We are sysadmins. We love numbers. They mean a lot to us. They are specific and clean.
We also like a lot of details. When someone asks what operating system we use, we rattle off all fifty we can think of. That includes the embedded OS we know is buried deep in our toaster. Why? Because when we discovered it has a serial port, we plugged in and watched the bootup messages. That's why.
However, when writing and speaking the number of things we list means something to the reader/listener more than the number. Controlling the number of items in the list is more important than being complete.
Lists of length 1, 2 3 and "4 or more" have particular meaning.
A list with one element means "Hey! Look at this! Remember it!". If you ask me what operating system I use at work, the complete answer is a list a mile long. If I want you to remember that I am a Linux sysadmin, you won't remember that if I list "Linux, Mac OS X, Windows, IOS, JunOS, Android and ChromiumOS". The word Linux gets lost in the noise, even if it is the first item of the list. If I simply say, "I administer Linux machines" then that is what people will remember. If you want someone to remember what you said reduce the list down to one item.
A list with two elements implies comparison. "I am knowledgeable about Windows and Linux." invites comparison. It implies that these are different things and emphasizes that I have two very different skill sets: the ability to run Windows, and the ability to run Linux. A reader unfamiliar with computers will understand that these are two different things and might ask questions that relate to how they compare. It is actually jarring to list two items that you don't want the user to compare in their minds. In fact, the more similar they are, the more someone will think about the differences. "I run Ubuntu 9.1 and 9.2" makes people wonder what is so different about them that I list them both. Think about how these phrases invite comparison: "At home and at work", "night and day", "HTML5 and Flash", "Ubunto 9.x and 10.x", "apples and oranges". If your point isn't to emphasize differences (good or bad) make sure your list doesn't contain two items. If you want to emphasize differences, make sure your list has exactly two items.
A list with three elements implies (a) that you expect the reader/listener to hold all three in their head while I discuss them, (b) that you will discuss them in that order, (c) that the order matters. A three-item list is short enough that the reader can hold them all in their brain for the duration of the discussion. You haven't made the statement so complex as to have overloaded them. When you "drill down" on the items in the list, cover each item in the same order as the original list. This parallel format helps the reader/listener understand the flow. Lastly, order the items with great care. Often we put the most important item first but I find that people most remember the last item the most, so put it last. If I want you to "reboot the machine, make sure it comes back up, and come to my desk when you are done" I am emphasizing the need for you to come back to me. When writing an article or giving a presentation the last item often gets the most discussion. If you have one complicated and two short topics, end with the complicated item. This lets you cover the first two briefly and then focus on the third item for the remainder of your time.
A list with four or more elements implies that the point isn't the contents of the list, but that the list is very long. I might tell you that I use a lot of operating systems: MacOS, Ubuntu, Redhat, Windows, Android, JunOS and IOS. The point I am making is that the list is very long. The contents of the list is not so important. The reader/listener walks away remembering "Tom knows a lot of operating systems". If this is not what you intend, reduce the list to be shorter than 4 items. You may have to summarize ("Tom knows Linux, Windows, and some lesser-known operating systems.") If you don't want someone to focus on the details of the list, make sure there are 4 or more items on it.
We are sysadmins. Numbers are important to us. However, it is important to remember that the number of items in a list tells people a lot more than just what is on the list:
- Remember me.
- Things to keep in your head
- The quantity is more important that the details.