Unplanned work and rework

All developers know that frequent context switching leads to less productivity. If we are disturbed while coding, it takes us some time to get back into the code and continue with the same productivity we had when the disturbance occurred. So, working on multiple projects or tasks also reduces productivity. In his book Quality Software Management: Systems Thinking, Gerald M. Weinberg presents the result of a study that concludes that when only working on two projects simultaneously, the performance drops by about 20% (Weinberg G.M. 1991). For each project you add, the performance drops 20% further (see Figure 2.1):

Figure 2.1 – Loss of productivity when context switching

Another study from 2017 shows that developers who work on two or three projects spend on average 17% of their effort on context switching (Tregubov A., Rodchenko N., Boehm B., & Lane J.A., 2017). I think the actual percentage may vary a lot from product...