If you are planning to buy a bunch of grapes, how will you choose them? By inspecting each one of them. Of course not! You will pick a few of those grapes as a sample & inspect them and then you will decide whether you should buy that lot or not. If those few grapes that you have randomly picked turned out to be sour, you would conclude that the entire lot is sour and will reject it. And if they turned out to be sweet you will conclude that the whole lot is sweet. The assumption underlying this entire process of selection and rejection of the lot by the sample drawn from them is that the sample represents the population (or in this case the lot).
Questions based on sampling pattern makes their appearance in GMAT now and then. These questions are the easiest to solve provided the student identifies the sample and population correctly. Many times it is not easy. The following example will give you an idea about the intricacies involved in this pattern.
Scientists compared 42 average-weight and 47 obese infants aged 7 to 9 months, on current daily nutrient intake, the ratio of formula or breast milk to solids in the diet, and maternal reliance on external feeding cues, such as time of day. Mothers completed a three-day full record at home before answering questions on current feeding practices. The researchers concluded from all of these data that, contrary to popular belief, the feeding practices of mothers of obese babies do not contribute significantly to their babies’ obesity.
Which of the following could be an assumption on which the researchers relied in drawing their conclusion?
In the above argument when the author of the argument refers to “all of these data,” he is referring to the 3-day record that he has of the feeding practices of mothers of fat babies. These 3- days he believes are representative of the 210 – 270 days that had passed before this survey was conducted. Therefore, In this question the sample is the 3- day record and the population is the 210-270 days that had passed before the survey was conducted.
Strengthening of arguments based on sampling pattern is done by showing that the sample represents the population and Weakening of arguments based on sampling pattern is done by showing that the sample doesn’t represent the population.
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