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Rick WicklinJanuary 18, 2022 0

Simulate events when some probabilities are zero

Several probability distributions model the outcomes of various trials when the probabilities of certain events are given. For some distributions, the definitions make sense even when a probability is 0. For other distributions, the definitions do not make sense unless all probabilities are strictly positive. This article examines how zero

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Programming Tips

Rick WicklinJanuary 12, 2022 3

How to assign a name to a color

Some colors have names, such as "Red," "Magenta," and "Dark Olive Green." But the most common way to specify a color is to use a hexadecimal value such as CX556B2F. It is not obvious that "Dark Olive Green" and CX556B2F represent the same color, but they do! I like to

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Analytics | Learn SAS | Programming Tips

Rick WicklinJanuary 10, 2022 0

12 blog posts from 2021 that deserve a second look

On this blog, I write about a diverse set of topics that are relevant to statistical programming and data visualization. In a previous article, I presented some of the most popular blog posts from 2021. The most popular articles often deal with elementary or familiar topics that are useful to

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Programming Tips

Rick WicklinJanuary 5, 2022 0

A block-Cholesky method to simulate multivariate normal data

You can use the Cholesky decomposition of a covariance matrix to simulate data from a correlated multivariate normal distribution. This method is encapsulated in the RANDNORMAL function in SAS/IML software, but you can also perform the computations manually by calling the ROOT function to get the Cholesky root and then

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Analytics | Data Visualization | Programming Tips

Rick WicklinJanuary 3, 2022 3

Top 10 posts from The DO Loop in 2021

Last year, I wrote almost 100 posts for The DO Loop blog. My most popular articles were about data visualization, statistics and data analysis, and simulation and bootstrapping. If you missed any of these gems when they were first published, here are some of the most popular articles from 2021:

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Analytics | Programming Tips

Rick WicklinDecember 6, 2021 1

The expected number of points on a convex hull

While discussing how to compute convex hulls in SAS with a colleague, we wondered how the size of the convex hull compares to the size of the sample. For most distributions of points, I claimed that the size of the convex hull is much less than the size of the

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Learn SAS | Programming Tips

Rick WicklinNovember 29, 2021 5

Caslibs and librefs in SAS Viya

When SAS 9 programmers transition to SAS Viya, there are inevitably questions about how new concepts in Cloud Analytic Services (CAS) relate to similar concepts in SAS. This article discusses the question, "What is the difference between a libref and a caslib?" Both are used to access data, but they

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Data Visualization | Programming Tips

Rick WicklinNovember 17, 2021 0

The order of vertices on a convex polygon

In a previous article, I showed how to use theCVEXHULL function in SAS/IML to compute the convex hull of a finite set of planar points. The convex hull is a convex polygon, which is defined by its vertices. To visualize the polygon, you need to know the vertices in sequential

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Data Visualization | Learn SAS | Programming Tips

Rick WicklinNovember 15, 2021 10

Two-dimensional convex hulls in SAS

Given a cloud of points in the plane, it can be useful to identify the convex hull of the points. The convex hull is the smallest convex set that contains the observations. For a finite set of points, it is a convex polygon that has some of the points as

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Analytics | Programming Tips

Rick WicklinOctober 18, 2021 2

Crossover and mutation: An introduction to two operations in genetic algorithms

This article uses an example to introduce to genetic algorithms (GAs) for optimization. It discusses two operators (mutation and crossover) that are important in implementing a genetic algorithm. It discusses choices that you must make when you implement these operations. Some programmers love using genetic algorithms. Genetic algorithms are heuristic

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