your formula is right on the money, but suppose your problem supplies no SD - see my recent message in this thread.
"Dennis Roberts" <[EMAIL PROTECTED]> wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > this is the typical margin of error formula for building a confidence > interval were the sample mean is desired to be within a certain distance of > the population mean > > n = sample size > z = z score from nd that will produce desired confidence level (usually > 1.96 for 95% CI) > e = margin of error > > so, typical CI for mu would be: > > samp mean +/- z times standard error of mean > > e or the margin of error here is z * stan error of the mean (let me > symbolize se) > > e = z * se > > for 95% CI .. e = 1.96 * se > > e = 1.96 * (sigma / sqrt n) > > now, what n might it take to produce some e? we can rearrange the formula ... > > sqrt n = (1.96 * sigma) / e > > but, we don't want sqrt n ... we WANT n! > > n = ((1.96 * sigma)/ e) ^2 > > so, what if we wanted to be within 3 points of mu with our sample mean the > population standard deviation or sigma were 15? > > n = ((1.96 * 5) / 3)^2 = about 11 ... > > only would take a SRS of about 11 to be within 3 points of the true mu > value in your 95% confidence interval > > unless i made a mistake someplace > > > At 09:54 AM 9/28/01 -0400, Randy Poe wrote: > >John Jackson wrote: > > > > > the forumla I was using was n = (Z?/e)^2 and attempting to express .05 > > as a > > > fraction of a std dev. > > > >I think you posted that before, and it's still getting > >garbled. We see a Z followed by a question mark, and > >have no idea what was actually intended. > > > > - Randy > > > > > >================================================================= > >Instructions for joining and leaving this list and remarks about > >the problem of INAPPROPRIATE MESSAGES 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 and remarks about > the problem of INAPPROPRIATE MESSAGES are available at > http://jse.stat.ncsu.edu/ > ================================================================= ================================================================= Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =================================================================
