since the t test or, the ci are based on the same information ....
it has to be: A) the standard ERROR of the difference in means, or B) the critical value for t being used (based on df)
the actual difference in means is a constant
so, these seem to be the only two options to examine
given the intervals you have ... -73 ... 5 .... wider .... -71 ... -3 ... narrower ... i am guessing in the second ... the df is based on n1-- + n2-1 ... rather than the 6 that you have used ... calculators and computers are not likely to use a shortcut rule for such a thing ... the n-1 of the SMALLER sample is just a rule of thumb ...
For example: x1bar = 349, x2bar = 383 (sample means), s1 = 19.6, s2= 39.5 (sample standard deviations), n1 = 10, n2 = 7 (sample sizes), find a 95% confidence interval for mu1 - mu2 (difference in population means). When I use the formula in the textbook with critical t = 2.447 (df = 6, using one less than the smaller sample size) from the table, I get 349 - 383 +/- 2.447*sqrt[19.6^2/10 + 39.5^2/7] which comes out to the interval (-73.56, 5.56). However, when I put the same information into the TI-83 (option "0" on the TESTS menu) and choose "no" for pooled, it gives me the interval (-71.21, 3.21). If I do a t-test with the same data, it comes out the same either way. What am I doing wrong?Thank you. Howard Wachtel Delaware Valley College [EMAIL PROTECTED] . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
_________________________________________________________ dennis roberts, educational psychology, penn state university 208 cedar, AC 8148632401, mailto:[EMAIL PROTECTED] http://roberts.ed.psu.edu/users/droberts/drober~1.htm . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
