Hi Consider the binomial distribution. One calculates an exact probability that for n = 20 and p = .5, p(x = 13) = .xx (whatever .xx is). If one now approximates this with a normal distribution, we find the area between 7.5 and 8.5 and report p = .yy (roughly). That is, we do not (cannot?) say that p is < .yy, as that would be nonsense. Considering now the tails of the distribution, if we want to approximate the p value for x >= 8, then we report (estimated) p between 7.5 and 20, and it would be nonsense to say p <= some value as it could actually be greater given it is an estimate.
Extending this thinking to the p value for a statistical results, we are calculating (estimating) the probability that our test statistic is greater than or equal to the observed value. That is, p = .05 = p(z >= 1.645) = an area or the interval between 1.645 and infinity. I guess another way to think about this is that our observed value may have noise associated with it, but the probability distribution does not. That is, given a specified value for z, t, F, whatever, the probability for that value is exact. Take care Jim Jim Clark Professor & Chair of Psychology 204-786-9757 4L41A -----Original Message----- From: Marc Carter [mailto:[email protected]] Sent: Monday, August 19, 2013 2:00 PM To: Teaching in the Psychological Sciences (TIPS) Subject: RE:[tips] Question... Hi, Jim -- But that's my point (no pun intended): if I report p = .xx, then I'm reporting a point, not an interval. If I report p < .xx, then I'm reporting an interval. Am I misunderstanding? (It's been known to happen...) m -- Marc Carter, PhD Associate Professor of Psychology Chair, Department of Behavioral and Health Sciences College of Arts & Sciences Baker University -- > -----Original Message----- > From: Jim Clark [mailto:[email protected]] > Sent: Monday, August 19, 2013 1:47 PM > To: Teaching in the Psychological Sciences (TIPS) > Subject: RE:[tips] Question... > > Hi > > I think this is a miss-application of that rule. The p value is an > area, not a point estimate. > > Take care > Jim > > Jim Clark > Professor & Chair of Psychology > 204-786-9757 > 4L41A > > -----Original Message----- > From: Marc Carter [mailto:[email protected]] > Sent: Monday, August 19, 2013 1:42 PM > To: Teaching in the Psychological Sciences (TIPS) > Subject: [tips] Question... > > Hi, All -- > > Hope your school years are off to a good start, if they've started (we > start Wednesday and I am SO not ready). > > But I have a question: am I being a pedant if I insist that my > students never report an exact _p_ value? IIRC from my calc days, the > probability of obtaining an exact value of a random variable is zero. > > I suppose I should just go with the flow, but it sort of rankles... > > What do you all think? > > m > > -- > Marc Carter, PhD > Associate Professor of Psychology > Chair, Department of Behavioral and Health Sciences College of Arts & > Sciences Baker University > -- > > > > The information contained in this e-mail and any attachments thereto > ("e-mail") is sent by Baker University ("BU") and is intended to be > confidential and for the use of only the individual or entity named > above. The information may be protected by federal and state privacy > and disclosures acts or other legal rules. If the reader of this > message is not the intended recipient, you are notified that > retention, dissemination, distribution or copying of this e-mail is > strictly prohibited. If you have received this e-mail in error please > immediately notify Baker University by email reply and immediately and > permanently delete this e-mail message and any attachments thereto. > Thank you. > > --- > You are currently subscribed to tips as: [email protected]. > To unsubscribe click here: > http://fsulist.frostburg.edu/u?id=13251.645f86b5cec4da0a56ffea7a891720 > c > 9&n=T&l=tips&o=27152 > or send a blank email to leave-27152- > [email protected] > > --- > You are currently subscribed to tips as: [email protected]. > To unsubscribe click here: > http://fsulist.frostburg.edu/u?id=13029.76c7c563b32ad9d8d09c72a2d17c90 > e > 1&n=T&l=tips&o=27153 > or send a blank email to leave-27153- > [email protected] The information contained in this e-mail and any attachments thereto ("e-mail") is sent by Baker University ("BU") and is intended to be confidential and for the use of only the individual or entity named above. The information may be protected by federal and state privacy and disclosures acts or other legal rules. If the reader of this message is not the intended recipient, you are notified that retention, dissemination, distribution or copying of this e-mail is strictly prohibited. If you have received this e-mail in error please immediately notify Baker University by email reply and immediately and permanently delete this e-mail message and any attachments thereto. Thank you. --- You are currently subscribed to tips as: [email protected]. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13251.645f86b5cec4da0a56ffea7a891720c9&n=T&l=tips&o=27154 or send a blank email to leave-27154-13251.645f86b5cec4da0a56ffea7a89172...@fsulist.frostburg.edu --- You are currently subscribed to tips as: [email protected]. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13090.68da6e6e5325aa33287ff385b70df5d5&n=T&l=tips&o=27156 or send a blank email to leave-27156-13090.68da6e6e5325aa33287ff385b70df...@fsulist.frostburg.edu
