Waterfall says Resource and Scope should be fixed and Time is the variable for delivering projects. Agile (or maybe Scrum) says Resource and Time is fixed and Scope is variable.

Nothing is fixed

Fixing resource is kind of nonsense when by ‘resource’ we mean people. People take days off, have good days and bad days, get more done on some days than others, so resource is constantly moving in both quantity and quality. In reality nothing is fixed.

Scope = quality

When we talk about Scope we’re really talking about the quality of the solution delivered. Sometimes, if the quality of solution required is too great for the fixed length of time then the quality (scope) is reduced to fit the fixed length of time.

Time = quantity

When we talk about fixing Time, such as in a two week sprint, we are really talking about the quantity of output, the amount of work that gets done.

Quality / quantity

This is why the Scope / Time discussion is really a quality / quantity discussion. If you reduce the Time available to work on a solution you have to reduce the Scope of what will be delivered. You can have it in one day and the solution will be good, or you can have it in one week and the solution will be better, or you can have it in a month and the solution will be the best. Sometimes ‘good’ is good enough, and sometimes ‘good’ is all you have time for, but by fixing Time (and so fixing quantity) you limit the ability to deliver the best solutions.

Perhaps allowing the team to decide what quality of solution needs to be delivered, and how long that solution will take to build sets them up to do great work rather than doing just what they can fit into an arbitrary length of time.