If I start with $100 and end up with $250, did that money grow 2 1/2 times?
A reporter and I are having a good-natured disagreement: He says yes, and I say no.
The increase in question (I've simplified it for this example) was 150 percent. There's no arguing that; it's just math. To me, that translates to growing 1 1/2 times. The reporter points out that growing 1 1/2 times sounds far less impressive than doubling-and-getting-halfway-to-tripling. At first glance, it sounds like a mere 50 percent increase. I see his point, and besides that, nobody would ever say something "grew 1 1/2 times." I would say the amount grew 150 percent. The writer proposed "more than doubled," which sounds more striking even though it's less precise, and even though the amount much more than doubled, and that was fine with me.
But he still thinks I'm wrong. Let's examine the point further. For starters, I'll add by to the sentence, so it can't be read as meaning that the amount grew on two or three occasions. Then let's raise the stakes. Because double is such a handy word, I think "grew two times" is almost as unlikely a phrase as "grew one time" or "grew 1 1/2 times." Triple and quadruple are viable words as well, if less common, and I'll skip quintuple just for good measure.
So let's say I started with $100 and ended up with $600. My money sextupled, but few would say that. Now then: Did it grow six times? It multiplied six times, and the result is six times the original amount, but it grew 500, not 600, percent. So which model does the grow by ___ times expression follow, the multiplication or the percentage change?
Here's how I do the math: Even though nobody would say something "grew by one time," that would have to mean it doubled -- which, inconveniently enough, means growing by 100, not 200, percent. So if doubling is growing one time, tripling is growing two times, quadrupling is growing three times, quintupling is growing four times and sextupling is growing five times. You don't get to count the return on your original investment as an increase, though that issue gets confused a little in the casino, where a big "99 percent" return on a slot machine means you're losing a dollar of every hundred you put in (note the use of the correct word return there).
My friendly adversary pointed me to a dictionary that defines the verb triple as meaning "to increase three times in size or amount." And there is the -fold model. A twofold increase is doubling, a threefold increase is tripling, and so on. To which I respond: None of the dictionaries on my shelves are that sloppy, and those shelves also hold an otherwise wonderful usage book in which the author is tripped up by -fold, insisting that tripling would be a twofold increase. (It's a special case, -fold, because "a onefold increase" is not only never used but also impossible. You can fold something in two or three or more, but you can't fold it in one.)
Whatever the answer, as I told the reporter, the fact that we're disagreeing should be a clue that such a reference would be unacceptably ambiguous. You have people like me, in the "Do the math" school, and you have people like him, in the "Aw, c'mon, everybody knows what that means" school. My bottom line, as with the dispute over "times more" vs. "times as much as," is that we're dealing with a confusing and ultimately bankrupt expression. When you're tempted to say "three times more," make it "three times as much as." When you're tempted to say "grew three times," say "multiplied three times" or "grew 200 percent" or "tripled."