"BobA5835" wrote in message
...
Hello All:

We use Excel spreadsheets to gather and calculate data for determining the
Reference Time (Geomean) for use in the formula: PT (patient's time in
seconds)/Reference Time (in seconds) raised to the power of the (reagent's)
ISI. This formula determines the Patient's INR which is used by the
patient's Physician to determine dosing of the drug coumadin in a
coagulation
assay called the Prothrombin Time.

Normally, we obtain 20 to 30 data points with the order of magnitude of
approximately 11.0 to 15.0 seconds, always two digits with one decimal
place.
Excel calculates the Geomean easily with this number of data points.

We have run into a situation where an institution has multiple sites and
the
data we gather can approach 400 data points, i.e., 20 data points by 20
different sites. This gives us approximately 300 to 350 usable
datapoints.
The problem is that Excel will not calculate the Geomean using two digits
and
a single decimal place beyond an 'n' of 274. And yes, we have observed
that
the 'n' is dependant on the order of magnitude of the numbers used.
Single
digits will calculate to a higher 'n' them say double or triple digits.

It appears that this phenomenon is referred to as "overflow" and the
Result
in the cell is #NUM!

I have two questions:

1. Is there a fix for this issue that will allow us to use Geomean to
calculate to a higher 'n' based on the numerical values that we must use?

And

2. And this might be beyond this newsgroup, but I don't know how to
contact
any real mathematicians, if we by groups obtain the Geomeans of an 'n' of
20-30, 20 times and then obtain the Geomean of those, is the final Geomean
obtained legit?

Thank you for any assistance,

Bob Allen


I don't know which version of Excel you are using, or indeed if it matters.
I am using 2000. When I type GEOMEAN into Help, it tells me that the
function can take from 1 to 30 arguments (what you called "data points"). So
I suppose any more than that and the result isn't defined. You could see if
your Excel version is the same. BTW, it's always worth typing the function
name into Help to check you are using it correctly.

To try to answer your second question, taking the GEOMEAN of multiple
GEOMEANS is valid ONLY if the number of data points used to calculate each
is the same. So, if you split your data into groups of exactly 20 (say), you
can calculate the GEOMEAN of each group, and then calculate the overall
GEOMEAN by finding the GEOMEAN of these results. Of course, this will only
work if you can split your data into equal-sized groups.

(It's like averaging averages. This gives a correct overall average only if
the individual averages are calculated from groups of the same size.)