now this is cross-sectional over time

这就是随时间推移的横向研究法
because the females let's say up here,

因为这里的这个女性群体
this group of females is different from that group of females.

这个女性群体不同于之前那个女性群体
If they were the same, and

如果她们是同一群体
you were following the same group of people over time,

那你们就是针对同一群体进行跟踪调查
then you'd have a longitudinal or a cohort study

那样就是纵向研究或者群组研究了
that I'll get to in a minute.

这个我一会儿说
If you're just taking this snapshot in time in 1978,

如果你们只在1978年做了一次研究
then you get another group,

然后就换了研究对象群体
let's say a random group of people from the population

比如从大众中随机选取的一组人
then you've got another group of females,

接着你们选取了另一个女性群体
another group of males,

另一个男性群体