It is important that people who are equal on the measured construct (e.g., mathematics ability, depression) have an approximately equal probability of answering a test item accurately or acknowledging the presence of a symptom. An example of an item on a mathematics test that might be used in the United Kingdom but not the United States could be the following: "In a football match two players get a red card; how many players are left on the pitch?" This item requires knowledge of football (soccer) to be answered correctly, not just mathematical ability. Thus, group membership can influence the chance of correctly answering items, as encapsulated in the concept of differential item functioning. Often tests are constructed for a specific population and the nature of that population should be taken into account when administering tests outside that population. If a test is invariant to one population (e.g. schoolchildren in the United Kingdom) it does not automatically mean that the test functions in much the same way in another population (e.g. schoolchildren in the United States).