Article: The Zeigarnik Effect Explained
In 1927, psychologist Bluma Zeigarnik noticed that waiters in a cafe could recall the order they had not yet delivered than those they had. The later conducted experiment suggested that a desire to complete a task can help it be retained in one's memory until it has been completed. Also, the completion of the tasks enables the process of forgetting it to take place. Zeigarnik's findings revealed that participants could recall details of interrupted tasks around 90% better than those that they had been able to complete undisturbed.
We might all experienced while preparing for an exam. After an exam, we often find it hard to remember the things we were studying and had good understanding previously since we no longer have an immediate use for the information.
This effect can also explain why when we start working on something but do not finish it, thoughts of the unfinished work will continue to pop into our find even when we have moved on to other things. We can find such application of the effect in the cliffhanger of a TV show or LinkedIn's user profile completion process where there's a progress bar telling users how close they are to complete their profile.
Though much research supports the Zeigarnik effect, it is noteworthy that the result can also be undermined by things like people's motivation to complete the task, how difficult that task is, and the timing of task interruption.