It is often important to provide information from various sources to build a complete picture of a topic, but there are instances where continuity of sources plays a role for presenting comparisons.
If you wanted to compare open land areas in 20 U.S. cities, the optimal sources of information would be those that count open land across differing cities by the same method: acreage, square miles, percent of overall land, city-owned versus privately-owned parcels, etc. Ideally, identifying one source that provides data for all the cities would make for the best comparability. This might be a U.S. government agency or an association that works on open land issues.
Frequently, however, such neat sources aren't available or don't cover every place or variable you need. Locating additional sources that quantify information the same way is a challenge but can be important for supporting the point you want to make.
Sometimes you may find sources that have contradictory information, in this case go back to authority and select information based on credentials of the author or organization publishing the information.