Should you drop outliers? Outliers are one of those statistical issues that everyone knows about, but most people aren’t sure how to deal with.  Most parametric statistics, like means, standard deviations, and correlations, and every statistic based on these, are highly sensitive to outliers. And since the assumptions of common statistical procedures, like linear regression […]

Two excellent resources about multiple imputation and missing data: Joe Schafer’s Multiple Imputation FAQ Page gives more detail about multiple imputation, including a list of references. Paul Allison’s 2001 book Missing Data is the most readable book on the topic. It gives in-depth information on many good approaches to missing data, including multiple imputation. It […]

The best article I’ve read about how to calculate power and sample sizes is Russell V. Lenth’s “Some Practical Guidelines for Effective Sample Size Determination” in The American Statistician (full reference  below).  It is written for statistical consultants who assist researchers who need to make sample size estimates, so it is just a bit on […]

There are probably many myths about statistics, but there is one that I believe leads to the most frustration in researchers (and students) as they attempt to learn and apply statistics. The Carpentry Class: A Fable There was once a man who needed to build a house. He had a big pile of lumber, and […]

SPSS has a nice little feature for adding and averaging variables with missing data that many people don’t know about. It allows you to add or average variables, while specifying how many are allowed to be missing. For example, a very common situation is a researcher needs to average the values of the 5 variables […]

Level is a statistical term that is confusing because it has multiple meanings in different contexts (much like alpha and beta). There are three different uses of the term Level in statistics that mean completely different things. What makes this especially confusing is that all three of them can be used in the exact same […]