# Eric’s Enlightenment for Monday, April 20, 2015

1. John D. Cook explains why $0!$ is defined to be equal to $1$.  This is also an excellent post on how definitions are created in mathematics.
2. Why are GDP estimates often unreliable?  (Jonathan Jones wrote this report for Britain, but it’s likely applicable to all countries.)
3. Rick Wicklin shares useful code for counting the number of missing and non-missing observations in a data set in SAS.
4. A potentially game-changing breakthrough in artificial photosynthesis may be able to solve the world’s carbon emission problem…