A "chart hit" is a recording, identified by its inclusion in a chart that uses sales or other criteria to rank popular releases, that ranks highly in popularity compared to other songs in the same time frame. "Chart-topper" and related terms (like "number one", "No. 1 hit", "top of the charts", "chart hit", and so forth) are widely used in common conversation and in marketing, and are loosely defined. Because of its value in promoting recording artists and releases, both directly to the consumer, and by encouraging exposure on radio, TV other media, chart positioning has long been a subject of scrutiny and controversy. Chart compilation methodology and data sources vary, ranging from "buzz charts" (based on opinions of various experts and tastemakers), to charts that reflect empirical data such as retail sales. Therefore, a "chart-topper" may be anything from an "insiders' pick" to a runaway seller. Most charts that are used to determine extant mainstream popularity rely on measurable data.