What does No possible value means?

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  • Updated 3 years ago
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Hi, may i ask what does "No possible value" means in the output of 2-way generated tests?

Thank you!

Helen
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Helen Noynay

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  • confused

Posted 8 years ago

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Sean Johnson, CTO

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Official Response
Helen,

Sure! I assume you have some invalid pairs in this test plan? What the "no possible value" is trying to tell you is that the test case is providing coverage for a needed pair, and that the way you have your invalid pairs setup then prevents this test from being able to provide a value for that parameter in light of that needed pair and the invalid pairs. That sounds confusing, I know, so an example is much easier to understand.

Let's say we have a test plan with 3 parameters, each with 2 values:

Fruit: Apple, Pear
Car: Toyota, Dodge
Dog: Collie, Mutt

And let's further suppose we have 2 invalid pairs:

if Fruit = Apple then Car != Toyota
if Car = Dodge then Dog != Mutt

To create 2-way coverage, Hexawise will ensure you've paired every parameter value with every other parameter value (unless an invalid pair says it shouldn't be paired), which in this case means that Hexawise will necessarily pair Fruit as Apple with Dog as Mutt in at least one test case, since that pairing could be the source of a bug. You probably already see the problem!

In the test case that has Fruit as Apple and Dog as Mutt we need to have a test value for the Car parameter. You can't have Car as Toyota, because Apple can't be paired with Toyota, and you can't have Car as Dodge, because Mutt can't be paired with Dodge. So what value can Hexawise provide for Car in this test case? It has no value to provide, there is none, so it provides "no possible value".

That's why you get these cases. Generally, given the real context of your actual test plan, it is clear what to do to resolve these. Sometimes your invalid pairs may need a bit of adjusting, sometimes you might want to introduce a "N/A" value for a parameter, etc.

If you have a real-life case where it's not so clear what to do, let us know and one of us would be more than happy to hop on the phone and talk through that test plan with you.

Cheers,
Sean
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Helen Noynay

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Hi Sean,

Yes, i got it now! I'll revise my factor table.
Thank you!
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Sean Johnson, CTO

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I'm happy to help. Good luck, and thanks for the good question.
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F.J.

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Hi Sean -
We're working with a fairly large model right now that's got a fairly large number of constraints.

In your example, it's pretty easy to see what's causing the inability to select a car.

In bigger models, it can be hard to figure out, just from a value of "no possible value" in a variable, what constraints led to that problem.

Would there be any way that additional info could be provided someplace, maybe a popup, but definitely included in exported files so that it could make it faster for me to zero in on the problem?

Thanks!
- FJ
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Sean Johnson, CTO

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FJ,

It's a good idea and having a tooltip with an explanation for the no possible value is something we've been working on for some time. It can be tricky for us humans in complicated cases to determine why a particular no possible value case exists, so the algorithm to do the same is also tricky to get right for all possible cases, but we really want a tooltip there to explain it.

Cheers,
Sean
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Rob

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For those of you sitting at your desk with an Odie confused look on your face....write down the above example in blue, "X" out the invalid pairs, the visual will allow you to see the light.
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Justin Hunter, Hexawise Founder, Founder and CEO

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This is one of the trickier issues that can arise when designing tests with this kind of pairwise testing approach (AKA allpairs testing / all pairs testing / orthogonal array testing / OATS / combinatorial testing / combination testing) approach.

After Sean answered this question in this user forum, he also wrote a more detailed blog post about Hexawise no possible value issues:

If you're still reading this far and want to read further, I recommend checking out Sean's more detailed blog post: https://hexawise.com/posts/whaddya-mean-no-possible-value