Public opinion is an expression of the general population's thoughts on a particular issue. It's a useful tool in politics. It is most often determined through public opinion polls, which are surveys commissioned by various groups in order to determine people's thoughts on particular matters. One such group is the Gallup Organization, which is an American company that publishes public opinion research on a multitude of topics through the use of the Gallup Poll. The Gallup Poll uses representative samples, which are polling sections that have the same basic characteristics as the general population. Many different factors affect public opinion, such as politicians, news media, socioeconomic status, major world events, and opinion leaders. Public opinions often change over time, or as these factors change.