CLO means continuous listening research
The CLO has been performed by the research firm Intomart GFK for many years. It is a continuous study with a panel of 15,000 people, previously it was kept with a paper diary, but nowadays the focus is on completing the Internet (completing on paper is still done, by people who do not have Internet access Have). Because there are many channels (about 45) and listened to the radio in many different places and way, many respondents are needed for a representative investigation. To prevent panel fatigue, 1/12 of the panel is replaced every month, so that a new panel is actually working every year. Each panelist fills a diary of one week per 2 months, one per quarter of which the channel is listened to and at which listening place. Every month, Intomart GFK comes up with the listening figures of the last two months.