Blog Archives

Two ways that correlation and stepwise regression can give different results

October 8, 2012
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In general, a correlation test is used to test the association between two variables (y and z). However, if there is a third variable (x) that might be related to z or y, it makes...

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More on fixed and random effects: Plotting and interpreting

September 7, 2012
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More on fixed and random effects: Plotting and interpreting

In a recent post I showed how plotting model fits can help to interpret higher-order polynomial terms. The key comparison there was between a model that did and did not have the higher order fixed effect terms. If you're going to use this strategy, you need to...

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Plotting model fits

August 29, 2012
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Plotting model fits

We all know that it is important to plot your data and explore the data visually to make sure you understand it. The same is true for your model fits. First, you want to make sure that the model is fitting...

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Customizing ggplot graphs

August 7, 2012
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There are many things I love about the R package ggplot2. For the most part, they fall into two categories:The "grammar of graphics" approach builds a hierarchical relationship between the data and the graphic, which creates a consistent, int...

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Crawford-Howell (1998) t-test for case-control comparisons

August 6, 2012
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Crawford-Howell (1998) t-test for case-control comparisons

Cognitive neuropsychologists (like me) often need to compare a single case to a small control group, but the standard two-sample t-test does not work for this because the case is only one observation. Several different approaches have been proposed and in a new paper just published in Cortex, Crawford and Garthwaite (2012) demonstrate that the Crawford-Howell (1998) t-test...

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