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Adding Variability in Test Case Design

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I love autumn! Yes, I am definitely a boy of summer and very much prefer warmer weather; however, there is something special about autumn. This past weekend my daughter, and my 2 friends Dongyi and her husband Yuning and I participated in the Rum Run sailboat fun race with an overnight raft up at Bainbridge Island’s Port Madison. Saturday morning was quite rainy, but the wind was blowing 15 knots with gusts to 25 knots and NOAA weather radio announcing gale force warnings in Puget Sound. Wow…what a ride! But, it was actually the rather relaxing sail back to my marina on Sunday morning that rekindled the beauty of autumn in my mind. The bright reds, golden yellows, and pastel browns of the foliage seemed to blend into a collage framed by the darkness of the waters of Puget Sound and the snow covered peaks of the Olympic mountains. The beauty of autumn reminds me about change. A sloughing of the old, the cleansing brought about by the pure white snows, eventually followed by the new and fresh growth that blossoms in spring.

Just as the earth goes through variable cycles of rejuvenation, we must also continually update our tests, and (more importantly) the test data we use in our test cases to prevent them from becoming stale. Trees shed their leaves in the autumn and new leaves emerge in the spring, but the tree is fundamentally still the same tree. Similarly, a well-designed test case has a unique fundamental purpose and by changing the variables we can grow the value of that test case. Of course, the cycle of change in test data should be dramatically shorter in duration as compared to the seasonal changes of mother earth.

Here is a simple example of how a well-designed test case using variable test data can increase the value of the information each  test iteration provides through increased confidence and also potentially reduce overall risk. In my role at Microsoft I am in a unique position to not only conduct controlled studies, but I can also implement ideas into practice on enterprise level software projects. One experiment I started about 2 years ago involved multiple groups of testers (sessions) located around the world divided into 3 separate control groups. Each control group tested the identical web page that would display the stock price if the user input a valid stock ticker symbol into a single textbox on the page and pressed the OK button. The only difference in the control groups was the instructions to perform single positive test case with the specific purpose of “ensure any valid stock ticker symbol displays the current stock price for the publicly traded stock specified by its symbol.” The purpose of the study was to determine if different cultural and experiential backgrounds impacted the test data used in a test based on the instructions for a test case. The study collected demographic information on the participants as well as specific inputs applied to the web page. Information on the oracle used by the students was collected anecdotally. Step one in each test was identical because we were not interested in how the tester launched the browser. (Of course this assumes there are other tests that test the multitude of ways to launch a browser and navigate to a URL. Also, if the browser failed to launch the test case is blocked.)

Group 1 was given the most vague instructions for the test case. The instruction was simply:

    1. Launch browser and navigate to [url address]
    2. Enter a valid stock ticker symbol and press the OK button and verify the accuracy of the returned stock price.

The instructions in the test case given to Group 2 were also somewhat vague, but provided a little guidance both on input and oracle.

    1. Launch browser and navigate to [url address]
    2. Enter a valid stock ticker symbol (e.g. “MSFT”)
    3. Press the OK button
    4. Verify the returned stock price is identical to the current stock price listed on the appropriate exchange

Group 3 had similar instructions to Group 2, but the group was given additional guidance as indicated below.

  1. Launch browser and navigate to [url address]

  2. Enter a valid stock ticker symbol from a publicly traded stock listed on any public stock exchange

    Listings of valid stock ticker symbols are on stock exchange web sites such as:
    https://www.nyse.com
    https://www.eoddata.com/Symbols.aspx
    https://www.nasdaq.com
    https://www.londonstockexchange.com

  3. Press the OK Button

  4. Verify the returned stock price is identical to the current stock price listed on the appropriate exchange

Results

The results were mostly not surprising, but rather reinforcing. For example, we expected Group 1 to be rather random, but mostly aligned with ticker symbols they were familiar with. Of course, the majority (90%) of stock ticker symbols entered was MSFT and there was no significant difference in cultural background, locale, experience or educational background. (As this study was conducted at Microsoft I am sure there was some bias as to the symbol entered.) What was most interesting was that testers with no formal training (no previous courses in testing, no CS degree, and read less than one discipline specific book) and with more than 2 years of test experience were approximately more likely (25%) to violate the purpose of the test and enter random or completely invalid data as their first action. In other words, instead of executing the required test their initial reaction was to immediately go on a bug hunt.

In group 2 99% of the participants simply entered the stock ticker symbol “MSFT.” But, what was even more surprising was the fact that one the next day, the same people in that group were given the same exact test, and 95% of them simply reentered MSFT. Perhaps this is laziness, perhaps this is related to the superficial nature of the study, or perhaps this is due to individuals taking the path of least resistance. The percentage of people who entered identical stock ticker symbols on consecutive days was not significantly different between group 1 and group 2.

It should be no surprise that group 3 had the greatest distribution of variable test data applied to the web page. Demographics had no impact on any of the people who were in group 3. The majority of people in group 3 (78%) would select the first stock exchange listed (regardless of what link it was) but there was no significant overlap in the selected stock ticker symbols. When asked to repeat the test on the next day 83% of the participants selected a different link and and a different symbol. Of those who selected the same link 97% selected a different stock ticker symbol. On the down side, approximately 4% of the people simply took the path of least resistance and input MSFT as the test data on both days of the experiment.

Conclusion

One of the most common problems I hear about ‘scripted,’ or pre-defined test cases is that they are too prescriptive and not flexible enough to allow the tester to try things. Of course, a well-designed test case is not simply a prescriptive set of steps inputting the same hard coded test data they run over and over. So, in this study we made the assumption that a scripted test case that specified “Enter MSFT in the textbox” would simply result in the tester entering “MSFT” without any thinking on the part of the tester. Hard-coding variable test data is often times the worse possible way to design a test case.

Vaguely written test cases added some level of variability, but also seemed to increase the probability of the tester executing context free tests outside the scope of the purpose of the test. In fact, what we found was some testers (approx 2%) simply went on a bug hunt and never actually input a valid stock ticker symbol at all during the session.

A test case that provided only one example that is representative of the type of test data required for the test case produced the least desirable results in this study. I am not sure this would be the case in practice. However, based on this study if I were to outsource execution of a test case similar to that used by group 2 the only thing I could guarantee is that MSFT would definitely be tested numerous times, and the variability of other test data would be extremely limited regardless of the number of testers executing that test or the number of iterations.

When faced with a virtually infinite number of possibilities for input variables as test data used in either positive or negative tests we need to test as many possibilities as possible given the available resources in order to increase test coverage and reduce overall risk. So, one way increase the coverage of test data while still achieving the specific purpose of the test case is to provide useful resources that help guide the tester while relying on the tester’s creative thinking skills and curiosity to expand the test coverage.

Of course, we can also increase variability of test data and capture the essence of the tester’s creativity using a similar approach in a well-designed automated test case as well. In fact, a similarly designed automated test case enables us to significantly increase the amount of variable test data that is exercised in order to expand test coverage and increase overall confidence.

Comments

  • Anonymous
    March 21, 2011
    Good point. Thanks for sharing this.