And the other thing you can do is take self reports like the ones you're doing, hopefully do enough of them, and a random enough sample of the population, that you can extrapolate from that sample to the whole population, and try to estimate what people are eating in that way.

还有其他可以做的,比如大家正在做的这种个体调查,希望能把它做的充分些,如果有充足的随机样本,就可以从样本推断出,整个总体,从而推断哪类人有这样的饮食习惯

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选

Covariance is--we'll call it--now we have two random variables, so cov... I'll just talk about it in a sample term.

协方差是...我们有两个随机变量,x和y的协方差是,从样本的角度来说

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