We come, at last, to the final post on when to use numerals and when to use words to express numbers. In some cases, you use both.
Rule 4.33 instructs one to use a combination of numerals and words to express back-to-back modifiers. The reason? Such a combination in many cases increases the clarity and readability of the construction.
In some cases this is easy to see, as the numbers are actually confusing at a glance if they are not clearly distinguished. For example, “There are 12 10 a.m. New York Megabusses” can very easily lead to a bus missed by 2 hr. “There are twelve 10 a.m. busses” is the better construction, and you are happily on your way to whatever future awaits in the City That Never Sleeps.
In case you are wondering, there is no specific rule that stipulates which number in the construction should be a numeral and which a word. The following, for example, are all fine:
- 2 two-way interactions
- ten 7-point scales
- twenty 6-year-olds
There is, however, some logic to it. When you have back-to-back numerical modifiers, start with the "base" (6-year-olds, 7-point scales, two-way interactions), with the number in the usual format (notice none of the examples give 20 six-year-olds or 10 seven-point scales).
There is a caveat to the general rule to combine numerals and words for these constructions that states that in some situations, readability may suffer if numerals and words are combined: “first two items” is preferable to “1st two items” and “first 2 items.”
At first blush, this can be a tough rule, as it is to a certain degree contingent on a subjective aesthetic interpretation of readability. That being said, trust your judgment. Look at the construction to see whether it is clear and readable as is or could be confusing. The great majority of cases will profit from changing to a combination of words and numerals. The relatively few cases in which the change isn’t warranted are likely to involve ordinal and low numbers. In the rare instance in which you are genuinely unsure of whether the construction should be changed, it’s probably fine either way.