Posts Tagged ‘ Data Analysis ’

A quantile definition for skewness

July 19, 2017
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A quantile definition for skewness

Skewness is a measure of the asymmetry of a univariate distribution. I have previously shown how to compute the skewness for data distributions in SAS. The previous article computes Pearson's definition of skewness, which is based on the standardized third central moment of the data. Moment-based statistics are sensitive to [...] The post A quantile definition for skewness appeared first on The DO Loop.

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3 ways to visualize prediction regions for classification problems

July 17, 2017
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3 ways to visualize prediction regions for classification problems

An important problem in machine learning is the "classification problem." In this supervised learning problem, you build a statistical model that predicts a set of categorical outcomes (responses) based on a set of input features (explanatory variables). You do this by training the model on data for which the outcomes [...] The post 3 ways to visualize prediction regions for classification problems appeared first on The DO Loop.

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Test for the equality of two proportions in SAS

July 5, 2017
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Test for the equality of two proportions in SAS

A SAS customer asked how to use SAS to conduct a Z test for the equality of two proportions. He was directed to the SAS Usage Note "Testing the equality of two or more proportions from independent samples." The note says to "specify the CHISQ option in the TABLES statement [...] The post Test for the equality of two proportions in SAS appeared first on The DO Loop.

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The average bootstrap sample omits 36.8% of the data

June 28, 2017
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The average bootstrap sample omits 36.8% of the data

Suppose you roll six identical six-sided dice. Chance are that you will see at least one repeated number. The probability that you will see six unique numbers is very small: only 6! / 6^6 ≈ 0.015. This example can be generalized. If you draw a random sample with replacement from [...] The post The average bootstrap sample omits 36.8% of the data appeared first on The DO Loop.

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Two ways to compute maximum likelihood estimates in SAS

June 14, 2017
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Two ways to compute maximum likelihood estimates in SAS

In a previous article, I showed two ways to define a log-likelihood function in SAS. This article shows two ways to compute maximum likelihood estimates (MLEs) in SAS: the nonlinear optimization subroutines in SAS/IML and the NLMIXED procedure in SAS/STAT. To illustrate these methods, I will use the same data [...] The post Two ways to compute maximum likelihood estimates in SAS appeared first on The DO Loop.

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Two simple ways to construct a log-likelihood function in SAS

June 12, 2017
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Two simple ways to construct a log-likelihood function in SAS

Maximum likelihood estimation (MLE) is a powerful statistical technique that uses optimization techniques to fit parametric models. The technique finds the parameters that are "most likely" to have produced the observed data. SAS provides many tools for nonlinear optimization, so often the hardest part of maximum likelihood is writing down [...] The post Two simple ways to construct a log-likelihood function in SAS appeared first on The DO Loop.

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Runs in coin tosses; patterns in random seating

June 5, 2017
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Runs in coin tosses; patterns in random seating

If you toss a coin 28 times, you would not be surprised to see three heads in a row, such as ...THHHTH.... But what about eight heads in a row? Would a sequence such as THHHHHHHHTH... be a rare event? This question popped into my head last weekend as I [...] The post Runs in coin tosses; patterns in random seating appeared first on The DO Loop.

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Sample quantiles: A comparison of 9 definitions

May 24, 2017
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Sample quantiles: A comparison of 9 definitions

According to Hyndman and Fan ("Sample Quantiles in Statistical Packages," TAS, 1996), there are nine definitions of sample quantiles that commonly appear in statistical software packages. Hyndman and Fan identify three definitions that are based on rounding and six methods that are based on linear interpolation. This blog post shows [...] The post Sample quantiles: A comparison of 9 definitions appeared first on The DO Loop.

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Quantile definitions in SAS

May 22, 2017
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Quantile definitions in SAS

In last week's article about the Flint water crisis, I computed the 90th percentile of a small data set. Although I didn't mention it, the value that I reported is different from the the 90th percentile that is reported in Significance magazine. That is not unusual. The data only had [...] The post Quantile definitions in SAS appeared first on The DO Loop.

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Quantiles and the Flint water crisis

May 17, 2017
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Quantiles and the Flint water crisis

The April 2017 issue of Significance magazine features a cover story by Robert Langkjaer-Bain about the Flint (Michigan) water crisis. For those who don't know, the Flint water crisis started in 2014 when the impoverished city began using the Flint River as a source of city water. The water was [...] The post Quantiles and the Flint water crisis appeared first on The DO Loop.

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