In article <[EMAIL PROTECTED]>, jim clark <[EMAIL PROTECTED]> wrote: >Hi
>On 21 Mar 2003, Dennis Roberts wrote: >> At 09:55 PM 3/21/03 +0000, Jerry Dallal wrote: >> >dennis roberts wrote: >> > > could someone give an example or two ... of how p values have really >> > > advanced our knowledge and understanding of some particular phenomenon? >> >Pick up any issue of JAMA or NEJM. >> no ... that is not sufficient ... just to look at a journal ... where p >> values are used ... does not answer the question above ... that's circular >> and just shows HOW they are used ... not what benefit is derived FROM there use >> i would like an example or two where ... one can make a cogent argument >> that p ... in it's own right ... helps us understand the SIZE of an effect >> ... the IMPORTANCE of an effect ... the PRACTICAL benefit of an effect >> maybe you could select one or two instances from an issue of the journal >> ... and lay them out in a post? >Am I missing something ... isn't it important to determine >whether an effect has a low probability of occurring by chance? >If an effect could have too readily occurred by chance, then its >size would not seem to matter much and there is no reason to >think that it has practical benefit in general. No one is saying >that p values are the be all and end all, but neither does that >mean they have no value for their intended purpose (i.e., >identifying outcomes that are readily explained by random >factors). This is the type of brainwashing which is accomplished by the classical approach. The practical benefit only depends on the size of the effect, and has nothing to do with the chance that something that extreme would have occurred if there was no effect at all. Here is an extreme version of a bad example; there is a disease which is 50% lethal. The old treatment has been given to 1,000,000 people and 510,000 have survived. There is a new treatment which has been given to 3 people, and all have survived. You find you have the disease; which treatment will you take? The first has a very small p-value; it is about 20 sigma out. The second has a probability of 1/8 of occurring by chance if the treatment does nothing. -- This address is for information only. I do not claim that these views are those of the Statistics Department or of Purdue University. Herman Rubin, Deptartment of Statistics, Purdue University [EMAIL PROTECTED] Phone: (765)494-6054 FAX: (765)494-0558 . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
