Alternating between tasks is slower and more error-prone than completing them in blocks, and part of the cost survives any amount of preparation.
Rogers and Monsell showed in 1995 that switch costs shrink with warning time but do not vanish, which points to a residual component incurred at the moment of the switch itself rather than to incomplete preparation. Jersild had measured the effect in 1927. The cost is per switch, so it scales with fragmentation rather than with the number of tasks.
In practice
- A developer interrupted four times an hour loses more than four interruptions' worth of time.
- A day of thirty-minute meetings on six subjects produces less than a day of three two-hour blocks.