[A] Difficulty of changing one's immigration status.　　[B] Extent of error in card completion.　　[C] Classification of border-crossers.　　[D] Uncertainty in categorization.　　[E] Computer-assisted measurement of illegal immigration.　　[F] Preventing illegal immigration.　　[G] Determining net flow of population.　　Whenever a person enters or leaves the island nation of Freedonia, he or she must fill out an arrival or departure card. The data collected from the cards are entered into a computer database, known as the Inflow/Outflow Record (IOR). The Immigration Bureau uses the IOR to monitor changes in the population of Freedonia, which was estimated in 1994 to be 14.4 million people.　　1. ___________________________　　The cards do more than just help count the number of people coming and going. The people who cross Freedonia's borders are put into one of several categories depending on how they fill out their card. The first category, labelled "Category M" by the Immigration Bureau, is made up of people, usually tourists and business travellers from abroad, whose stay in Freedonia is less than 6 months. In "Category N" are citizens and residents of Freedonia who go abroad for a similar period of time. "Category P" includes foreigners who stay in Freedonia for a period longer than 6 months, while Freedonians who leave the country for more than 6 months are put in "Category Q". Then there are the people who migrate permanently to Freedonia, known as "Category R", and those who permanently emigrate from the island state, who are placed in "Category S". Emigrants, it should be noted, are sometimes former immigrants to Freedonia. 　　2. ___________________________　　One problem with maintaining the IOR is that the departure and arrival cards ask for people's intentions, and intentions do not always become reality. Freedonia's population includes many people who originally entered the country on a temporary visa but who later applied for and were given permanent status; in this way, someone who was Category M becomes Category R. This is not too great a problem as changes in migration status inside the country can easily be tracked and entered into the IOR. It becomes difficult to make accurate categorizations, though, when Freedonians move overseas with plans to return--whether in less than 6 months or after a longer period—but do not, in fact, come back. Similarly, Freedonians who claim to be emigrating to other countries may change their minds and return to Freedonia.　　3. ___________________________　　People may also make mistakes when filling out the cards. In 1994, a study was made of 21,730 arrival and departure cards filled out by people leaving from Freedonia's major airports and seaports. The study showed that one in five cards had errors. A total of 4,008 passengers who were citizens of Freedonia mistakenly said that they were temporary entrants to Freedonia. Of these,18 percent were, in fact, emigrating or Category Q leavers. The study's most important finding was the lack of certainty expressed by departing Freedonians about when they planned to return to Freedonia. The arrival and departure cards were redesigned by the Immigration Bureau after the 1994 study, but while the new cards have been in use for over a decade, no new research has been done.　　4. ___________________________　　The unrecorded movements of people from one category to another make it hard to measure the flow of population, but it should be said that Freedonia is the only nation with high overall immigration that keeps reliable records of departures. In this way, the Immigration Bureau is able to keep track of departing native Freedonians as well as former settlers. By monitoring both immigration and emigration, the Bureau is able to maintain a record of net migration, the total gain or loss of people over a period of time. In other countries with high level of immigration, the issue of net migration has often been neglected.　　5. ___________________________　　One final benefit of the IOR is the help it gives in determining the level of illegal immigration to Freedonia. People who enter Freedonia saying they will stay in the country for under six months will appear automatically in the database as "Category T" if they have not left the country after the end of that period. Unlike countries such as the United States that have little idea of the true extent of illegal immigration entering their borders, Freedonia's Immigration Bureau has shown it is able to keep a fairly accurate count.