Boundary Values Testing with example

What is “Boundary Values Testing”?
 
“Boundary Values Testing” is a method that tests the boundary whether it's an input, an output or a performance boundary. Our tests focus on the boundaries values and not on the entire range of data. We will use it when we have a field that can contain a range of values as an input, an output or as a requirement.

How to use “Boundary Values Testing”?
 
If you have a range: (a to b), you will test the following:

Test Case	Value	Expected Result
1	a-1	Invalid
2	a	Valid
3	a+1	Valid
4	b-1	Valid
5	b	Valid
6	b+1	Invalid

According to the ISTQB in "Boundary Values Testing" we only test the following:

Test Case	Value	Expected Result
1	a-1	Invalid
2	a	Valid
3	b	Valid
4	b+1	Invalid

Why ISTQB has less test cases, because we can say that if 'a' and 'a-1' are working well, then we can assume that 'a+1' is working well. The same claim will be about the upper bound. If 'b' and 'b+1' are working well, then we can assume that 'b-1' is working well.
Any option of implementing the "Boundary Values" method you will use is good. Instead of testing the entire range, you can tests 6 or 4 cases and still have confiedence that the software works well.
Here are some more rules from my experience that you can take in consideration:

• Always test 0 if it is inside the range and sometimes even if it's out of range because 0 has special effect on software (like dividing in 0).
• Always test the empty string if it is inside the range and sometimes even if it's out of range.
• Sometimes you can test a value that exists inside the range and not in the boundary just in case…(It allows you to sleep deeper at night…).

Now, let us practice the "Boundary Values Testing" method.
Practice 1
You are testing inventory software that contains a field of the quantity of items. The field can contain any value between 10 to 100 units.

• What is the max number of test cases you will need in order to test the field?
• What is the minimum number of test cases you will need in order to test the field, using boundary testing?

Try to solve it and then continue to read the answer.
Practice 1 - answer
The field contains a range of 10 to 100 (10-100).

• The max number of test cases you will need in order to test the field will be 93 test cases (9, 10, 11, 12…97, 98, 99, 100, 10) or 94 test cases if we include the value of 0.
• The minimum number of test cases you will need in order to test the field, using boundary testing will be 6 test cases (9, 10, 11, 99, 100, 101) or 7 test cases if we include 0.
• Note that we need to add more tests like alphabetic chars and special chars like %, *.

Practice 2
You have a password field that can contain up to 8 characters and must have at least 3 characters. What is the minimum number of test cases you will need in order to test the field? (Pay attention to the requirement that specifies the field's length and not what kind of chars it can get! In the real world we can't ignore it but in order to simplify the example we will ignore it).
Try to solve it and then continue to read the answer.

Practice 2 - answer
6 test cases: length 2, length 3, length 4, length 7, length 8, length 9 or 7 test cases Length 2, length 3, length 4, length 6, length 7, length 8, length 9. We can add a test case that test the empty string. 

Practice 3
You have a field that can contain up to 5 digits and can have 0 digits. The value of the field can be in the range of (-5148 to +3544) What is the minimum number of test cases you will need in order to test the field using "Boundary" testing?
Try to solve it and then continue to read the answer.

Practice 3 - answer

• For the length we have 5 test cases: 0 length, 1 length, 4 length, 5 length and 6 length.
• For the range we have 7 test cases: -5149, -5148, -5147, 0, +3543, +3544, +3545.
• Total of 12 tests cases.

we reduce the amount of test cases to be less then 12 (try to solve it and continue to read)?
Well, we can reduce it to 10 test cases by combine the value testing of the field with the length testing of the field.

Case #	Length	Value	Expected Result
1	0	None	Valid
2	1	0	Valid
3	4	3543	Valid
4	4	3544	Valid
5	4	3545	Invalid
6	5	-5149	Invalid
7	5	-5148	Valid
8	5	-5147	Valid
9	6	123456	Invalid
10	6	-45322	Invalid

This was an example of why you must use your head all the time, because sometimes you can combine methodologies with each other or combine them with your common sense and reduce the amount of testing or create smart tests that will reveal beautiful and important bugs.