Equivalence Class Partitioning Part 3 - The tests

In the last post we decomposed the set of characters in the ANSI character set into valid and invalid class subsets for use in a base filename component on the Windows Xp operating system. The second part of the testing technique of equivalence class partitioning is to then use this information in tests that will adequately evaluate the functional capabilities of the base filename parameter used by COMDLG32.DLL in Windows Xp.

(Note: It is appropriate here to differentiate between a test case and a test. A test case is the hypothesis or assertion that I am either trying to prove or disprove. A test is the process or procedure I am using to prove or disprove my hypothesis. In other words, a test case in this example might be "Validate a base filename can be saved using valid characters in the ANSI 1252 character set." In order to prove or disprove this hypothesis, I will execute multiple tests using the various subsets outlined in the previous post , and as described below. If any test fails, the test case fails. )

There are basically 2 procedures for defining the tests that use the data from an equivalence class table. Paul Jorgensen describes a procedure for robust equivalence class testing in which each valid class subset is tested individually. Glenford Myers suggests a procedure in which valid class subsets are combined in a test until all valid class subsets have been covered. (Jorgensen refers to this as weak equivalence class testing.) Both authors agree that subsets in the invalid subsets must be tested in a way that only one parameter is invalid and any other parameters use valid variables. Using Myers' approach (or what Jorgensen describes as weak equivalence class testing) the tests for the base filename parameter are illustrated in the table below.

[Edited 11/30] I appearantly made too many assumptions about conducting these tests, so I must clarify some of those assumptions.  

• Any time a key is pressed there is essentially a state change. The primary state change we are concerned with for this particular functional test is whether or not a file is saved to the file system. However, in cases of invalid input there are other state changes that may be interesting. In most well designed Windows applications an erroneous input in an input control will be highlighted (usually after an error message is displayed). This is indeed an expected state change. The only time no noticable state change occurs (there is still a state change because the machine is processing WM_Key messages) is with Test #5. But, another noticable state change (the one primarily identified in the table below) is the state change to the listview control.
• There are 3 ways to effectively affect the Save button. One way is to press enter, another is to press the Alt + s key mnemonic, and thirdly to left mouse click the button control (including simulating any of those actions via automation). In some cases the visible state changes may vary; however, the purpose of these tests it so verify the existance of a file name in the file system for valid cases, and the non-existance and no abhorrent side effects in the case of members of the invalid class subsets. So, regardless of the visible state changes, the tester can use any of these procedures to affect the Save button.
• Application of techniques (systematic procedures) at a granular level is very different than simply trying things to see what happens, exploratory testing, guessing, or wild speculation. I made the assumption that readers are familiar with designing and executing atomic functional tests at a granular level in which certain variables are controlled. For example, once we enter an invalid string in the file name text box and press the save button the Save As... dialog is now dirty; meaning that the next input could produce an error but I will not know if the error is a result of an error with the base filename parameter, or with the Save As... dialog state. Equivalence class partitioning is a low level functional test of a single parameter, so in order to test the base filename parameter the tester (or automated test) should minimally close and reinstantiate the Save As... dialog on each test.

Reducing tests while increasing breadth of coverage

The total number of possible tests of valid string combinations for just the base filename parameter using only characters within the ANSI 1252 character set is 214²⁵¹ + 214²⁵⁰ + 214²⁴⁹ + ...214² + 214¹. This number of tests, of course, is a virtual impossibility, so by employing the equivalence class partitioning testing technique we are able to systematically produce a minimum baseline set of tests that has a high probability of both proving or disproving our hypothesis or test purpose, as well as providing great variability in the test data to increase breadth of data coverage. The minimum possible number of valid tests determined by combining at least one element from each valid class subset is only 4 tests. But, let's look at each valid test a little more closely.

Test #3 is probably a red-herring! This is only an invalid filename on Windows NT 4.0 and below. So, if your application is ported from that time frame, and you are using a custom function for your file save functionality rather than Windows APIs then you might consider running this test once. If if passes, you can probably forget running this again ever again on that product. Test #1 evaluates the literal strings in valid subset V1 can be listed in an array or enumeration and one element can be selected at random throughout the development lifecycle, or each literal string can be tested once and the probability of failure in a later build is most likely less than .001%. Test #4 is also a test that probably doesn't require a great deal of retesting of various combinations of elements from subsets V2, V3, V4, V5 , & V7. Elements from the valid class subsets described in Test #2 are most interesting and this is the test that we will probably want to automate and run repeated throughout the development lifecycle because it provides great breadth of coverage. Remember this is the minimum number of valid tests. What isn't covered in this set is common or 'real-world' data sets which we would certainly want to include. Additionally, Test #2 relies on at least one element from each indicated subset. We might also want to consider additional tests that focus on subsets V4 and V5 only. Also, we might also consider testing valid class subset V6 as a special case if we suspected a private function excluded code point values that were not assigned character glyphs. However, if these 4 valid tests pass the probability of failure of any combination of these data sets used in this parameter is minimal. Random selection of elements for Test #2 (and possibly Test #4) may slightly increase the probability of exposing a defect in the base filename parsing routine. Tests #5 through #26 are tests for invalid filenames, or in the case of Test #19 and #20 where the expected result is to truncate the leading or trailing space character.

This of course only analyzes (or tests) the base filename parameter and assumes a nominal valid 3-letter extension, valid filenames do not preexist on the file system, and within the specific context described in the first post. If the context changes, (e.g. this example does not apply to Unix platforms, or Windows Vista, or other parameters) then this set of tests (assuming we would include at least Tests #2, and #5 through #26 as part of our regression suite ran on each new build) would provide a high level of confidence in the functional capability of this specific parameter.

Next, we would also decompose the data for the file extension parameter (which is different than the base filename parameter) because in the File name textbox we can enter either the base filename or the base filename and an extension, and once we verify the functionality of the base filename component, we can then proceed to the next step in analyzing the functionality of the arguments passed to the File name textbox parameter which we shall examine at a later date.

It is important to note the this and any other techniques are simply systematic procedures designed to help us wrap our minds around complex problems. They are not the only approach to testing (only a fool would advocate a single approach to testing), but when used in conjunction with various other techniques, methods, and approaches EQ can help to establish a strong foundation of confidence in low level operations. Of course, as has been previously stated, the limiting factor of this and other functional techniques is the knowledge of the professional tester to think critically and rationally analyze the "overall system" within the appropriate context.

Comments

• Anonymous
  November 29, 2007
  I can think of another way to think about eq classes. In the table you have listed few possible expected results. Let us take one, say "No error message and no state change". How about thinking about all those values for basefile name that produce this result. Let us say if this subset is having a million values, from the principle of equivalance classes, system (windows) treats all them them identicailly producing the result - "No error message and no state change". In this case, using ECP, I am reducing (asserting that) million values to one test that produces an expected result ... What do you say?

• Anonymous
  November 30, 2007
  The comment has been removed

• Anonymous
  December 01, 2007
  The comment has been removed

• Anonymous
  December 02, 2007
  The comment has been removed

• Anonymous
  December 03, 2007
  >>> In your example above "We never know what will happen if we do "X" with the software..." is true. Of course we won't. Because there is NO CONTEXT! Here is one possible rephrase : "We never know what will happen if we do "X" with the software..." is true. Of course we won't. Because there are so many contexts possible, hence the answer would depend upon who is asking and in what context. Context driven software testers are aware of this and constantly practice probing around context. Welcome to context driven testing. !!! Shrini

• Anonymous
  December 03, 2007
  Shrini, I know you have been taught to try to rephrase other peoples words; however, I usually choose my words very carefully with clear intent and purpose. They have definitive and specific meaning. There is no need to rephrase them.  Thus, when I state "there is no context" in your given example, I mean exactly that you have provided a vague and arbitrary example without any context (a set of circumstances or facts that surround a particular event or situation) whatsoever. Since you made the statement "We never know what will happen if we do "X" with the software...", I am asking for you to define the context of "X" and the software. One more time since you've obviously missed the definition of context being the set of circumstances or facts that surround a particular event or situation. So, when you put something in context it actually means to define the set of circumstances, that are possible. Your explaination of "constantly practice probing around context" actually inplies not putting things into context (the denotative meaning), but actually infers guessing about random possibilities in the vicinity of, or near the  set of circumstances (context) for a particular event or situation. So, if you can come back to reality and provide a context for your example then we can have a meaningful conversation. If you can't, then this conversation is pointless psycho-babble. Also, I really hate to burst your bubble, but software testing has been contextually based well before the neologism "context-driven."