High level estimates are guesses (usually very bad and over optimistic).
In Agile, we accept that our initial high level estimates are not to be trusted. Of course, budgets need to be created and expectations need to be set.
Relative sizing means to size our stories relatively to each other and then measure how fast the team can go.
How to do Planning Poker
This is a team based estimation and building consensus technique in Agile. This is a game where the development team estimates stories individually first (using a deck of cards with numbers like one, three, five points on them) and then compares the results collectively together after.
If everyone’s estimates is more or less the same, the estimate is kept. If there are differences, the team discusses them and estimates again until consensus is reach.
This technique works because:
- The people doing the estimating are the ones doing the job
- The estimates are done based on discussions and consensus
Read the full series of articles about Agile Methodology: The Basics, Who and Why, How to Deliver, User Stories, Estimation and Planning.
Read more: Jonathan Rasmusson. “The Agile Samurai: How Agile Masters Deliver Great Software (Pragmatic Programmers)” Pragmatic Bookshelf
Carlose Lopez 
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