Plan-Do-Check-Act (aka PDCA or Plan-Do-Study-Act) cycle is a four-step method used for continuous improvement processes, solving problems and managing change.
The 5 Whys method uses a series of questions to understand the layers of a problem. The 5 Why technique assists in finding the root cause of a problem.
An impact/difficulty matrix is used to compare choices and help a team with decision making based on the impact and difficulty of implementing an idea.