At 04:14 AM 12/10/01 +0000, Jim Snow wrote: >"Ronny Richardson" <[EMAIL PROTECTED]> wrote in message >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > A few weeks ago, I posted a message about when to use t and when to use z. > >I did not see the earlier postings, so forgive me if I repeat advice already >given.:-) > > 1. The consequences of using the t distribution instead of the normal >distribution for sample sizes greater than 30 are of no importance in >practice.
what's magical about 30? i say 33 ... no actually, i amend that to 28 > 2. There is no good reason for statistical tables for use in practical >analysis of data to give figures for t on numbers of degrees of freedom over >30 except that it makes it simple to routinely use one set of tables when >the variance is estimated from the sample. with software, there is no need for tables ... period! > 3. There are situations where the error variance is known. They >generally arise when the errors in the data arise from the use of a >measuring instrument with known accuracy or when the figures available are >known to be truncated to a certain number of decimal places. For example: > Several drivers use cars in a car pool. The distance tavelled on each >trip by a driver is recorded, based on the odometer reading. Each >observation has an error which is uniformly distributed in (0,0.2). The >variance of this error is (0.2)^2)/12 = .003333 and standard deviation >0.0578 . To calculate confidence limits for the average distance travelled >by each driver, the z statistic should be used. this is pure speculation ... i have yet to hear of any convincing case where the variance is known but, the mean is not _________________________________________________________ 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/ =================================================================
