1、 Introduction to Absence Valuation
Absence的种类：
病假 ：An employee is sick for a long period of time.
休假：An employee takes a vacation.
产假：An employee goes on maternity leave.
私假：An employee takes unpaid leave.
methods for valuating absences：
用平均值或者常量：Valuating absences using averages or constants
假定：The As If principle