Then in systematic time sampling, our observations might be made during five 20-minute periods, beginning every hour.

在系统的时间采样中，我们的观察可能在每小时开始的20分钟内进行，共观察5个时间段。

One point I'd like to make is systematic and random time sampling are not always used in isolation. They are often combined in studies.

我想说的一点是系统的和随机的时间抽样并不总是分开使用。在研究中，它们经常结合在一起。