Blog Archives

Example 2014.10: Panel by a continuous variable

August 18, 2014
By
Example 2014.10: Panel by a continuous variable

In Example 8.40, side-by-side histograms, we showed how to generate histograms for some continuous variable, for each level of a categorical variable in a data set. An anonymous reader asked how we would do this if both the variables were continuous. ...

Read more »

Example 2014.9: Rolling averages. Also: Second Edition is shipping!

August 11, 2014
By
Example 2014.9: Rolling averages.  Also: Second Edition is shipping!

As of today, the second edition of "SAS and R: Data Management, Statistical Analysis, and Graphics" is shipping from CRC Press, Amazon, and other booksellers. There are lots of additional examples from this blog, new organization, and other features ...

Read more »

Example 2014.8: Estimate power for an interaction, by simulation

June 30, 2014
By
Example 2014.8: Estimate power for an interaction, by simulation

In our last entry, we demonstrated how to simulate data from a logistic regression with an interaction between a dichotomous and a continuous covariate. In this entry we show how to use the simulation to estimate the power to detect that interaction. ...

Read more »

Example 2014.7: Simulate logistic regression with an interaction

June 24, 2014
By
Example 2014.7: Simulate logistic regression with an interaction

Reader Annisa Mike asked in a comment on an early post about power calculation for logistic regression with an interaction. This is a topic that has come up with increasing frequency in grant proposals and article submissions. We'll begin by showing how to simulate data with the interaction, and in our next post...

Read more »

Example 2014.6: Comparing medians and the Wilcoxon rank-sum test

June 12, 2014
By
Example 2014.6: Comparing medians and the Wilcoxon rank-sum test

A colleague recently contacted us with the following question: "My outcome is skewed-- how can I compare medians across multiple categories?" What they were asking for was a generalization of the Wilcoxon rank-sum test (also known as the Mann-Whitney-Wilcoxon test, among other monikers) to more than two groups. For the record, the answer...

Read more »

Example 2014.5: Simple mean imputation

April 25, 2014
By
Example 2014.5: Simple mean imputation

We're both users of multiple imputation for missing data. We believe it is the most practical principled method for incorporating the most information into data analysis. In fact, one of our more successful collaborations is a review of software for ...

Read more »

Example 2014.4: Hilbert Matrix

April 14, 2014
By
Example 2014.4: Hilbert Matrix

Rick Wicklin showed how to make a Hilbert matrix in SAS/IML. Rick has a nice discussion of these matrices and why they might be interesting; the value of H_{r,c} is 1/(r+c-1). We show how to make this matrix in the data step and in R. We also show t...

Read more »

Example 2014.3: Allow different variances by group

February 27, 2014
By
Example 2014.3: Allow different variances by group

One common violation of the assumptions needed for linear regression is heterscedasticity by group membership. Both SAS and R can easily accommodate this setting. Our data today comes from a real example of vitamin D supplementation of milk. Four sup...

Read more »

Example 2014.2: Block randomization

January 22, 2014
By
Example 2014.2: Block randomization

This week I had to block-randomize some units. This is ordinarily the sort of thing I would do in SAS, just because it would be faster for me. But I had already started work on the project R, using knitr/LaTeX to make a PDF, so it made sense to continue the work in R. RAs is...

Read more »

Example 2014.1: "Power" for a binomial probability, plus: News!

January 14, 2014
By
Example 2014.1: "Power" for a binomial probability, plus: News!

Hello, folks! I'm pleased to report that Nick and I have turned in the manuscript for the second edition of SAS and R: Data Management, Statistical Analysis, and Graphics. It should be available this summer. New material includes some of our more po...

Read more »