Can you explain a pair-wise defect?

Answer

Orthogonal array is a two-dimensional array in which if we choose any two columns in the array and all the combinations of numbers will appear in those columns. The following figure shows a simple L₉(3⁴) orthogonal array. In this the number 9 indicates that it has 9 rows. The number 4 indicates that it has 4 columns and 3 indicates that each cell contains a 1, 2, and 3. Choose any two columns. Let's choose column 1 and 2. It has (1,1), (1,2), (1,3), (2,1), (2,2), (2,3), (3,1), (3,2), (3,3) combination values. As you can see these values cover all the values in the array. Compare the values with the combination of column 3 and 4 and they will fall in some pair. This is applied in software testing which helps us eliminate duplicate test cases.