stata中允许的四种权重： 1.  fweights, or frequency weights, are weights that indicate the number of duplicated observations.
    2.  pweights, or sampling weights, are weights that denote the inverse of the probability that the
        observation is included because of the sampling design.

    3.  aweights, or analytic weights, are weights that are inversely proportional to the variance of an
        observation; that is, the variance of the jth observation is assumed to be sigma^2/w_j, where w_j
        are the weights.  Typically, the observations represent averages and the weights are the number
        of elements that gave rise to the average.  For most Stata commands, the recorded scale of
        aweights is irrelevant; Stata internally rescales them to sum to N, the number of observations in
        your data, when it uses them.

    4.  iweights, or importance weights, are weights that indicate the "importance" of the observation in
        some vague sense.  iweights have no formal statistical definition; any command that supports
        iweights will define exactly how they are treated.  Usually, they are intended for use by
        programmers who want to produce a certain computation.
什么含义，特别aweight表示什么意思？具体应用到什么情况下？例如CGSS数据中weight变量的数值为0.9275444，1.334958，0.1913974等等，那么这个权重怎么用？