Speaking of sources, if you used a statistic, you need to provide your audience with additional information including where the statistic came from. You should be wary of statistics that seem to appear out of nowhere.
· A poor example: The ten largest cities in the U.S. comprised 54% of the total U.S. population.
· A good example: According to the United States Census Bureau, in 2000, the ten largest cities in the U.S. comprised 54% of the total U.S. population.
In the second example, your audience knows exactly where the statistic comes from (if they don't believe your statistic, they can go and check themselves) and it comes from a reputable source (the U.S. Census Bureau).
This information is from http://owl.english.purdue.edu/owl/resource/672/02/. Visit the OWL website for more information on writing.
