Permutation Tests and Bootstrapping
Never worry about a normal distribution again!
• See our free webinar "Never Worry About a Normal Distribution Again" Webinar page
• This material is now part of our Applied Environmental Statistics online courses. Online Training Center
One of the hardest things in environmental statistics has been to test differences in group means, and compute useful confidence intervals, when data are strongly skewed. Traditional t-tests and analysis of variance look for differences in means, but have low power when data are strongly skewed. Transformation to logarithms often addresses skewness, but the resulting tests on logs evaluate differences in geometric means, not means. Rank-based nonparametric tests look for differences in medians. What’s a scientist to do?
Permutation alternatives to t-tests, analysis of variance, and bootstrap confidence intervals allow inferences about the mean, and confidence intervals around the mean, to be made without assuming normality and without transforming variables.
** Currently these topics have been fully integrated into our Applied Environmental Statistics course, so we are not teaching a separate course on this topic.
What you will learn in AES:
• What are the advantages of permutation tests over parametric tests?
• How do permutation tests work?
• Power of parametric vs nonparametric vs permutation tests
• What is bootstrapping and how does it work?
• Computing bootstrapped confidence intervals
• Computing bootstrapped regression equations
• Permutation tests for paired data
• Permutation tests for two groups (t-test situation)
• Permutation tests for 3+ groups (ANOVA situation)