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Rick WicklinMay 13, 2019 0

Write to a SAS data set from inside a SAS/IML loop

In SAS/IML programs, a common task is to write values in a matrix to a SAS data set. For some programs, the values you want to write are in a matrix and you use the CREATE FROM/APPEND FROM syntax to create the data set, as follows: proc iml; X =

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Analytics | Learn SAS

Rick WicklinMay 1, 2019 1

Encodings of CLASS variables in SAS regression procedures: A cheat sheet

SAS regression procedures support several parameterizations of classification variables. When a categorical variable is used as an explanatory variable in a regression model, the procedure generates dummy variables that are used to construct a design matrix for the model. The process of forming columns in a design matrix is called

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Learn SAS | Programming Tips

Rick WicklinApril 29, 2019 4

The normal mixture distribution in SAS

Did you know that SAS provides built-in support for working with probability distributions that are finite mixtures of normal distributions? This article shows examples of using the "NormalMix" distribution in SAS and describes a trick that enables you to easily work with distributions that have many components. As with all

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Learn SAS | Programming Tips

Rick WicklinApril 8, 2019 0

Use the FLOOR-MOD trick to allocate items to groups

Suppose you need to assign 100 patients equally among 3 treatment groups in a clinical study. Obviously, an equal allocation is impossible because the second number does not evenly divide the first, but you can get close by assigning 34 patients to one group and 33 to the others. Mathematically,

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Learn SAS | Programming Tips

Rick WicklinApril 3, 2019 0

Convergence in mixed models: When the estimated G matrix is not positive definite

I've previously written about how to deal with nonconvergence when fitting generalized linear regression models. Most generalized linear and mixed models use an iterative optimization process, such as maximum likelihood estimation, to fit parameters. The optimization might not converge, either because the initial guess is poor or because the model

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Learn SAS | Programming Tips

Rick WicklinApril 1, 2019 0

Matrix operations and BY groups

Many SAS procedures support the BY statement, which enables you to perform an analysis for subgroups of the data set. Although the SAS/IML language does not have a built-in "BY statement," there are various techniques that enable you to perform a BY-group analysis. The two I use most often are

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Data Visualization | Learn SAS

Rick WicklinMarch 20, 2019 3

Truncate response surfaces

An analyst was using SAS to analyze some data from an experiment. He noticed that the response variable is always positive (such as volume, size, or weight), but his statistical model predicts some negative responses. He posted the data and asked if it is possible to modify the graph so

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Learn SAS | Programming Tips

Rick WicklinMarch 11, 2019 1

How to detect SAS data sets that contain (or do not contain) character variables

A SAS programmer posted an interesting question on a SAS discussion forum. The programmer wanted to iterate over hundreds of SAS data sets, read in all the character variables, and then do some analysis. However, not every data set contains character variables, and SAS complains when you ask it to

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Data Visualization | Learn SAS

Rick WicklinMarch 6, 2019 3

Use PROC BOXPLOT to display hundreds of box plots

A previous article shows how to use a scatter plot to visualize the average SAT scores for all high schools in North Carolina. The schools are grouped by school districts and ranked according to the median value of the schools in the district. For the school districts that have many

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Data Visualization | Learn SAS

Rick WicklinFebruary 27, 2019 4

3 ways to create nested box plots in SAS

Box plots are a great way to compare the distributions of several subpopulations of your data. For example, box plots are often used in clinical studies to visualize the response of patients in various cohorts. This article describes three techniques to visualize responses when the cohorts have a nested or

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