Irrelevant interruptions and their cost to thinking

​Everyone knows that it’s dangerous to use a cell phone while driving. The danger lies in distraction; even with a hands-free phone, the conversation saps attentional resources that should be devoted to the road.

But what if you’re not driving? Suppose you’re a student working a multi-step math problem and an announcement comes in over the school PA system. 

Lab studies provide some disquieting answers. Coming back to your main task after an interruption carries a cost both in time lag and in error rates…in one field study errors were observed where the main task was the administration of medication.

In a new study Erik Altmann and his colleagues investigated the effect of the duration of the interruption. They were especially interested in duration because delay is known to have a robust effect on memory, but is likely to have an effect on attention. (The importance of delay is explained below.)

​In this experiment, the multi-step main task involved a display of a digit and a letter, about which subjects had to make seven judgments, in a particular sequence. The graphic below explains the task. 

​Interruptions occurred randomly, on average every 6 trials (but with a lag of at least three trials). Subjects saw a letter string that they were to type on the computer keyboard. Duration of the interruption was controlled by the number of strings and the number of characters in each string. 

Let’s return to the importance of the interruption delay. Suppose you’re executing a multi-step procedure and you’re interrupted. When you return to your main task any problems you have might be due to memory or attention. If the problem is one of memory (but your attention is fully back on task) we’d expect that you might forget where you are in the sequence, e.g., you were supposed to the “R” task in the UNRAVEL sequence, but you forget and repeat the “N” task. But we wouldn’t expect you’d necessarily make a mistake when answering the “N” task, because attention is fully back on task.

But if the problem is that your distracted, we’d think that both types of problems would be more likely—an interruption would make you lose your place in the sequence (sequence errors) and make you more prone to mistakes in when answering (answer errors).

​The results were quite clear, as shown in the figure

The interruption made subjects forget where they were in the sequence, but it did not seem to affect attention much once they returned to the task, because the interruption didn’t make them more error prone.

​As in other experiments, this one showed that an interruption incurred a cost to response time upon return to the main task. In this experiment, this cost increased with delay, and in light of the error data, we’d interpret this effect as being due to subjects struggling to remember where they were in the sequence. 

This memory-based account of the effect of interruptions on sequential tasks is consistent with decades of experimental work showing that the contents of short term memory is compromised by delay.  These results don’t mean attention is not a relevant factor in interruptions, but they do speak to the relative roles of memory and attention in sequenced tasks.

So administrators...if you haven't set a policy that the PA system is silent during class, think about doing so!