On 19 Feb 2004 18:52:35 -0800, [EMAIL PROTECTED] (Michael Eglinton) wrote: > Hello all > > I work for an organisation that receives counts of all 'notifable' > diseases from around NZ. > > I would like to compare this years figures with last years and in the > past we have used the Mantel-Haenszel chi-square test to test if there > has been a linear relationship between years (thus indicating a > change). However we have a problem with this test when cell counts > are small. > > But my question is > > Should I even be using this test at all - to my mind we are using > population data so any differences are real and therefore significant.
The trick is this: "Population differences" are real, so long as it is *administrative* -- you only care about who wins the vote, or how many shoes are needed. The minute that you start thinking about *meaning*, you have to bring in some imagined 'universe' of comparisons. Is there a declining trend? - Yes, if it measures that way. Is the decline something that would happen by chance? - If you ask the question about 'chance', then you are going to say something more meaningful than the previous 'Yes'. It might not be *enormously* more meaningful, but the decline meets a minimal test for being 'interesting' if it *is* larger than chance. > We do have non-sampling error in our estimates (e.g. people who do > not go to a doctor, some diseases are not always recorded because of > the number of cases received e.g. campy in Auckland) but I do not > believe that we have a good handle on the size of this error and it > may be similar from year to year. Is there any change to *explain*? I hope that you don't have a politician on hand who merely wants to claim that there was a decline, and the Party deserves the credit, for being there. Statistics provides a tool, a way to state more concretely something that our intuitions can suggest. By the way, I wonder how complete the record is, for 'notifiable diseases'? - in NZ, or anywhere? Do the doctors properly report all that they see, and do the doctors *see* all, the cases, and do the doctors successfully *diagnose* all the cases that they see? For some diseases, I'm pretty sure that the first success of a public health campaign will be a big increase in the number of reported cases. A measured decline may indicate that reporting is worse. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html "Taxes are the price we pay for civilization." . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
