Using a statistic or figure
Whenever you need a fact, statistic or figure to back your ideas up or to launch your ideas from, simply footnote or bracket your source.

An example:
There were 20,000 people at the demonstration (NY City Parks Department estimation, NY Times, Late Edition, 4/23/98). Although so many people turned out, very few were hopeful it would make a difference to the mayor who is determined to make education cutbacks.

This is also an example of where a footnote would be better than a bracketed note. Note that the information slows the writing down.