What Does a Twofold Increase Mean?

Date: 01/23/2004 at 04:57:51
From: Laura
Subject: 2-fold increase

What does a onefold increase mean?  Is a twofold increase simply twice
the amount?  What about a threefold increase?  I'm not used to this
terminology of using "fold."  Does it refer to powers or multiples?

Date: 01/23/2004 at 09:40:45
From: Doctor Peterson
Subject: Re: 2-fold increase

Hi, Laura.

The suffix "-fold" refers to multiples.  "Onefold" means a whole, 
"twofold" means twice, "threefold" means three times as much.

According to Merriam-Webster (m-w.com), "twofold" means

  1 : having two parts or aspects
  2 : being twice as great or as many

and the suffix "-fold" means

  1 : multiplied by (a specified number) : times -- in adjectives
      <a sixfold increase> and adverbs <repay you tenfold>
  2 : having (so many) parts <threefold aspect of the problem>

Although some people might quibble over "twofold increase," arguing 
that it has to mean the _increase_ itself is twice the original 
value, and thus a tripling, a quick web search makes it clear that 
the phrase universally refers to a mere doubling: an increase _to_ 
twice the original amount.  See this page for a discussion of the 
quibble with reference to a similar, but more controversial, phrase:

  Percent Greater Than vs. Increased
    http://mathforum.org/library/drmath/view/61774.html 

I have never heard of a "onefold increase," and don't find the phrase 
on the web.

If you have any further questions, feel free to write back.

- Doctor Peterson, The Math Forum
  http://mathforum.org/dr.math/