List all the potential causes through tools like Fishbone diagram, Brainstorming, etc.,
Verify and Prioritize all the potential causes through Hypothesis testing, PARETO ANALYSIS, FMEA and other tools.
Once Statistically significant difference is proved for Critical Causes, Check whether it is Practically significant(Since sample size affects the result. Always Check the outliers for proper result as that may be affecting results)
Root cause should be both Statistically and Practically significant.