Equivalence Class Testing
Today I thought I would talk about Equivalence Class Testing.
Equivalence Class Testing is when you create sets of input that have equivalent expected behavior. You do this until all possible inputs show up in one of the groupings. Now when you test one item from each of the inputs you should be covering all the possible inputs in that grouping.
Some types of inputs lend themselves really well to this type of testing – the first is a range of values. You break it down into values within the valid range, invalid values above the valid range and invalid values below the valid range. Lets have an example to better illustrate what I am trying to describe. We are going to test the appointment-making part of the program. An appointment has four fields:
- Time
- Day
- Month
- Year
Lets look at the month input field. Depending on how it is implemented it could take valid values from 1-12 or it could take the months in a string format. Lets look at the input of values from 1-12 and what equivalence classes world be necessary to test this input.
- valid range of 1-12
- invalid range below the valid range <1
- invalid range above the valid range >12
If the input was a string you would end up with two sets. One that had the valid inputs in it and one that had every thing else you could think to put in it.
So along with the month input you would also have a day, year, and time input. The program is written in such a way that they all have to be tested at the same time. So you list all the groupings for all the other inputs fields and test them at the same time. You will notice that you can test most of the valid test cases at the same time, but you will have to test the error cases one field at a time so you can make sure the error is handled appropriately.
This kind of testing will allow you to keep the number of test cases small, and you could use this type of testing at any level of testing from unit all the way up to acceptance testing. One of the limiting factors of this type of testing is that it assumes identical behavior for all items in a grouping. This is best suited to testing programs where inputs are within ranges or values in sets.