Monthly Archives: March 2010

Validating credit card numbers in SAS

March 16, 2010
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Validating credit card numbers in SAS

Major credit card issuing networks (including Visa, MasterCard, Discover, and American Express) allow simple credit card number validation using the Luhn Algorithm (also called the “modulus 10″ or “mod 10″ algorithm). The following code demonstrates an implementation in SAS. The code also validates the credit card number by length and by checking against a short

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In search of a random gamma variate…

March 16, 2010
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In search of a random gamma variate…

One of the most common exersices given to Statistical Computing,Simulation or relevant classes is the generation of random numbers from a gamma distribution. At first this might seem straightforward in terms of the lifesaving relation that exponential and gamma random variables share. So, it’s easy to get a gamma random variate using the fact that

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Nutritional supplements, ranked

March 16, 2010
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Nutritional supplements, ranked

One of my favourite shows on TV right now is The Big Bang Theory. For those who haven't seen it: it's like Friends, except instead of New York yuppies, it's PhD physicists and engineers at CalTech. It's nice to see geeks and smart people be the focus (rather than the comic relief) of a sitcom. Also, the equations on...

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DICOM-to-NIfTI Conversion

March 16, 2010
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DICOM-to-NIfTI Conversion

Now that the two packages oro.dicom and oro.nifti have been released, we can put them together and perform the much sought after conversion from DICOM format to NIfTI format (entirely in R).  Why?  Because DICOM is the international "standard" for medical imaging data coming off the scanners, but it's not the easiest thing to manipulate on...

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Rcpp 0.7.10

March 15, 2010
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Versions 0.7.7 to 0.7.9 of Rcpp contained a bug: protecting paths with quotes was supposed to help with Windows builds, but did the opposite at least in 'backticks mode' for getting path and/or library information. Using the shQuote() function instead...

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Solving the rectangle puzzle

March 15, 2010
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Solving the rectangle puzzle

Given the wrong solution provided in Le Monde and comments from readers, I went to look a bit further on the Web for generic solutions to the rectangle problem. The most satisfactory version I have found so far is Mendelsohn’s in Mathematics Magazine, which gives as the maximal number for a grid. His theorem is

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Robert Brown and Pollen Particles

March 15, 2010
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Robert Brown and Pollen Particles

In 1827, the botanist Robert Brown was studying pollen particles as they floated in water. When viewed through a microscope, he observed that the particles seemed to move around as if the were alive. Although he couldn’t have known at the time, the seemingly random motion was caused by the collision of water molecules

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Visualizing droughts with R

March 15, 2010
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Visualizing droughts with R

Physicist and weather scientist Joe Wheatley used R to design and create a useful visual representation of how drought affects a region over long time-scales. Instead of charting absolute rainfall (or lack thereof), he instead charts the Standardized Precipitation Index (SPI), where extreme values (above 2 or below -2) indicate extreme wetness or dryness compared to the usual precipitation...

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Weighting model fit with ctree in party

March 15, 2010
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Weighting model fit with ctree in party

Conditional inference trees (ctree) in package party allows weighting which is useful when one classification outcome is more important than another. Useful examples are not difficult to imagine: in a marketing direct mailing, a false positive (non-res...

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The Price of Calculation

March 15, 2010
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In a world in which the price of calculation continues to decrease rapidly, but the price of theorem proving continues to hold steady or increase, elementary economics indicates that we ought to spend a larger and larger fraction of our time on calculation.1 Over the next ten years, I hope that more and more mathematically

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