The DO Loop

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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Analytics | Learn SAS | Programming Tips

Rick WicklinSeptember 13, 2021 8

The partition problem

The partition problem has many variations, but recently I encountered it as an interactive puzzle on a computer. (Try a similar game yourself!) The player is presented with an old-fashioned pan-balance scale and a set of objects of different weights. The challenge is to divide (or partition) the objects into

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Analytics | Programming Tips

Rick WicklinAugust 30, 2021 0

Bootstrap correlation coefficients in SAS

You can use the bootstrap method to estimate confidence intervals. Unlike formulas, which assume that the data are drawn from a specified distribution (usually the normal distribution), the bootstrap method does not assume a distribution for the data. There are many articles about how to use SAS to bootstrap statistics

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Learn SAS | Programming Tips

Rick WicklinAugust 25, 2021 0

Convert a symmetric matrix from wide to long form

For graphing multivariate data, it is important to be able to convert the data between "wide form" (a separate column for each variable) and "long form" (which contains an indicator variable that assigns a group to each observation). If the data are numeric, the wide data can be represented as

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Learn SAS | Programming Tips

Rick WicklinAugust 11, 2021 0

More on the SWEEP operator for least-square regression models

One of the benefits of using the SWEEP operator is that it enables you to "sweep in" columns (add effects to a model) in any order. This article shows that if you use the SWEEP operator, you can compute a SSCP matrix and use it repeatedly to estimate any linear

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Learn SAS | Programming Tips

Rick WicklinAugust 9, 2021 0

Never multiply with a large permutation matrix

Do you ever use a permutation matrix to change the order of rows or columns in a matrix? Did you know that there is a more efficient way in matrix-oriented languages such as SAS/IML, MATLAB, and R? Remember the following tip: Never multiply with a large permutation matrix! Instead, use

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Programming Tips

Rick WicklinJuly 26, 2021 4

Compare the default definitions for sample quantiles in SAS, R, and Python

A SAS programmer recently asked why his SAS program and his colleague's R program display different estimates for the quantiles of a very small data set (less than 10 observations). I pointed the programmer to my article that compares the nine common definitions for sample quantiles. The article has a

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Learn SAS

Rick WicklinJuly 21, 2021 0

Operations on lists in SAS/IML

To get better at something, you need to practice. That maxim applies to sports, music, and programming. If you want to be a better programmer, you need to write many programs. This article provides an example of forming the intersection of items in a SAS/IML list. It then provides several

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Analytics | Data Visualization | Programming Tips

Rick WicklinJuly 19, 2021 1

Copulas and multivariate distributions with normal marginals

After my recent articles on simulating data by using copulas, many readers commented about the power of copulas. Yes, they are powerful, and the geometry of copulas is beautiful. However, it is important to be aware of the limitations of copulas. This article creates a bizarre example of bivariate data,

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Advanced Analytics

Rick WicklinJuly 5, 2021 18

An introduction to simulating correlated data by using copulas

Do you know what a copula is? It is a popular way to simulate multivariate correlated data. The literature for copulas is mathematically formidable, but this article provides an intuitive introduction to copulas by describing the geometry of the transformations that are involved in the simulation process. Although there are

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