I've just received a memo which says (effectively)
As more people leave, there will be less people available.
I want that word to be fewer. Are there guidelines for which word ought to be used when?
Ah, less vs. fewer. Another arrow in the prescriptivist’s quiver of pointless pedantry.
According to Merriam-Webster's Dictionary of English Usage, a usage guide that looks carefully at the history of usage advice, the rule creating a clear separation for less and fewer was invented in 1770 by Rober Baker in his book Reflections on the English Language, where he wrote in a comment on less:
The Word is most commonly used in speaking of a Number; where I should think Fewer would do better. No Fewer than a Hundred appears to me not only more elegant than No less than a Hundred, but more strictly proper.
The Merriam-Webster Dictionary of English Usage authors then comment:
Baker’s remarks about fewer express clearly and modestly—“I should think,”, “appears to me”—his own taste and preference. It is instructive to compare Baker with one of the most recent college handbooks in our collection:
Fewer refers to quantities that can be counted individually.… Less is used for collective quantities that are not counted individually… and for abstract characteristics. —Trimmer & McCrimmon 1988
Notice how Baker’s preference has here been generalized and elevated to an absolute status, and his notice of contrary usage has been omitted. This approach is quite common in handbooks and schoolbooks; many pedagogues seem reluctant to share the often complicated facts about English with their students.
How Baker’s opinion came to be an inviolable rule, we do not know. But we do know that many people believe it is such.
They then give many examples of usage of less for countable quantities, and add finally:
The examples above show native speakers and writers of English using less of count nouns in various constructions. Fewer could have been used in many of them—at times it might have been more elegant, as Robert Baker thought—but in others no native speaker would use anything but less.
With regards to the example in the original question, either fewer or less would be perfectly grammatical, but so many people are under the spell of the rule that less must never be used with countable nouns that anyone who doesn’t follow the rule may be subject to criticism.
The Cambridge Grammar of the English Language also weighs in on less vs. fewer:
The relation between less and fewer is fairly complex. In non-count singulars only less is possible: Kim has less/*fewer money than Pat. In plural NPs we have:
i. She left less than ten minutes ago.
ii. Less/Fewer than thirty of the students had voted.
iii. He made no less/fewer than fifteen mistakes.
iv. You pass if you make ten mistakes or less/?fewer.
v. He took less/*fewer pains to convince us than I’d expected.
vi. He made fewer/less mistakes than the others.
Both [i] and [ii] have than + numeral. In [i] ten minutes expresses an amount of time rather than a number of individuated units, and in such cases fewer is virtually impossible—just as few would be in a comparison of equality: She left as little/*few as ten minutes ago. Similarly with We paid less than thirty dollars for it; She’s less than forty years old; We were going at less than ten miles an hour. In [ii] we are concerned with countable individuals and little cannot be used in a comparison of equality (*as little as thirty of the students); nevertheless, for inequality less is more common than fewer in this construction. The same applies with percentages: Less/Fewer than 30% of the students had voted. Construction [iii] has the comparative form following no: though the interpretation is count plural, less is here again more common than fewer. Construction [iv] has or after a numeral: less is the usual form here, with fewer quite marginal; this construction is widely seen in supermarkets, with the fast checkout labelled eight items or less, or the like. In [v] pains is plural but non-count rather than count (we can’t ask how many pains he took), and here only less is possible. Finally in [vi] (as also in [15ii]) the comparative occurs directly with a count plural noun: both forms are found, but less is subject to quite strong prescriptavist disapproval, so that fewer is widely preferred in formal style, and many speakers in informal style too.
[Usage manuals are divided on the issue of less vs. fewer. Some uncompromisingly brand such forms as less mistakes as incorrect, while others note that though commonly condemned they are often used by speakers of Standard English. Before the Early Modern English period (beginning around 1500) more was restricted to non-count NPs with moe used as the comparative of many. At that time less was used along with fewer for count NPs, but came to be stigmatised and quite rare in this use: it is only within the last generation or so that it has become frequent. The current revival seems inexorable, given the strong pressure of analogy with more.]
The New Oxford American Dictionary has a note about this:
USAGE Fewer versus less: strictly speaking, the rule is that fewer, the comparative form of few, is used with words denoting people or countable things ("fewer members"; "fewer books"; "fewer than ten contestants"). Less, on the other hand, is used with mass nouns, denoting things that cannot be counted ("less money"; "less music"). In addition, less is normally used with numbers ("less than 10,000") and with expressions of measurement or time ("less than two weeks"; "less than four miles away"). But to use less with count nouns, as in "less people" or "less words," is incorrect in standard English.
Grammar Girl wrote about this, and putting it very succintly, the differences between "few" and "less":
Less and fewer are easy to mix up. They mean the same thing—the opposite of more—but you use them in different circumstances. The basic rule is that you use less with mass nouns and fewer with count nouns.
So, what is a count noun, and a mass noun?
A count noun is just something you can count. I'm looking at my desk and I see books, pens, and M&M's. I can count all those things, so they are count nouns and the right word to use is fewer. I should eat fewer M&M's.
Mass nouns are just things that you can't count individually. Again, on my desk I see tape and clutter. These things can't be counted individually, so the right word to use is less. If I had less clutter, my desk would be cleaner. Another clue is that you don't make mass nouns plural: I would never say I have clutters on my desk or that I need more tapes to hold my book covers together.
I have added that emphasis because it points out a lot. This makes identifying Mass nouns very easy.
What about your example, in which you said "Less than 9 sausages?" Isn't that, a violation of this rule? Shouldn't "few" be used?
The fact is, there is the word than, which changes everything. When there is a "than", the sentence is changed into a comparison, and "less" is used when making comparisons. According to the Chicago Style Manual:
Less than is used before a plural noun that denotes a measure of time, amount, or distance: less than three weeks; less than $400; less than 50 miles.
Oxford Dictionaries Online surprisingly helps us with this:
Less is also used with numbers when they are on their own and with expressions of measurement or time, e.g.:
His weight fell from 18 stone to less than 12.
Their marriage lasted less than two years.
Heath Square is less than four miles away from Dublin city centre
That's why "less" was used, when referring to "9 sausages".
Examples of usage of "less than" includes:
I have less pencils than you.
We all have less reputation than Robusto.
The increase was less than the decrease.
I have less than 5 dollars in my pocket.
We have less than 2 hours to accomplish this.
Finishing off, with a quote from Grammar Girl, this notes the exceptions to the rules:
There are exceptions to these rules; for example, it is customary to use the word less to describe time, money, and distance. For example, you could say, “That wedding reception lasted less than two hours. I hope they paid the band less than $400.” So keep in mind that time, money, and distance are different, but if you stick with the quick and dirty tip that less is for mass nouns and fewer is for count nouns, you'll be right most of the time.
Hope that helps.
British Supermarkets are split on whether to describe express checkouts as '10 items or less' (Tesco) or '10 items or fewer' (Marks & Spencer). Seemingly the more upmarket chains use 'fewer' and the more mainstream chains using 'less'.
As to which is the correct usage I would agree with Scott Handelman:
"Fewer" is used when the actual number of objects is quantifiable. Since you can count people, "fewer" is correct.
"Less" is used when the amount is not easily quantifiable. You can put less salt in your soup, for example.
Strunk's 1918 edition of The Elements of Style confirms that less is for multitude nouns and fewer is for countable nouns in the Misused Expressions section. So does the Chicago Manual of Style (to which I cannot link, because it's subscription only).
Of course there are disagreements as detailed above by @nohat, but those are a couple of well-respected style guides that can give some guidance to those of a prescriptivist bent.
The reality is that less is being used where fewer was once common. It will become dominant and then invariant within the next 10-20 years. I see less + countable nouns all the time in print and on TV lately. Language change in action. http://languagelog.ldc.upenn.edu/nll/?p=2819
In this particular case the correct word to use is 'less', since 'people' is a mass/collective noun. It is commonly used as the plural form of 'person'. I consider this grammatically incorrect (undoubtedly it was so traditionally) though modern usage has adopted it as a sort of plural form.
Oxford Dictionaries Online explains the usage cases of 'less' and 'fewer' pretty well:
Use fewer if you’re referring to people or things in the plural (e.g. houses, newspapers, dogs, students, children). For example:
* People these days are buying fewer newspapers.
* Fewer students are opting to study science-related subjects.
* Fewer than thirty children each year develop the disease.
Use less when you’re referring to something that can’t be counted or doesn’t have a plural (e.g. money, air, time, music, rain). For example:
* It’s a better job but they pay you less money.
* People want to spend less time in traffic jams.
* Ironically, when I’m on tour, I listen to less music.