In practice, it often happens that data is missing for some reason. If you take Mathematica's data about the countries of the world, not all of them will have updated information on population, GDP, and other parameters. In order to check the data for completeness, you can use the
This formula computes how many countries do not contain the information of the
PopulationGrowth parameter. The
Map function substitutes as an argument of the
CountryData function values for all the countries that are stored in the database. The
_Missing filter computes only those countries whose information is missing.