请教关于future forecasitng的问题 growth, future, industry

Polonez

题目中遇到说 forecast sale use growth of industry sale而不选use GDP growth,  notes上有说 the "top down" approach to forecasting sales is used, analyst begins with a forecast of GDP growth,.......   请问应该怎么forecast sale呢，top down approach是什么呢？ 谢谢老师