I need to know how to write out 2 1/2 years in a formal document. Do I write out the numbers? If so, do I include hyphens, or not, or just for the one-half? I've consulted the American Psychological Association Publication manual and have found somewhat conflicting advice.
1. "Hyphenate a phrase used as an adjective when it precedes the term it modifies." This makes me think it would be: two-and-a-half years.
2. "express numbers in figures that represent statistical or mathematical functions, fractional or decimal quantities, percentages, ratios, and percentiles. eg., 2 1/2 times as many." This makes me think it would be written as: 2 1/2 years.
3. it also says that "numbers that represent time; dates; ages; population size, specific numbers of subjects; scores and points on a scale; exact sums of money; and numerals as numerals should be written as figures. eg., in about 3 years."
4. It also says that "common fractions should be written out as words" such as half, fifth, fourth.
So which rule trumps the other? Do I write, two-and-a-half years, two and a half years, 2 and a half years, or 2 1/2 years???
The first rule would be for a phrase like this:
It was a two-and-a-half-year study.
In your case, I see no need to hyphenate. It depends on what the time represents. If it is just a period of time, then I'd write is as words, but if it's a result or statistic, I'd use numbers. I don't like '2 and a half years' much as it seems to be a mixture.