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Tips for figuring out test coverage

Determining testing coverage is about figuring out what you’re going to test in the application. When I start this process, I start with a coverage outline. And while I like to develop coverage outlines in Excel, you can use just about any application you’d like. A lot of people use mind mapping tools, word, or a graphing tool like Visio or OmniGraffle.

I’ll often start by developing a generic list of items to cover while I’m testing. I typically do this by working through the elements of the SFPDO mnemonic to get things started. The SFPDO mnemonic comes from James Bach, and it’s a heuristic to help you figure out what you need to test. If you are not familiar with the SFDPO heuristic, it addresses the following:

  • Structure: what the product is
  • Function: what the product does
  • Data: what it processes
  • Platform: what it depends upon
  • Operations: how it will be used

Within each of those areas, there are specific factors you can look at. For example the following list details out what’s included in Structure – often ignored area of test coverage:

  • Code: the code structures that comprise the product, from executables to individual routines.
  • Interfaces: points of connection and communication between sub-systems.
  • Hardware: any hardware component that is integral to the product.
  • Non-executable files: any files other than multimedia or programs, like text files, sample data, or help files.
  • Collateral: anything beyond software and hardware that is also part of the product, such as paper documents, web links and content, packaging, license agreements, etc.

Using the SFDPO mnemonic, I’ll cover each area in detail to identify what I believe I should be testing. Once I have my initial list, I put it down and walk away from it. I do this for a couple of reasons. Normally, it’s because I’m tired, but also to give myself time away from the list to see if anything new occurs to me while I keep it in the back of my thoughts.

A second approach I use to identify coverage is to look at what test data I already have. I’ll see if there is any data I have access to that’s ready to use, or could be ready to use with very little work. Is there test data lying around from past projects or production that I can use? What coverage does that give me? Is there test data I can create easily with tools or automation? What coverage does that give me? If I find anything interesting, or if the data I find sparks any ideas, I’ll go back and add that to the coverage outline.

Finally, a their approach is to think about specific risks related to the product I’ll be testing. Sometimes I’ll use bug taxonomies to spark my thinking if I have a hard time getting started. These normally help me with generic risks. The one I reference most is the appendix to Kaner, Falk, and Nquyen’s Testing Computer Software. Once the taxonomy gets me going, I can normally think of some additional risks that are more specific to my application.

Regardless of where the ideas come from and how I develop it, once I have a coverage outline I work to get it reviewed with various project stakeholders. That typically involves dialog and trade-offs. I cut out a bunch of the stuff I wanted to test and add a bunch of stuff I didn’t think of. Over time, this outline evolves as my understanding of the application and the risks to the project evolve.

For more information on SFDPO, check out Bach’s original article or his methodology handout which details the specific product elements covered with the mnemonic. Also, if you don’t have a copy of Testing Computer Software, you can pick one up here.

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