Many people think that they can multi-task, or rather that they can be just as effective switching between multiple tasks on a regular basis rather than concentrate on one task at a time. While I am certain that there are people that can do this effectively, for knowledge work in particular most people are not good at context switching. The effect of context switching is pretty high. Consider the diagram:

The summary of this slide is:

Switching context between two tasks reduces your effective capacity by 20%; between three tasks doubles that.

Perhaps you don't think you are effected in this way? Here is a simple exercise to find out. Our task is to time ourselves as we produce a page with the numbers from 1 to 10, the letters from A to J, and the roman numerals from I to X:

The first time we time ourselves we will write the numbers down in column order, so 1 through 10, then A through J, and finally I through X. We will then turn over the page and try it again, but this time we will write down each row. So we will produce 1, A, I then 2, B, II until we complete the 10th row.

If the time recorded for the first round versus the second round are basically the same then congratulations you are one of a small group of people (2%) that seem to be effective when multi-tasking. If you are like the rest of us, you find a significant difference and that difference will look similar to the data in the diagram above. To calculate:

Interesting, isn't it?

In addition to the time, the other thing to note is the error rate. My experience is that when I do this the second time around and I go back and validate what I've produced, that there are more mistakes in the second round of processing.

• Why Should We Focus on Doing One Thing To Completion at a Time? for additional reasons that we should not multi-task.