要理解这一方法，我们首先要清楚美国的大学分为三类-公立，私立和非盈利型；私立学校为了利润而经营。
To understand this, it helps to know that America has three types of university-public ones; private, not-for-profit institutions; and private schools run for profit.
描述性研究法一共分为三类：观察法，案例分析法以及调研法。
There are three main types of descriptive methods: observational methods, case-study methods and survey methods.
供求决定论、斯拉法模型和劳动价值论是三类价格决定理论。
The theory of supply and demand, Sraffa's model and the labor theory of value are the three kinds of price theories.
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