Cortez说的有关leading indicator的缺点，which is input are not static是不是econometrics的问题？
还有看到leading indicator讲义上的缺点好像没有提到这个input，讲义上的缺点包括：
- may provide little more than binary
- can provide false signals
- history subject to frequent revision
- current data not reliable as input for historical analysis (难道是这个？）
- Overfitted in sample. Likely overstates forecast accuracy
