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Table 1 Characteristics of story points and T-shirt sizes as estimating units as seen by team members

From: Can FOSS projects benefit from integrating Kanban: a case study

Criteria Story points T-shirt sizes
Simplicity/Complexity Estimation depends on the reference issue(s), which has to stay the same over time. Otherwise estimations are not meaningful. Simpler, intuitive, not interdependent, relates to working time, easier. Everybody can do it.
Level of detail More exact. If it is not correctly done, it is less meaningful. It is more complicated to assign issues a correct value. It is less detailed. Estimation and reality are closer to each other.
Usefulness Useful if it is done correctly. More experience needed to estimate correctly. If there are too many different units, estimating becomes impossible. Usefulness depends on project and team size. Our estimation procedure delivered useless estimates. Always useful. A rough estimation is always helpful. It is sufficient if a more detailed planning is not necessary.
Informative value It is more exact but also more error-prone. One needs to know the estimation system. A finish date can be determined more exactly. Less granular.
Makes use of learning effects Estimates become more accurate with experience. No.
Time expenditure It is much more time-consuming, especially in the beginning. It is faster.