12.4 T-Shirt Sizing

Nontechnical stakeholders often want (and need) to make decisions about project scope during the wide part of the Cone of Uncertainty. A good estimator will refuse to provide highly precise estimates while the project is still in the wide part of the Cone. Sales and marketing staff will say, "How can I know whether I want that feature if I don't know how much it costs?" And a good estimator will say, "I can't tell you what it will cost until we've done more detailed requirements work." It would appear that the two groups are at an impasse.

This impasse can be broken by realizing that the goal of software estimation is not pinpoint accuracy but estimates that are accurate enough to support effective project control. In this case, nontechnical stakeholders are typically not asking for an estimate in staff hours. They are asking whether a specific feature is a mouse, rabbit, dog, or elephant. This observation leads to a very useful estimation approach called t-shirt sizing.

In this approach, the developers classify each feature's size relative to other features as Small, Medium, Large, or Extra Large. In parallel, the customer, marketing, sales, or other nontechnical stakeholders classify each feature's business value on the same scale. These two sets of entries are then combined, as shown in Table 12-13.

Table 12-13: Example of Using T-Shirt Sizing to Classify Features by Business Value and Development Cost
Feature	Business Value	Development Cost
Feature A	Large	Small
Feature B	Small	Large
Feature C	Large	Large
Feature D	Medium	Medium
Feature E	Medium	Large
Feature F	Large	Medium
Feature G	Small	Small
Feature H	Small	Medium
…
Feature ZZ	Small	Small

Creating this sort of relationship between business value and development cost allows the nontechnical stakeholder to say things like, "If the cost of Feature B is Large, I don't want it, because the value is only Small." This is a tremendously useful decision to be able to make early in the life cycle of that feature. If you were instead to carry that feature through some amount of detailed requirements, architecture, design, and so on, you would be expending effort on a feature that ultimately isn't cost justified. In software, a quick "No" answer has great value. T-shirt sizing allows for early-in-the-project decisions to rule out features so that you don't need to carry those features further into the Cone of Uncertainty.

The discussion about what to carry and what to cut is easier if the feature list can be sorted into a rough cost/benefit order. Typically, that is done by assigning a net business value number (another unitless measure) based on the combination of development cost and business value. Table 12-14 shows one possible scheme for assigning net business value. You can use this scheme or come up with one that seems to more accurately reflect the values in your environment.

Table 12-14: Net Business Value Based on Ratio of Development Cost to Business Value
Development Cost
Business Value	Extra Large	Large	Medium	Small
Extra Large	0	4	6	7
Large	-4	0	2	3
Medium	-6	-2	0	1
Small	-7	-3	-1	0

This sort of net business value lookup table allows you to add a third column to the original value/cost table (Table 12-13) and to sort that table by net business value, as shown in Table 12-15.

Table 12-15: Example of Sorting T-Shirt Sizing Estimates by Approximate Net Business Value
Feature	Business Value	Development Cost	Approximate Net Business Value
Feature A	L	S	3
Feature F	L	M	2
Feature C	L	L	0
Feature D	M	M	0
Feature G	S	S	0
Feature ZZ	S	S	0
Feature H	S	M	-1
Feature E	M	L	-2
…
Feature B	S	L	-3

Remember that the Approximate Net Business Value column is an approximation. I don't suggest just counting down the list and drawing a line. The value of sorting by approximate business value is that it supports getting some quick "definitely yes" answers for the features at the top of the list and some quick "definitely no" decisions for the features at the bottom. That allows discussion to focus on the middle of the list, which is where the discussion will be most productive anyway.

Tip #59

Use t-shirt sizing to help nontechnical stakeholders rule features in or out while the project is in the wide part of the Cone of Uncertainty.