Y (Output) is a Function of X (Input) and hence always focus on Xs while DMAIC project. Listing all possible Xs is very much important and it should not be basis one individual's thought/assumption.
Under DMAIC methodoly, there are certain tools, which we use during the Analysis phase to list all possible Xs
1. Brainstorming
2. 5 WHY's Analysis
3. Affinity Diagram (It is used, when the ideas/suggestions are too many, to diffuse the confusion after a brainstorming session by organizing the multiple ideas generated through the sessions. This categorizes a large amount of Ideas/Suggestions into logical groupings.
4. Cause & Effect Diagram
Thanks