We hear about the benefits of multi-tasking but often fail to understand when it is more beneficial to focus on a single task. The difference lies in cognitive load and the amount of effort it takes to complete tasks. Some types of tasks require too much focus and effort to complete while others don't require much attention and are beneficial for multi-tasking. Knowing the difference can help.
When To Focus: Focus is best when the task is complicated and it takes considerable effort to complete. Your brain can only handle so many tasks at once. Ensuring you are reducing distractions and competing projects helps in using all your energy in completing your primary work.
When to Multi-Task: Work that is routine, doesn't require much thought and small mistakes dont matter. Let us assume you are filing papers, cleaning your desk and having a routine conversation on the door. Pretty simple. Your cognitive engagement on each task is low and therefore you have the capacity to do multiple work.