Posts Tagged ‘ Statistical Programming ’

Reduced models: A way to choose initial parameters for a mixed model

June 27, 2018
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Reduced models: A way to choose initial parameters for a mixed model

This article describes how to obtain an initial guess for nonlinear regression models, especially nonlinear mixed models. The technique is to first fit a simpler fixed-effects model by replacing the random effects with their expected values. The parameter estimates for the fixed-effects model are often good initial guesses for the

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Use a grid search to find initial parameter values for regression models in SAS

June 25, 2018
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Use a grid search to find initial parameter values for regression models in SAS

When you fit nonlinear fixed-effect or mixed models, it is difficult to guess the model parameters that fit the data. Yet, most nonlinear regression procedures (such as PROC NLIN and PROC NLMIXED in SAS) require that you provide a good guess! If your guess is not good, the fitting algorithm,

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The bootstrap method in SAS: A t test example

June 20, 2018
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The bootstrap method in SAS: A t test example

A previous article provides an example of using the BOOTSTRAP statement in PROC TTEST to compute bootstrap estimates of statistics in a two-sample t test. The BOOTSTRAP statement is new in SAS/STAT 14.3 (SAS 9.4M5). However, you can perform the same bootstrap analysis in earlier releases of SAS by using

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Video: A new syntax for lists in SAS/IML

June 8, 2018
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I recently recorded a short video about the new syntax for specifying and manipulating lists in SAS/IML 14.3. This is a video of my Super Demo at SAS Global Forum 2018. The new syntax supports dynamic arrays, associative arrays ("named lists"), and hierarchical data structures such as lists of lists.

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Random permutations without duplicates

April 16, 2018
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Random permutations without duplicates

A colleague and I recently discussed how to generate random permutations without encountering duplicates. Given a set of n items, there are n! permutations My colleague wants to generate k unique permutations at random from among the total of n!. Said differently, he wants to sample without replacement from the

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Distance correlation

April 4, 2018
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Distance correlation

Correlation is a statistic that measures how closely two variables are related to each other. The most popular definition of correlation is the Pearson product-moment correlation, which is a measurement of the linear relationship between two variables. Many textbooks stress the linear nature of the Pearson correlation and emphasize that

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Find the distance between observations and a target value

March 28, 2018
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Find the distance between observations and a target value

Suppose you want to find observations in multivariate data that are closest to a numerical target value. For example, for the students in the Sashelp.Class data set, you might want to find the students whose (Age, Height, Weight) values are closest to the triplet (13, 62, 100). The way to

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Compute with combinations: Maximize a function over combinations of variables

March 19, 2018
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Compute with combinations: Maximize a function over combinations of variables

About once a month I see a question on the SAS Support Communities that involves what I like to call "computations with combinations." A typical question asks how to find k values (from a set of p values) that maximize or minimize some function, such as "I have 5 variables,

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