The biggest advantage of constraining time is that it’s always unambiguous. If you decide to work for three hours and then stop, there’s no confusion there.

Disadvantage: time constraints can encourage a sloppier attitude towards work. For example, you might decide to spend all day studying in the library—but without tasks to constrain your productivity, you end up checking your phone.

• It’s unclear the time and effort required to complete the task.
• The work itself is ambiguous and may require a lot of trial-and-error.
• The work is continuous and can’t be easily divided into discrete chunks.

The advantage of constraining tasks is that it focuses directly on the object of productivity: whatever you’re trying to accomplish: you can't fool yourself into believing you’re working hard but you’re not actually accomplishing much.

Disadvantage: tasks can often be ambiguous or hard to predict. If you fail to predict properly you might create to-do lists that are unachievable or those that are trivial.

• Tasks are discrete and fairly predictable.
• You might be tempted to fill up time without making real progress
• The tasks are frequently repeated, and therefore easier to estimate.