If you go to Costco and try a sample of the pan-fried brand X dumplings, it's called a "sample" because you are not capable (or willing or allowed) to eat all the brand X dumplings in the store. So a sample is a selected representative of a large group of something. If you like the sample of the pan-fried dumplings, you then assume that all the dumplings of the same brand sold in the store are equally good and you decide to buy a pack. The whole inventory of brand X dumplings in the store is the "population" from which your "sample" comes from. The sample naturally cannot be called a population or vice versa.
An interesting sampling problem is that your sample may have been good for a number of reasons, not just because of the dumpling are made by company X. You may think it's good because...
a) You had to fight the hungry mob surrounding the stand just to get a sample.
b) You happened to be very hungry and everything just tasted better than usual.
c) The sample was cooked with a nice table-top grill and just the right amount of one particular type of oil.
d) The store staff has become an expert in cooking dumplings.
None of these conditions can be replicated when you eat the brand X dumplings you take home. As a result, your sample ends up not quite representing the population of brand X dumpling sold by Costco.
Here's another potential sampling problem. Even though you like the sample at Costco, you decide to buy it elsewhere because the Costco package has enough dumplings for a whole football team and there are only four of you in the family. When you find brand X dumplings in a supermarket, however, the packaging looks different even thought the content seems to be the same. But you assume that your sample should be similar enough to all the same dumplings made by company X and you purchase a bag. You may be disappointed when you eat it at home because it turns out that brand X provides special batches of these dumplings through different packaging process for Costco. And somehow those procedural differences made the Costco version of the dumplings better tasting than the supermarket version.
These considerations are the same for a research project. Let's say I am interested in whether using a tablet in the fourth-grade classroom can enhance the learning outcome of reading comprehension. And I find out that one of the school X fourth-grade teachers allows all their students to use their tablets (from home or provided by the school) to read during a daily reading period while another teacher in the same grade in the same school does not allow the use of tablets during the same reading period. So I compared the tablet group and the no-tablet group in terms of their progress made during the school year based on a reading test administered before and after the school year. These fourth-grade students in the four teachers' classrooms in this particular school constitute my "sample" for the research project, but what I would really like to find out is whether using tablet in the classroom would be beneficial to all fourth graders in the U.S., which would be my "population." Alternatively, I might even be interested in all fourth graders in the world as my "population. So the question is whether or to what extent my sample of fourth-graders can represent my target population. Many factors then become relevant: The particular demographics of these students, the particular teachers' experience... etc.
If school X has many fourth-grade classrooms and teachers, I could say that my target population is simply the fourth graders in this particular school so that my sample of students would be a pretty good representative of the population. But then what is the research value of finding out whether using tablet is beneficial only for this particular school or not?
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