In contrast, the least productive programmers scored low on having a quiet workspace, very low on ability to divert telephone calls, and very high on how frequently they were interrupted. It is clear that interruptions are a major cause of low productivity among programmers. The problem is not the time needed to handle the interruptions themselves, but the time needed to get back into the programming problem. Everybody, no matter what they do, face a reorientation time when they return to their work after an interruption. When you are reading a magazine article and look up to answer a question, it takes you longer to read the next paragraph than if you had been reading continuously.
Programmers have it hard.
They have to build up a complex mental model of the programming problem, the state of the different variables, and so on. That's why most people do not make good programmers, but it is also why even the best programmer gets into trouble when he or she is interrupted. Down falls the carefully constructed mental house of cards. Building the mental state back up to the level required for efficient programming can easily take fifteen minutes. Thus, a two-minute telephone call kills productivity for seventeen minutes or so.