Blog Archives

Example 8.16: Exact logistic regression

November 30, 2010
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Example 8.16: Exact logistic regression

In example 8.15, on Firth logistic regression, we mentioned alternative approaches to separation troubles. Here we demonstrate exact logistic regression. The code for this appears in the book (section 4.1.2) but we don't show an example of it there. ...

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SAS and R joins SAS-x

November 29, 2010
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SAS and R joins SAS-x

Tal Galili, organizer of the R-bloggers blog aggregator, has opened a new aggregator for people blogging about SAS. If you're unfamiliar with the concept, an aggregator is a single blog which republishes (with permission, in this case) the entries fro...

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Example 8.15: Firth logistic regression

November 22, 2010
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Example 8.15: Firth logistic regression

In logistic regression, when the outcome has low (or high) prevalence, or when there are several interacted categorical predictors, it can happen that for some combination of the predictors, all the observations have the same event status. A similar e...

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Example 8.13: Bike ride plot, part 2

November 8, 2010
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Example 8.13: Bike ride plot, part 2

Before explaining how to make and interpret the plot above, Nick and I want to make a plea for questions--it's hard to come up with useful questions to explore each week!As shown in Example 8.12, data from the Cyclemeter app can be used to make interes...

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Example 8.12: Bike ride plot, part 1

November 1, 2010
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Example 8.12: Bike ride plot, part 1

The iPhone app Cyclemeter uses the phone's GPS capability to record location and other data, and infer speed, while you ride. I took a ride near my house recently, and downloaded the data. I'd like to examine my route and my speed. A simple plot of ...

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Example 8.9: Contrasts

October 12, 2010
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Example 8.9: Contrasts

In example 8.6 we showed how to change the reference category. This is the natural first thought analysts have when their primary comparisons aren't represented in the default output. But our interest might center on a number of comparisons which don...

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Example 8.8: more Hosmer and Lemeshow

October 5, 2010
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Example 8.8: more Hosmer and Lemeshow

This is a special R-only entry.In Example 8.7, we showed the Hosmer and Lemeshow goodness-of-fit test. Today we demonstrate more advanced computational approaches for the test.If you write a function for your own use, it hardly matters what it looks l...

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Example 8.7: Hosmer and Lemeshow goodness-of-fit

September 28, 2010
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Example 8.7: Hosmer and Lemeshow goodness-of-fit

The Hosmer and Lemeshow goodness of fit (GOF) test is a way to assess whether there is evidence for lack of fit in a logistic regression model. Simply put, the test compares the expected and observed number of events in bins defined by the predicted p...

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Example 8.6: Changing the reference category for categorical variables

September 21, 2010
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Example 8.6: Changing the reference category for categorical variables

How can we change the reference category for a categorical variable? This question comes up often in a consulting practice.When including categorical covariates in regression models, there is a question of how to incorporate the categories. One simpl...

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Example 8.5: bubble plots part 3

September 14, 2010
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Example 8.5: bubble plots part 3

An anonymous commenter expressed a desire to see how one might use SAS to draw a bubble plot with bubbles in three colors, corresponding to a fourth variable in the data set. (x, y, z for bubble size, and the category variable.) In a previous entries...

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