A loaf of bread is, in most cases, improved by slicing it. This is because a loaf of bread is more than you need; you can make a whole thing (a sandwich) by using a part of the full loaf. Also, the cost of the slicing in terms of lost bread is very low, assuming you use a relatively sharp knife.
This is not true for people. While you may be able to get a whole thing from a part of a person’s time, the cost of slicing is large.
This is because there is a cognitive burden when switching tasks and arriving at a constructed mental state. I worked with a designer who described getting into a state of “Photoshop Zen” where he had awareness of what the layers of a given drawing were, and which he had active at any given time, allowing him to know what the effect of his actions would be. Getting to that place could take anywhere from a few minutes to more than an hour, depending on the complexity of the drawing. Something as simple as answering a question could cost him that constructed state, resulting in a loss of all the time he’d spent getting there and necessitating spending it *again* to get back there.
Understandably, he didn’t like being interrupted. And once I understood what it cost me (as the one depending on him to finish things), I didn’t like him being interrupted either, and I took steps to make sure that I didn’t do it, or let other people do it.
This idea of “flow state” or “maker time” is fairly common, and many of the agile teams that I’ve worked with in the past few years have evolved ways of dealing with interruptions to try to minimize the impact on team members. This is a very good thing.
I think management is still catching up to this idea, though. While we get that interrupting people is bad, and we should give them no-meeting time to focus, we’re still tempted to slice people up, spreading them across multiple projects. “We need her expertise on both of these – let’s have her split time between them” is something that I’m still hearing.
And it’s wrong.
When you cut someone’s time in half, you don’t get 50% of their productivity on each subject. You get maybe 40%, because of the time lost to constructing a mental state and context for each topic as you switch. The cost of switching is incurred at each switch, and doesn’t just represent lost time, it represents lowered effectiveness. So if you have someone trying to handle two things over the course of a day, they may never get to the point where they’re able to be truly effective – never get to “flow state.” This is especially bad if you’re doing it to the really important person you just had to spread across two efforts – they’re going to have a hard time meeting your needs, and they’re going to be really frustrated at not being able to get to doing real work.
You can minimize the cost of switching by minimizing the amount of it – have dedicated full days on each subject, or better yet full weeks – but you can’t get rid of it. The more switches you have, the more time you lose. Which means that splitting someone three ways is worse, and so on.
I should note that I’m talking about non-management roles here – the people who spend their time in meetings are doing a different kind of work. We still need “flow state” for things like writing up proposals, but when that’s not your primary deliverable, time-slicing has fewer consequences. That doesn’t mean it has none, mind you. Just fewer.
For people who need “maker time” to do their work (programmers, designers, writers, and the like), don’t slice them up across multiple simultaneous efforts. If you have to – and ask yourself really, really hard if you actually have to – then minimize the amount of task-switching by using full-day increments.