Method to identify the real root cause of problems by iteratively asking "why" for the answers. Usually, the first answer does not provide the real root cause. The 5 Whys help to investigate further explanations. Use the fishbone (or Ishikawa) diagram to define possible root causes. Then determine actions and countermeasures. Make sure that similar products/processes are not affected.
Example: Problem = Warehouse is full.
Real cause: Purchasing levels are not calibrated to sales volumes.
Action: Define sales volume over one, two, or three months; change frequency of procurement.
Countermeasure: Check stock levels (in terms of purchasing) on weekly basis; Purchasing frequency and amount.