Multiple Passes

We could make the pictures more realistic by reversing the drying order in the last one (since the dishes are stored on a stack), or by adding a putting-away step at the end, but that would lead us in the wrong direction. In fact, I'm really only interested in the first two options, which I'll refer to as “one at a time” and “in multiple passes”.

For dishes, it's easier to use multiple passes, but in general, which option is better depends on the situation. If you're learning how to do something, or making it up as you go, you probably won't even have more than one item, so of course you'll do one at a time. (Actually, in that case the options are indistinguishable.) If you're called upon to do the same thing again for another item, you'll have to do that one by itself too. But, as you're called upon to do the same thing more and more often, at some point you'll probably find that it's more efficient to organize the items into groups and go through each group in multiple passes.

It doesn't always become more efficient to work in multiple passes. If it's hard to switch from one item to another—i.e., if the cost in time or effort is high—then it'll always be better to work on one at a time. As a silly but conclusive example, postmen don't start the day by going around and opening all the mailboxes.

By the way, here's a funny thing. If we replace the idea of one person doing all the work with the idea of several people working together in parallel, then the difference between the two options turns into the difference between craftsmen and assembly-line workers!

There's one other benefit of multiple passes that I'd like to talk about, but I think it'll be easier to start with an example than an explanation.

One of the many things I found when I was cleaning my room was a stack of comic strips I'd cut out of the newspaper during my first few weeks in Boulder. Of course I wanted to look them over before I threw them out, so I quickly sorted them by type and date, then set out to perform the following three steps.

1. Write down the type and date, for the record.
2. Read the comic.
3. Decide whether the comic is worth keeping.

I initially tried to go through the comics one at a time, but that was a disaster. I'd get a few done with no trouble, but then something in a new comic would make me question my decisions on the old ones, so I'd dig them back up, spend some time looking at them, and then get totally confused about which ones I'd done and which ones I hadn't. Then I'd start over … but as soon as I got past the point where I'd stalled, the same thing would happen again. Several cycles later, it finally occurred to me to switch to multiple passes, and after that, it was easy.

I've managed to confuse and un-confuse myself in the same way in other situations, so I do think there's a general rule in there … which is, using multiple passes is less confusing. It's not a great rule, though. I don't know if it applies to other people; maybe I'm just easily confused. I also don't know in which situations it applies. Here are some guesses that I came up with after not too much effort.

• If the items are related (as the comics were related by story continuity), then information about future items has a legitimate backward influence on past decisions. Using multiple passes doesn't fix that, but it does move the decisions closer together.
• If the steps don't form a logical progression, it'll be easy to forget one, and I'll always be worrying that I've done so.
• If one of the steps takes a long time, I won't be able to tell what I've done by referring to short-term memory.
• If the items aren't modified along the way, I won't be able to tell what I've done by referring to the state of the items.