If someone ever cites the "magic number seven" rule at you, most likely they don't know what the hell they're talking about.
You've probably heard the rule before. If you give people a list of items to memorize, most people will recall between 5 and 9 items, hence it's often called the "7 +/- 2" rule or "magic number seven" after the academic paper where the finding was published.
Lots of people, designers and non-designers alike, cite this in web design as the maximum number of categories you can have at the top level of a website. I'm sure it's used in other places too.
Unfortunately, those are bullshit applications of the rule. The rule refers to only one thing: memorizing the exact sequence of digits like a phone number. Most people can remember seven numbers in order before running out of short term memory.
But when you're memorizing words instead of numbers, recall goes down to 5 plus-or-minus 2 (or less for long words). And again, this is a very limited application of short term memory -- exact, ordered recall.
Thankfully, web pages are different than phone numbers, and other researchers have done experiments that test the limits of memory. Let's try this one; I hand you a list and give you a minute to memorize it:
Then I ask you to recreate the list as best as you can. You might get half of them right; you'd definitely get more than 5 +/- 2. In fact, you would probably remember a clump of words at the beginning and a clump at the end, with some others scattered in between.
This is due to how our short term memory works. We remember the first items we encounter ("primacy") and the most recent items we encounter ("recency") better than the middle crap.
Let's try a slightly different experiment. Instead of asking you to recreate the list, I ask you to recall where specific items showed up. "Where did 'trombone' appear in the list?" You would remember even more than the previous test.
I'd say this kind of prompted recall is the closest to how people use web pages -- "where is that logout button? "I think I saw 'checkout' over here." "The search bar was somewhere at the top of the page." If that's the case, then "7 +/- 2" is total BS as a design rule because people are capable of memorizing much, much more with the right help.
Put memory to work for you
This leads to an interesting question. What can you do to improve experiences by taking advantage of how memory works? Here are a few good tips.
Awesome beginnings and endings
Because we remember the first and last items we've encountered better than other stuff, you should put your best effort into creating awesome beginnings and endings. Your audience will remember the beginning and end long after they've forgotten all that crap in the middle.
When you need someone to remember something, like a specific series of steps to accomplish a goal, use prompts to nudge them along. A little prompt (i.e. "Where is 'trombone' in the list?", "Step 1... Step 2...") can invoke a chain of memories that people would have otherwise forgotten.
I bet you can recall all the lyrics to your favorite songs you listened to over and over again as a teen. People recall items better after they've been repeatedly exposed to them. As time goes on, those items migrate to your long term memory. After a while, you'll have the lyrics to "Mexican Radio" embedded in your brain for life.
Memorize this list:
I bet you could easily do it. Our brains "chunk" similar kinds information together. We can memorize chunks that we already know, like movie lines or NFL teams, faster and better than others.
If you remember anything from this post...
By now, I'm sure you're wondering what to tell your boss when he insists that you need to cut your website's categories down to "seven plus or minus two." I'd say the limit is based on how many you can lay out without overwhelming the user. Try something, test it, try something else, then test it again. Repeat until all people - users and bosses alike - are satisfied.
If all else fails, just do what Amazon does. (They have 12 top level categories.) And if that doesn't work, show that person the hundreds of other interactive items on your website. People remember how to use the site just fine despite the fact that "hundreds" vastly exceeds our short term memory limit of "seven plus-or-minus two."
Have any memory tips appropriate for design? Please add them to the comments on this post. In the mean time, don't be afraid to tell someone that they have no fucking idea what they're talking about if they spew that magic number seven crap at ya.