Definition: Reference series
OECD - Business tendency surveys
Cyclical indicator systems are constructed around a reference series. For a system of leading indicators, the reference series is the economic variable whose cyclical movements it is intended to predict. Indices of industrial production and gross domestic product are often used as reference series.
Organization for Economic Cooperation and Development (OECD), "Business Tendency Surveys - A Handbook", OECD, Paris, 2003