Multitasking and Task Switching: Why Switching Has a Cost

What counts as multitasking?

"Multitasking" covers two different things, and research treats them separately:

• Doing two things at the same time, such as talking on the phone while driving. Researchers call this dual-tasking.
• Switching back and forth, such as answering a message halfway through writing a report. This is task switching.

A third idea, media multitasking, describes using more than one medium at once, such as messaging while watching a video. It is usually measured with a questionnaire about the share of your media time in which you use more than one medium at the same time ^[6].

Switching between mental sets, called shifting, is one of three often-studied executive functions ^[8]. What are executive functions? explains how it fits with inhibition and working memory.

Doing two things at once

People often have trouble doing even two simple tasks at the same time. A classic review of the evidence pointed to a "stubborn bottleneck" in choosing actions, and probably in retrieving information from memory, which limits how far two tasks can be handled at once ^[1].

That limit is not fixed for every task. In three experiments with simple choice-reaction tasks, some participants reached virtually perfect time sharing after relatively modest practice ^[9]. How much the tasks interfered also depended on instructions about which task had priority, and on whether people preferred to do the two at once or one after the other ^[9].

Those were simple, well-practised laboratory tasks. They show that practice can reduce the cost of one particular pair of tasks, not that writing an email while following a meeting becomes free.

Switching back and forth: the switch cost

In a classic set of experiments, people classified either a digit (odd or even) or a letter (consonant or vowel), changing task predictably on every second trial ^[3]. Switching had a substantial cost. Giving people more time to prepare, up to about 0.6 seconds, reduced it, but even with 1.2 seconds to prepare a large cost remained, and only on the first trial of the new task ^[3].

In four further experiments, with undergraduates classifying geometric shapes or solving arithmetic problems, switching cost more time when the rules were complex, and less when a cue showed which task came next ^[10]. The authors proposed two stages of control behind each switch: shifting your goal, then activating the rules of the new task ^[10].

A review of this research concluded that responses are substantially slower, and usually more error-prone, immediately after a switch ^[2]. The chance to prepare reduces this switch cost but does not eliminate it, and the cost seems to come both from leftovers of the previous task and from the time needed to reconfigure for the new one ^[2].

These experiments measure the cost of a single switch between very simple tasks. They show that switching is never free, but they cannot tell you how much time a day of checking email costs you.

Interruptions: switches you did not choose

Even very short interruptions can break a train of thought. In a lab task that required people to keep their place in a sequence of steps, interruptions averaging 2.8 seconds doubled the rate of sequence errors afterwards, and interruptions averaging 4.4 seconds tripled it ^[11].

People can make up some of the lost ground, at a price. In a study of interrupted work, people who were interrupted finished their tasks in less time with no difference in quality, but reported more stress, frustration, time pressure and effort ^[12]. For other everyday causes of lost focus, see Why can't I focus?

Does heavy multitasking change your attention?

In 2009, researchers gave a media-multitasking questionnaire to 262 students and then compared heavy and light media multitaskers on laboratory tasks, in groups of 15 to 22 people ^[5]. The heavy multitaskers were more affected by irrelevant items, and, surprisingly, had larger switch costs (by 167 milliseconds), which the authors linked to a weaker ability to filter out the task that no longer mattered ^[5]. The groups did not differ on a stop-signal task, which measures holding back a response ^[5].

The finding has not replicated well:

• Two replication studies ran 14 tests with an average statistical power of 0.81. Only five showed more distractibility in heavier media multitaskers, and only two of those held up in a more conservative Bayesian analysis ^[6]. The same paper's meta-analysis of 39 effect sizes found a weak association that was no longer significant after correcting for small-study effects ^[6].
• A larger meta-analysis of 118 assessments found a small overall association between media multitasking and cognitive control, which depended on how both were measured. Its authors concluded that, ten years on, researchers were no closer to understanding the link ^[13].
• A 2018 review co-written by one of the original authors described the evidence as sparse, with both convergent and divergent findings. On balance, heavier media multitaskers did worse in a number of cognitive domains, although many studies found no difference, and the differences often involved sustained, goal-directed attention ^[7]. Whether multitasking causes such differences remains an open question ^[7].

The cause could also run the other way. Among 277 university students, those who media-multitasked more, and those who used a phone more while driving, scored lower on the operation span task, which the authors used to measure executive control and multitasking ability (r = −0.19 and r = −0.15) ^[4]. On average, students placed their own multitasking ability at the 63rd percentile, and 193 of the 277 rated themselves above average ^[4]. The authors suggest that people often multitask because they are less able to block out distractions and focus on one task ^[4]. This was one correlational study.

Multitasking in a lecture

In a simulated university lecture, 40 students took notes on their laptops, and half of them, chosen at random, were also asked to complete online tasks during the lecture ^[14]. On a comprehension test afterwards, the multitaskers answered 55% of the questions correctly, against 66% for the students who only took notes ^[14].

In a second experiment, 38 students took notes on paper. Those seated where they could see the laptop screens of two classmates who had been recruited to multitask scored 56%, against 73% for students who could not ^[14].

These were two small experiments with one lecture each, and the multitasking was assigned by the researchers. They suggest that a busy screen costs the person using it and the people who can see it.

Supertaskers, and the phone at the wheel

Is anyone good at it? In a study of 200 people in a high-fidelity driving simulator, participants drove with and without a demanding spoken working-memory task ^[15]. Most got significantly worse when doing both, but 2.5% showed no decline at all; the authors called them supertaskers ^[15]. That is about one person in forty, in a single study, and people's own sense of their multitasking ability is a poor guide ^[4].

For everyone else, the phone matters. In simulated driving, listening to the radio or to an audiobook did not disrupt performance, but conversations on a handheld or a hands-free phone doubled the rate of missed traffic signals ^[16]. A meta-analysis of 33 studies, with about 2,000 participants, found that phone tasks slowed drivers' reactions by about a quarter of a second on average, with similar costs for handheld and hands-free phones; drivers did little to compensate by keeping more distance or slowing down ^[17].

Can you train switching?

Practice makes switching faster on the practised task. In a study of students, 31 practised a cued switching task for 21 days while 29 did an active control task ^[18]. Training reduced switch costs, mostly within the first four to six sessions, although some cost remained, and it helped on another switching task ^[18]. There was no benefit on tasks measuring interference control, response inhibition, working memory or general intelligence ^[18].

So a smaller switch cost in a game is a real change in that game. It is not evidence that juggling email and a report has become free, and it is not a reason to multitask more.

Measure your own switch cost

On LaserMind's Switch the Rule, a digit from 1 to 9 (never 5) appears under a rule. For ODD / EVEN, odd goes to F and even to J. For LESS / MORE than 5, less goes to F and more to J. On a touch screen you tap the two answer buttons instead. The rule appears half a second before the digit, so you can prepare, and each digit waits up to 3 seconds.

• Two modes. A session has 48 trials, and you choose how often the rule changes: "Switches sometimes" (on 25% of trials) or "Switches often" (on 50%). Difficulty does not change during a session.
• Errors. Wrong answers, timeouts and answers faster than 150 milliseconds count as errors.
• Your main number is the switch cost: your median response time on trials where the rule changed, minus trials where it stayed the same, counting correct answers only. Lower means a change of rule slowed you down less. You also see your accuracy and median response time.

Compare your switch cost with your own earlier sessions in the same mode. Times also depend on your device and keyboard, which is why the optional leaderboards (opt-in, adults only) rank each mode separately on each kind of device. The task switching and attention switching method pages describe offline versions with cards or a row of digits.

What the research says

Frequently asked questions

Are some people simply good at multitasking? A few may be: 2.5% of drivers in one simulator study showed no cost from adding a second task ^[15]. But in another study, the students who multitasked most scored lower on a test of executive control, and most rated their own multitasking ability above average ^[4].

Does practice make multitasking free? Practice reduces the cost of the particular combination you practise. Some people reached near-perfect time sharing on simple lab tasks ^[9], and switching training reduced switch costs, but the gains stayed with switching tasks ^[18].

Will Switch the Rule make me better at multitasking at work? That has not been shown. Practice usually shrinks your switch cost on this task; whether that carries over to everyday multitasking is not established. If juggling tasks is the problem, How to focus better covers ways to reduce switching in your own work.

References

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