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Systematic sampling and random sampling are often used in combination.

Definition: selection of different locations 

Reason: humans’ or animals’ behaviour ______（ 填空不写）across circumstances




However, in random sampling, these five 20-minute periods may be distributed randomly over the course of the day. That is to say, intervals between observation periods could vary—some longer others shorter. 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.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. Now let's come to situation sampling. Then, what is situation sampling? It involves studying behaviour in different locations and under different circumstances and conditions. By sampling as many different situations as possible researchers can reduce the chance that their observation results will be peculiar to a certain set of circumstances and conditions. Why? Because people or for that manner animals do not behave in exactly the same way across all situations.
然而,在随机抽样中,这5个20分钟的时间段可以是一天内的任何时候。也就是说,观测时间间隔将不再是一个小时---而是有的长有的短。我想强调的一点是有系统的时间抽样和随机的时间抽样并不总是单独使用的。在研究中我们通常将这两种方法结合使用。例如,尽管观测时间间隔是有系统规定的,但一个时间段内要何时进行观察是不固定的。这意味着研究者可以在一个20分钟的时间段内随意选取个时间点开始，然后只要每隔15秒观察一次即可。接下来我们说说情景抽样。那么,什么是情况抽样呢?它包括在不同的地点,在不同的环境以及不同的条件下研究行为。在情景抽样中，如果有越多不同的情景，研究人员就可以更好地避免他们的观察结果受限于某种特定的环境和条件。为什么?因为人类或动物的行为方式在各种情况下都并不是一模一样的。 翻译by sugarle