我在学习statistics with stata (updated for version 9)时，其第34--35页有一段话：Occasionally we encounter a string variable where the values are all or mostly numbers. To convert these string values into their numerical counterparts, use the real function. For example, the variable siblings below is a string variable, although it only has one value. “4 or more,” that could not be represented just as easily by a number.但我在运用原书数据包进行操作时，发现在canada2.dta 中，并没有变量 siblings, 底下的describe 和 list 应当如何进行？