A group of experts (or decision making units) whose opinions are sought periodically. Ideally, the composition of the panel remains constant over time. In practice, it is not easy to ensure this, so rules must be set up in advance on replacing panel members. Alternatively, one can start with a large panel and then analyze only responses from those who remain for all periods. Panels are used in the Delphi technique, and they can also be used in intentions surveys and for retail sales forecasts, where periodic reports are obtained for sales at representative stores.