If I want to measure how much business a shopping mall gets, I can stand by an entrance way and count the number of people who go into the mall. If I stop every tenth person on the way out and frisk them for their receipts, I can estimate the per-person amount of money spent, and multiply that by ten to get an idea of how much business is represented by the particular entrance way I am sampling. This would work.
This post excerpts energy and climate material from the Media Balance Newsletter, a free fortnighly published by physicist John Droz Jr., founder of the Alliance for Wise Energy Decisions. The complete Newsletter for this post can be found here.