For example, while observation intervals are scheduled systematically, observations within an interval are made at random times.

例如，观察的时间间隔有系统的安排，而时间间隔内的观察却是在随机时间内进行的。

For example, while observation intervals are scheduled systematically, observations within an interval are made at random times. That means the researcher might decide to observe only during 15-second intervals that are randomly distributed within each 20-minute period.

例如，虽然观察间隔是系统安排的，但间隔内的观察是在随机时间进行的。这意味着研究人员可能决定仅在 15 秒的间隔内进行观察，这些间隔在每个 20 分钟的时间内随机分布。