At 04:07 PM 1/13/2003, Rich Ulrich wrote:
are you saying that a 95% CI is a 50% CI? if so, explainExactly on this topic, I see a serious mis-statment in the document, under "Planning a study to yield a certain margin of error (M)".Where it says, "how large of a SAMPLE would I need, with 95% confidence, to produce an interval ...." - it ought to say, "... with 50% confidence, ..." since it is doing only the simplest extrapolation from the point-estimate of the SD the pilot study. That 95% was pulled out of thin air, or borrowed by accident (I guess) from the size of the CI. It is unaccounted for; it is a mistake. (But it is not surprising. This power stuff is tricky, and I think that I remember making that same mistake, back when I first started worrying about these, and before I found that "anchor.")
the handout was a follow up from a 95% CI example in the book ... it was not pulled out of thin air ... it is fairly standard ... how many books show you the process of building 50% CIs?
it was not ... it was given as the estimate of the sd based on sample data ... and that is stated (see reference to "have some approximation to the SD in the population")Note that *if* the original SD was given as a population's exact value, then the original CI could be in terms of z; and reducing the size would be an exact formula, where cutting it in half requires exactly 4 times the relevant degrees of freedom. That is to say, it would be an EXACT formula, so the outcome for the CI would be guaranteed, rather than being "95%", or some other quantity.
where is power discussed in this handout? now, i am not suggesting that power is an unrelated matter but .... this handout is not about power ...At the end of the document, it seems to recognize that, indeed, you can do something complicated (a power analysis) to accommodate the observed variability, to get to 95%. In addition, the document overlooks the tentativeness of most real power statements; it misses the "iffy-ness." The statement of outcomes, I think, should be revised with attention to the difference between *observing* an effect-size of 0.1 mpg (What are these terms?), and *hypothesizing* something as being an *underlying* effect. That is: We know that the test with the CI of +/- 0.05 will REJECT if the observed effect is 0.05, not to mention 0.10.
_________________________________________________________-- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html . . ================================================================= 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/ .
=================================================================
