A Fun Presentation on a Powerful Software Test Design Approach

Combinatorial Software Test Design – Beyond Pairwise Testing


I put this together to explain combinatorial software test design methods in an accessible manner.  I hope you enjoy it and that, if you do, that you’ll consider trying to create test cases for your next testing project (whether you choose our Hexawise test case generator or some other test design tool).


Where I’m Coming From

As those of you know who read my posts, read my articles, and/or have attended my testing conference presentations, I am a passionate proponent of these approaches to software test design that maximize variation from test case to test case and minimize repetition.  It’s not much of an exaggeration to say I hardly write or talk publicly about any other software testing-related topics.  My own consistent experiences and formal studies indicate that pairwise, orthogonal array-based, and combinatorial test design approaches often lead to a doubling of tester productivity (as measured in defects found per tester hour) as compared to the far more prevalent practice in the software testing industry of selecting and documenting test cases by hand.  How is it possible that this approach generates such a dramatic increase in productivity? What is so different between the manually-selected test cases and the pair-wise or combinatorial testing cases?  Why isn’t this test design technique far more broadly adopted than it is?

A Common Challenge to Understanding: Complicated, Wonky Explanations

My suspicion is that a significant reason that combinatorial software testing methods are not much more widely adopted is that many of the articles describing it are simply too complex and/or too abstract for many testers to understand and apply.  Such articles say things like:

A. Mathematical Model

A pairwise test suite is a t-way interaction test suite where t = 2. A t-way interaction test suite is a mathematical structure, called a covering array.

Definition 1 A covering array, CA(N; t, k, |v|), is an N × k array from a set, v, of values (symbols) such that every N × t subarray contains all tuples of size t (t-tuples) from the |v| values at least once [8].

The strength of a covering array is t, which defines, for example, 2-way (pairwise) or 3-way interaction test suite. The k columns of this array are called factors, where each factor has |v| values. In general, most software systems do not have the same number of values for each factor. A more general structure can be defined that allows variability of |v|.

Definition 2 A mixed level covering array, MCA (N; t, k, (|v1|,|v2|,…, |vk|)), is an N × k array on |v| values, where

| v |␣ ␣k | vi | , with the following properties: (1) Each i␣1

column i (1 ␣ i k) contains only elements from a set Si of size |vi|. (2) The rows of each N × t subarray cover all t-tuples of values from the t columns at least once.

– “Construct Pairwise Test Suites Based on the Bak-Sneppen Model of Biological Evolution” World Academy of Science, Engineering and Technology 59 2009 – Jianjun Yuan, Changjun Jiang

If you’re a typical software tester, even one motivated to try new methods to improve your skills, you could be forgiven for not mustering up the enthusiasm to read such articles.  The relevancy, the power, and the applicability of combinatorial testing – not to mention that this test design method can often double your software testing efficiency and increase the thoroughness of your software testing – all tend to get lost in the abstract, academic, wonky explanations that are typically used to describe combinatorial testing.  Unfortunately for pragmatic, action-oriented software testing practitioners, many of the readily accessible articles on pairwise testing and combinatorial testing tend to be on the wonky end of the spectrum; an exception to that general rule are the good, practitioner-oriented introductory articles available at combinatorialtesting.com.

A Different Approach to Explaining Combinatorial Testing and Pairwise Testing

In the photograph-rich, numbers-light, presentation embedded above, I’ve tried to explain what combinatorial testing is all about without the wonky-ness.  The benefits from structured variation and from using combinatorial test design  is, in my view, wildly under-appreciated.  It has the following extremely important benefits:

  • Less repetition from test case to test case
    • In the context of discussing testing’s “pesticide paradox” James Bach, I believe, used the analogy that following in someone’s footsteps is a very good way to survive traversing through a mine field but a generally lousy way to find software defects efficiently.
    • Maximizing variation from test case to test case, as a general rule, is an absolutely spectacular way to find defects quickly.
    • There are thousands, if not trillions of relevant combinations to select from when identifying test cases to execute; computer algorithms will be able to solve the problem of “how can maximum variation be achieved?” far better than human brains can.
  • More coverage of combinations of test inputs
    • Most of the time, since awareness of pairwise and combinatorial testing methods remain low in the software testing community, combining all possible pairs of values in at least one test case is not even a conscious goal of testers.
    • Even if this were a goal of their test design strategy, testers would have a tremendous challenge in trying to achieve such a goal: with hundreds, thousands or tens of thousands of targeted combinations to cover, losing track of a significant number of them and/or forgetting to include them in software tests is virtually a foregone conclusion unless a test case generator is used.
    • More thorough coverage leads to more defects being found.
  • Efficiency (Testers can “turn the coverage dial” to achieve maximum efficiency with a minimal number of tests)
    • The efficiency and effectiveness benefits of pairwise testing have been demonstrated in testing projects every major industry.
    • I wanted to prominently include the message that testers using test case generators have the option to dramatically increase the testing thoroughness levels of the tests they generate because it is a topic that often gets ignored in introductions to pairwise testing case studies and introductions
  • Thoroughness – (Testers can also “turn the coverage dial” to achieve maximum thoroughness if that is their goal)
    • Too often, tester’s view pairwise as a technique that focuses on a very small number of curiously strong tests; that is only part of the story.
    • This can lead to the /false/ impression that combinatorial testing methods are inappropriate where high levels of testing thoroughness are required.
    • You can create very different sets of tests that are as thorough as possible (given your understanding of what you are testing) no matter whether you have 1 hour to execute tests or one month to test.

Other Recommended Sources of Information on Pairwise and Combinatorial Testing:

Questions or Comments?

If you have questions or comments, please leave a note below.  I’d love to hear about people’s experiences using these test design approaches.  Thank you.

Too Many Tests and No Computer to Run Them; Wil Shipley’s Mac Cops an Attitude

A friend passed me this set of recent tweets from Wil Shipley, a Mac developer with 11,743 followers on Twitter as of today. Wil recently encountered the familiar problem of what to do when you’ve got more software tests to run than you can realistically execute.

I love that. Who can’t relate?

Now if only there were a good, quick way to reduce the number of tests from over a billion to a smaller, much more manageable set of tests that were “Altoid-like” in their curious strength. 🙂 I rarely use this blog for shameless plugs of our test case generating tool, but I can’t help myself here. The opening is just too inviting. So here goes:

Wil,

There’s an app for that… See www.hexawise.com for Hexawise, a “pairwise software test case generating tool on steroids.” It eats problems like the one you encountered for breakfast. Hexawise winnows bazillions of possible test cases down in the blink of an eye to small, manageable sets of test cases that are carefully constructed to maximize coverage in the smallest amount of tests, with flexibility to adjust the solutions based upon the execution time you have available. In addition to generating pairwise testing solutions, Hexawise also generates more thorough applied statistics-based “combinatorial software testing” solutions that include tests for, say, all possible 6-way combinations of test inputs.

Where your Mac cops an attitude and tells you “Bitch, I ain’t even allocating 1 billion integers to hold your results” and showers you with taunting derisive sneers, head-waggling and snaps all carefully choreographed to let you know where you stand, Hexawise, in contrast, would helpfully tell you: “Only 1 billion total possibilities to select tests from? Pfft! Child’s play. Want to start testing the 100 or so most powerful tests? Want to execute an extremely thorough set of 10,000 tests? Want to select a thoroughness setting in the middle? Your wish is my command, sir. You tell me approximately how many tests you want to run and the test inputs you want to include, and I’ll calculate the most powerful set of tests you can execute (based on proven applied statistics-based Design of Experiments methods) before you can say “I’m Wil Shipley and I like my TED Conference swag.”

More info at:
hexawise.tv/intro/
or
https://hexawise.com/Hexawise_Introduction.pdf
free trials at:
http://hexawise.com/signup

– Justin Hunter

What is Agile? What is not Agile?

An unusually hectic work-schedule has been keeping me hopping lately.  I returned this weekend from a great two-week trip to the UK in which I visited with 5 testing teams using our Hexawise tool to design test cases for applications being used in two banks, a consulting and systems integration firm, a grocery store chain, and a telecoms company.

Every product manager worth his or her salt will tell you it is a good idea to go meet with customers, listen to them, and watch them as they use your application.  Even though everyone I know agrees with this, I find it difficult to make happen as regularly as I would like to.  This trip provided me with a reminder of how valuable in-depth customer interactions can be.  The two weeks of on-site visits with testing teams proved to be  great way to: (a) reconnect with customers, (b) get actionable input about what users  like / don’t like about our tool, (c) identify new ways we can continue to refine our tool, and even (d) understand a couple unexpected ways teams are using it.

Bret Petticord’s tweets on “What is Agile?” / “not Agile?” prompted me to write this quick post.  I like them a lot.

When we first created our Hexawise tool, we followed the 4 steps Bret lays out in his description of “What is Agile?”  My experience in the UK over the last two weeks was the start of one of many “Repeat” cycles.

I admire people who can succinctly summarize wisdom into bite-sized quips like Bret achieved with his two tweets.  Another guy who excels at creating sound-bites is James Carville.  Love him or hate him, he has that skill in spades.  When I watched the movie “War Room,” I felt like I was watching the “master of the sound-bite” in his element.  Me?  I’m more of a rambling, meandering, verbose communicator.  I’ve just taken 332 words and a screen shot with Bret’s tweets when all I set out to do in starting to write this post was to share Bret’s 32 words with you.

25 Great Quotes for Software Testers

All the quotes below are from the inside cover of Statistics for Experimenters written by George Box, Stuart Hunter, and William G. Hunter (my late father).  The Design of Experiments methods expressed in the book (namely, the science of finding out as much information as possible in as few experiments as possible), were the inspiration behind our software test case generating tool.  In paging through the book again today, I found it striking (but not surprising) how many of these quotes are directly relevant to efficient and effective software testing (and efficient and effective test case design strategies in particular):

  • “Discovering the unexpected is more important than confirming the known.”
  • “All models are wrong; some models are useful.”
  • “Don’t fall in love with a model.”
  • How, with a minimum of effort, can you discover what does what to what?  Which factors do what to which responses?
  • “Anyone who has never made a mistake has never tried anything new.” – Albert Einstein
  • “Seek computer programs that allow you to do the thinking.”
  • “A computer should make both calculations and graphs.  Both sorts of output should be studied; each will contribute to understanding.”  – F. J. Anscombe
  • “The best time to plan an experiment is after you’ve done it.” – R. A. Fisher
  • “Sometimes the only thing you can do with a poorly designed experiment is to try to find out what it died of.”  – R. A. Fisher
  • The experimenter who believes that only one factor at a time should be varied, is amply provided for by using a factorial experiment.
  • Only in exceptional circumstances do you need or should you attempt to answer all the questions with one experiment.
  • “The business of life is to endeavor to find out what you don’t know from what you do; that’s what I called ‘guessing what was on the other side of the hill.'”  – Duke of Wellington
  • “To find out what happens when you change something, it is necessary to change it.”
  • “An engineer who does not know experimental design is not an engineer.”  – Comment made by to one of the authors by an executive of the Toyota Motor Company
  • “Among those factors to be considered there will usually be the vital few and the trivial many.”  – J. M. Juran
  • “The most exciting phrase to hear in science, the one that heralds discoveries, is not ‘Eureka!’ but ‘Now that’s funny…'” – Isaac Asimov
  • “Not everything that can be counted counts and not everything that counts can be counted.” – Albert Einstein
  • “You can see a lot by just looking.”  – Yogi Berra
  • “Few things are less common than common sense.”
  • “Criteria must be reconsidered at every stage of an investigation.”
  • “With sequential assembly, designs can be built up so that the complexity of the design matches that of the problem.”
  • “A factorial design makes every observation do double (multiple) duty.”  –  Jack Couden

Where the quotes are not attributed, I’m assuming the quote is from one of the authors.  The most well known of the quotes not attributed, above, “All models are wrong; some models are useful.” is widely attributed to George Box in particular, which is accurate.  Although I forgot to confirm that suspicion with him when I saw him over Christmas break, I suspect most of them are from George (as opposed to from Stu or my dad); George is 90 now and still off-the-charts smart, funny, and is probably the best story teller I’ve met in my life.  If he were younger and on Twitter, he’d be one of those guys who churned out highly retweetable chestnuts again and again.

Related thoughts

As you know if you’ve read my blog before, I am a strong proponent of using the Design of Experiments principles laid out in this book and applying them in field of software testing to improve the efficiency and effectiveness of software test case design (e.g., by using pairwise software testing, orthogonal array software testing, and/or combinatorial software testing techniques).  In fact, I decided to create my company’s test case generating tool, called Hexawise, after using Design of Experiments-based test design methods during my time at Accenture in a couple dozen projects and measuring dramatic improvements in tester productivity (as well as dramatic reductions in the amount of time it took to identify and document test cases).  We saw these improvements in every single pilot project when we  used these methods to identify tests.

My goal, in continuing to improve our Hexawise test case generating tool, is to help make the efficiency-enhancing Design of Experiments methods embodied in the book, accessible to “regular” software testers, and more more broadly adopted throughout the software testing field.  Some days, it feels like a shame that the approaches from the Design of Experiments field (extremely well-known and broadly used in manufacturing industries across the globe, in research and development labs of all kinds, in product development projects in chemicals, pharmaceuticals, and a wide variety of other fields), have not made much of an inroad into software testing.  The irony is, it is hard to think of a field in which it is easier, quicker, or immediately obvious to prove that dramatic benefits result from adopting Design of Experiments methods than software testing.  All it takes is for a testing team to decide to do a simple proof of concept pilot.  It could be for as little as a half-day’s testing activity for one tester.  Create a set of pairwise tests with Hexawise or another t00l like James Bach’s AllPairs tool.  Have one tester execute the tests suggested by the test case generating tool. Have the other tester(s) test the same application in parallel.  Measure four things:

  1. How long did it take to create the pairwise / DoE-based test cases?
  2. How many defects were found per hour by the tester(s) who executed the “business as usual” test cases?
  3. How many defects were found per hour by the tester who executed the pairwise / DoE-based tests?
  4. How many defects were identified overall by each plan’s tests?

These four simple measurements will typically demonstrate dramatic improvements in:

  • Speed of test case identification and documentation
  • Efficiency in defects found per hour

As well as consistent improvements to:

  • Overall thoroughness of testing.

A Suggestion: Experiment / Learn / Get the Data / Let the Efficiency and Effectiveness Findings Guide You

I would be thrilled if this blog post gave you the motivation to explore this testing approach and measure the results.  Whether you’ve used similar-sounding techniques before or never heard of DoE-based software testing methods before,  whether you’re a software testing newbie or a grizzled veteran, I suspect the experience of running a structured proof of concept pilot (and seeing the dramatic benefits I’m confident you’ll see) could be a watershed moment in your testing career.  Try it!  If you’re interested in conducting a pilot, I’d be happy to help get you started and if you’d be willing to share the results of your pilot publicly, I’d be able to provide ongoing advice and test plan review.  Send me an email or leave a comment.

To the grizzled and skeptical veterans, (and yes, Mr, Shrini Kulkarni / @shrinik who tweeted “@Hexawise With all due respect. I can’t credit any technique the superpower of 2X defect finding capability. sumthng else must be goingon” before you actually conducted a proof of concept using Design of Experiments-based testing methods and analyzed your findings, I’m lookin’ at you),  I would (re)quote Sophocles: “One must try by doing the thing; for though you think you know it, you have no certainty until you try.” For newer testers, eager to expand your testing knowledge (and perhaps gain an enormous amount of credibility by taking the initiative, while you’re at it), I’d (re)quote Cole Porter: “Experiment and you’ll see!

I’d welcome your comments and questions.  If you’re feeling, “Sounds too good to be true, but heck, I can secure a tester for half a day to run some of these DoE-based / pairwise tests and gather some data to see whether or not it leads to a step-change improvement in efficiency and effectiveness of our testing” and you’re wondering how you’d get started, I’d be happy to help you out and do so at no cost to you.  All I’d ask is that you share your findings with the world (e.g., in your blog or let me use your data as the firms did with their findings in the “Combinatorial Software Testing” article below).

– Justin

Related: (Introductory Hexawise video overview showing 6.5 trillion possible tests reduced, using Design of Experiments techniques to the 37 tests most likely to find defects)

Related: (Article explaining behind Design of Experiments-based software testing techniques such as pairwise, OA, and n-wise testing: Combinatorial Software Testing by Kuhn, Kacker, Lei, and Hunter (pdf download)

Related: (Prior blog post) “In Praise of Data-Driven Management (AKA “Why You Should be Skeptical of HiPPO’s”)”

Related: (My brother’s blog: he’s in IT too and is also a strong proponent of using Design of Experiments-based software test design methods to improve software testing efficiency and effectiveness).

Defect Seen >10 Million Times and Still not Corrected…

I responded to a recent blog post written by Gareth Bowles today and was struck – again – that a defect that must have been seen >10 million times by now has still not been corrected. When anyone responds to a blog post on Blogger.com, the stat counter says “1 comments” instead of correctly stating “1 comment.” What’s up with that?

The clothing company Lands’ End (with the apostrophe erroneously after the s instead of before it) has a bizarre but somewhat logical explanation for why they have printed their grammatical-mistake-laden brand name on millions of pieces of clothing. According to one version of the story I have heard, they printed their first brochures with the typo and couldn’t afford to get it changed. I also remember reading a more detailed explanation in a catalog in the late 80’s to the effect that by the time the company management realized their mistake and tried to get trademark protection on “Land’s End” they discovered that another firm already had trademarked rights to that name. Quick internet searches can’t verify that so perhaps my memory is just playing tricks on me. But I digress. Here’s the defect I wanted to highlight with this post:

For Blogger.com to leave the extra “s” in has me stumped for several reasons. First, this defect has been seen by a ton of people as Blogger.com is, according to Alexa’s site tracking, the world’s 7th most popular site. Second, Blogger.com is owned by Google (among the most competent, quality-oriented IT wizards on the planet) and no trademark protection is preventing the correction. Third, it would seem to be such an easy thing to fix. Fourth, other sites (like wordpress) don’t make the same mistake. Fifth, it doesn’t seem like a “style preference” issue (like spelling traveled with one “L” or two); it seems to me like a pretty clear case of a mistake. It would be a mistake to say “one cars,” “one computers,” or “one pedantic grammarians”; similarly, it is a mistake to say “one comments”. What gives? Anyone have any ideas?

For anyone wondering where the “>10 million times” figure came from, it is pure conjecture on my part. If anyone has a reasoned way to refute or confirm it or propose a better estimate, I’m all ears.

What Software Testers Can Learn from the Game of 20 Questions

Dave Whalen posted a good piece here asserting that software testing, done well, requires a blend of “Science” and “Art”. I recommend it. (He also has a good post about testing databases here).

He includes the statement below which I agree with. If you are a software tester and any doubts about whether all of these methods work (pairwise software testing in particular), I would encourage you to conduct a pilot project on your own and measure the results achieved with and without the technique applied.

From the scientific side, testing can include a number of proven techniques such as equivalency class testing, boundary value analysis, pair-wise testing, etc. These techniques, if used properly, can reduce test times and focus on finding the bugs where they tend to hang out – much like a porch light on a summer night.

My response to Dave’s post, included below, is not especially profound or even well-written, but, hey, I’m in a hurry in the pre-Thanksgiving rush and the topic hit close to home so I couldn’t resist jotting a little something. Enjoy. Please let me know your thoughts / reactions if you have any.

Dave,

Very well said!

I wholeheartedly, enthusiastically agree with your premise. I also wish that more people saw things the same way.

My father co-wrote Statistics for Experimenters which describes the “art and science” within the Design of Experiments (“DoE”) field of applied statistics. Well-run manufacturing companies use DoE techniques in their manufacturing processes. Many companies, such as Toyota see them as an absolutely fundamental part of their processes (yet unfortunately, software testers, who could use DoE techniques such as pairwise and other forms of combinatorial testing, are often ignorant about how to use them properly and the software testing industry as a whole dramatically under-utilizes such techniques…. but I digress).

I brought the book up because it opens up with a good example relevant to the points you made. To win at the game of 20 questions, it is useful to know “the science” of game theory and DoE; choose questions so that there is a 50/50 chance that the answer will be Yes. Someone who knows this technique, all else being equal, will be win more because of their “scientific” approach than someone who doesn’t know the technique. And yet… other stuff (whether the subject matter expertise in this example, or subject matter expertise and “artistic” Exploratory Testing in your example) is indispensable as well.

You can’t truly excel at either 20 Questions or software testing unless you have a good mix of “science” (governed by mathematical principles, proven methods of DoE, etc.) and and “art” (governed by experience, instincts, and subject matter expertise).

– Justin

Update on testing.stackexchange.com

On October 6th, I informally launched testing.stackexchange.com as “the stackoverflow.com for Software Testing” without much hoopla. So far, less than a month later, with no advertising other than word of mouth, the initial results are very promising. We’ve had approximately:

* 70 new users join as members and contributors
* 50 software testing questions
* 160 answers to those questions
* 2,200 views of the questions and answers

The most important development is not reflected in the numbers above. More important, by far, than the number of the participants have joined is the quality of people who are contributing. Members of the forum include some prominent experts including: Jason Huggins (creator of Selenium and cofounder of Sauce Labs, Alan Page and Bj Rollison (of “How we Test Software at Microsoft” fame), Michael Bolton (the testing expert, not the singer), Fred Beringer, Elisabeth Hendrickson, Joe Strazzere, Adam Goucher, Simon Morley, Rob Lambert, Scott Sehlhorst, etc. etc.). Given the high quality people the site has attracted, the quality of the answers delivered has been quite high. Perhaps the quality is also above average because people answering know that their answers will be analyzed by thoughtful testers and voted up (or down) based on how good they are. In short, testers are asking good questions and getting them answered which is why I created the site in the first place. I’m cautiously optimistic about the future: if the site

Members so far include:

The most viewed questions so far include:

The most recent questions being asked and answered are:

I’d like to extend special thanks to Alan Page (who likes the idea so much that has volunteered to join me as a co-manager/Moderator of the site), to Shmuel Gershon, Jason from NC, and Joe Strazzere for being particularly active and to Alan Page, Corey Goldberg, Shmel Gershon, and Konstantin for helping to get the word out about the form through their blog posts telling the world about testing.stackexchange.com. Without their combined help, we’d be nowhere. With their help and support, we’re building a place where software testers can seek and receive high-quality, peer-reviewed answers to their testing questions.

Please help us succeed by spreading the word, asking a few questions, answering a few, and voting on the best answers.

Thanks everyone!

– Justin