Estimating with Tee Shirt Sizes

I occasionally encounter the use of t-shirt sizes (Small, Medium, Large, or so on) in use as estimating units by teams. T-shirt sizes are an OK approach to getting started with relative estimating, but they suffer from two severe weaknesses:

  • They aren't additive. You cannot tell a boss you'll be done in 3 mediums, 4 larges, and 2 petites.
  • Your view of an XL may not match mine. You may think it's 50% bigger than an L; I may think 25% bigger.

Teams get around both weaknesses with some underlying assumptions (hopefully stated) about size. They may, for example, state, “Let's call a medium a 5 and a large a 10.”

The primary advantage to t-shirt sizes is the ease of getting started. If a team needs to start with t-shirt sizes, that's fine. Later, though, that team will be better off using numbers directly. I might forget that an XL is 33% bigger than an L. I won't forget that a 10 is twice a 5. (If I do, the team has a different problem!)

T-shirt sizes can be a great way of becoming accustomed to relative estimating. So, start with them if your team finds that easier. But minimally put some underlying numbers on them (e.g., Medium=5) and then gradually shift to using the numbers directly.

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About the Author

Mike Cohn is the founder of Mountain Goat Software, a process and project management consultancy that specializes in helping companies adopt and improve their use of Agile processes and techniques. He is the author of User Stories Applied for Agile Software Development, Agile Estimating and Planning, and Succeeding with Agile. Mike is a co-founder of the Agile Alliance. He is also a co-founder and current board member of the Scrum Alliance. He can be reached at info@mountaingoatsoftware.com or connect with Mike on Google+.