How to Compare Apples to Oranges
- Many industries use the Sigma Quality Level (SQL) to measure the performance of processes. The SQL score of a process is an indicator of how many defects the process has and hence enables us to compare two processes how so ever diverse they may be. In this article I describe how to calculate the SQL score for beginners.
- To find the SQL score of a process we start with ascertaining as to what all or how many defects that can be possible in a process. If our process separates out the good apples from bad apples, one defect can be that a bad apple is selected, and the other defect can be that a good apple is not selected. Therefore, this process can have two defects, D=2. The SQL of our process sorting 1000 apples in a time frame will be as below:-
- Defect opportunities per unit – D (2)
- Total units produced in a time frame – U (1000).
- Total defect opportunities per time frame – D x U, 2 x 1000 = 2000.
- Total actual defects reported – A (suppose 5 defects are reported).
- Defects per million opportunities (DPMO) – A ÷ (D x U) x 1 million = 2500.
- Once we get a DPMO value, we read a SQL score from SQL table below, we select the value closest to our DPMO and read the corresponding SQL. In this case our SQL 4.3, which indicates a very good process. The maximum SQL is 6.
