1、Random sampling （in large numbers）provides the best chance ofan unbiased representative sample of a target population。
2、A deliberate effort is made to identify the characteristic of a sample most important for it to be representative of the target population。
3、It is quick，convenient and often the most economicalmethod of sampling。
It has，therefore，been the most common type of sampling。
4、Self-selecting samples are relatively convenient and，if volunteering is made on thebasis of informed consent ，ethical。
1、The larger the target population ，the more difficult it is to sample randomly ，since compiling a selection list ofeveryone becomes more impractical。Ture random sampling is，therefore，very rare。
2、Stratified sampling can be very time consuming，since the subcategories have to
be identified and their proportions in the target population calculated。
3、Opportunity sampling gives very unrepresentative samples and is
often biased on the part of the researcher who may choose subjects who
will be “helpful”。
4、Self-selecting samples are often unrepresentative – being biased on the part of the subject。Volunteers are unlike nonvolunteers in many ways。
能不能不来机器翻译的。。？
能不能继续给翻译翻译啊？您翻译的挺好的。。
追答2 通过慎重的工作，确保最重要的样本代表目标人群。
3 ,它是快速、方便和通常最经济的抽样方法，因此是最常见的取样方法。
4 如果志愿者了是在知情同意、知晓伦理的基础上，那么自选样本是相对方便的。
更大的目标人群，使随机抽样更困难。因为编选一个包含每个人的样本列表显然不切实际。真证随机抽样是非常罕见的。
分层抽样可能非常耗时,因为这些子类必须被识别和按比例计算目标人群。
Opportunity sampling gives very unrepresentative samples and is
often biased on the part of the researcher who may choose subjects who
will be “helpful”。
随意取样提供了很具代表性的样本，这是对一部分选择“有用”部分的研究人员产生偏见。
4、Self-selecting samples are often unrepresentative – being biased on the part of the subject。Volunteers are unlike nonvolunteers in many ways。
因为偏颇的部分问题，自我选择的样本往往不具代表性的。这是由于在许多方面是志愿者和非志愿者是不相同的。