Look at the 2 graphs below.
If we look only at the first graph, it seems as if America has a serious problem: For the last thirty years, the amount of money America owes to other countries has increased every year. But before we reach any conclusions, we need to ask:
What additional data should I look for to help me understand these data?
The second graph shows us a very different picture. Although America's debt is increasing steadily, this amount is also steadily decreasing in relation to her gross domestic product (gdp - value of all goods and services produced inside the borders of a country in a year). This second graph gives us the background we need to interpret the first graph.
In addition to asking if we need more data before reaching a conclusion, we also need to ask if data are reliable. The two graphs below come from Wikipedia - a source that anyone can edit and change. This means that we need to check the original source of these data, and Wikipedia gives a link to this source:
USA Government,http://www.whitehouse.gov/omb/budget/fy2009/pdf/hist.pdf