You are essentially paying for the expertise of the reviewer and the cache of the review — a type of external validation of sorts. (badredheadmedia.com)
Our culture's obsession with external sources of entertainment — TV, movies, the Internet, video games — may also play a role in increasing boredom. (scientificamerican.com)
Previous climate model projections of climate change accounted for external forcing from natural and anthropogenic sources but did not attempt to predict internally generated natural variability. (realclimate.org)