The short answer is that there is no way to ensure the sample is representative of the population. The best technique is random selection but even that is not a guarantee. It merely guarantees that any differences between the sample and the population is a matter of chance. Since your population is in fact a population, you should not do two "sample" t tests, but one sample t tests using the population mean for the null. Unfortunately, with a sample of size 10, you will not have much power to detect differences between the sample mean and the population mean. But even if you find no differences, that doesn't mean they do not exist. The short of it is, trust random selection, although the smaller the sample the larger the sampling fluctuations will tend to be.
Paul R. Swank, Ph.D. Professor, Developmental Pediatrics Medical School UT Health Science Center at Houston -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Jeremy Bauer Sent: Tuesday, March 04, 2003 11:16 PM To: [EMAIL PROTECTED] Subject: Testing whether my subsample represents my population...help! Good Day, Out of a population of 220 children, I need to randomly select 10 for use in a pilot experiement. The selection of 10 subjects is based on financial reasons, not on any power calculations. My question is, how do I make sure that the subsample of 10 subjects represent my population of 220? The question seems basic at face value, but I'm just not comfortable with any solutions. It is important that the subsample have similar age, height & weight. Do I just run one t-test for each of the 3 dependent variables? While the sample sizes are unequal, am I correct in saying that unequal sample sizes are "ok" as long as the variances are equal? Any help will be very appreciated! Thank you! Jeremy . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . ================================================================= . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
