https://english.stackexchange.com/questions/91263/why-is-a-100-increase-the-same-amount-as-a-two-fold-increase
The most articulate criticism of "twofold = doubling" can be found at https://www.future-science.com/doi/pdf/10.4155/ipk-2016-0007
Several defenders contradict themselves. Take, for example, this defense, from hunker.com ( https://www.hunker.com/13414935/how-to-calculate-fold ):
"A fold change is basically a ratio. It indicates the number of times something has changed in comparison to an original amount. A twofold increase indicates that an amount doubled." Huh? If an amount has doubled, it has changed only once.
I'm all for idiom, provided it's unambiguous. "Increased twofold" ain't.