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# How do I ensure that the sample that I obtained is a random sample? Give an example,...

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How do I ensure that the sample that I obtained is a random sample? Give an example, thx.

Posted by nightpast on August 25, 2013 at 1:48 PM via web and tagged with how do i ensure that the sample that i obtained is a random sample?, math

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If you are asking how to select a random sample:

Suppose you want a random sample of people. First you select the population you want to study. Then you assign each person in the population a number.

Use a table (book) of random numbers.(It is important to know that the table of random numbers is truly random. A random number generator on a computer does not create random numbers, but pseudo-random numbers. If you truly need random numbers they should be generated by a nondeterministic process like atomic decay or detection of cosmic rays, etc...) Open the book randomly and choose a number to start at randomly. (This is harder to do than it seems -- people will tend to choose pages in the middle and select a number in the middle of the page, so you will need a way to choose these randomly. Perhaps some nondeterministic event like rolling a set of colored die, etc...)

If you want to choose 25 people from a population, starting at your chosen number read off numbers two at a time. (02=2,etc...) If you get a repeat number you skip to the next 2 digit number until you have the 25 test subjects.

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If you already have a sample and want to know if it was a random sample it is a much harder question.

You can test to see that each digit is as likely to appear as any other digit. (For a large sample, this is necessary but not sufficient. It is easy to come up with a list like this that isn't random e.g. 12345678910111213141516...will fit the bill for a large sample.)

Probably in real life you would ask to see how the sample was created. If there was no nondeterministic mechanism, the list is not random.

Posted by embizze on August 25, 2013 at 5:35 PM (Answer #1)