ruby

http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/
http://thoughts.codegram.com/understanding-class-instance-variables-in-ruby/
http://www.devalot.com/articles/2008/09/ruby-singleton
http://rubymonk.com/learning/books/4-ruby-primer-ascent/chapters/45-more-classes/lessons/110-instance-variables

https://ruby-china.org/wiki/coding-style
http://www.alanmacdougall.com/blog/2012/06/08/interactive-debugging-with-pry/

Mobile TA

http://www.vogella.com/tutorials/Robotium/article.html

https://code.google.com/p/robotium/

Histogram

http://toyoizumilab.brain.riken.jp/hideaki/res/histogram.html

http://pubs.research.avayalabs.com/pdfs/ALR-2007-003-paper.pdf

http://stats.stackexchange.com/questions/6753/when-to-use-equal-frequency-histograms