The root-cause technique that repeatedly asks “why” and is associated with Sakichi Toyoda is Five whys.
Five whys is an iterative problem-solving method: each answer becomes the basis for the next “why?” The aim is to move past an immediate symptom and identify a process failure that can be corrected. The method is linked to Sakichi Toyoda and became part of the Toyota Production System.
Taiichi Ohno, a key architect of the Toyota Production System, described repeated questioning as central to Toyota’s scientific approach. The technique later spread through kaizen, lean manufacturing, lean construction, and Six Sigma. It is not a rigid rule that exactly five questions must always be asked; complex problems may require fewer or more.
A common mix-up is treating Five whys as identical to a fishbone diagram. A fishbone, or Ishikawa, diagram branches possible causes visually, while Five whys usually follows a causal chain. They can also be used together, and Five whys is only as reliable as the evidence and reasoning behind each answer.