https://en.wikipedia.org/wiki/Meta-analysis
http://scholar.google.es/scholar_url?url=http://onlinelibrary.wiley.com/doi/10.1111/nep.13225/full&hl=en&sa=X&scisig=AAGBfm1ZFAXoiMGu3BtPCBVKY4Xo9V1LtQ&nossl=1&oi=scholaralrt
https://en.wikipedia.org/wiki/Meta-analysis