RE: Assigning occasions to sparse data
Mike
There is no requirement that the number of occasions is the same for all
individuals in the study or for all studies in your dataset. So I don't see
why you want the number of occasions to be the same for all ID numbers.
On a similar note - I believe that the division of within subject
variability (WSV) into between occasion (BOV) and within occasion (WOV),
although helpful and somewhat intuitive, is also arbitrary. Indeed if you
took an asymptotic example where you made the occasion to equal to (say) 1
year and your study had samples over 2 years then BOV would turn out to be
rather inconsequential since the parameter values would be averaged over
each occasion and the random variability between occasions is likely to be
quite small.
So, if WSV = BOV + WOV then as the occasion duration tends to infinity then
WSV --> WOV and BOV --> 0. In converse, as occasion duration tends to zero
then WSV --> BOV and WOV --> 0 [assuming you can accurately estimate BOV].
The value of BOV is therefore design specific and hence any benefit from
interpretation of BOV can only be gained if the occasion is set a priori to
a clinically or biologically meaningful interval (e.g. 1 dose interval).
[i.e. To be (perhaps inappropriately) provocative, if large estimated BOV is
bad for your drug then all you need to do is make the occasion duration to
be very long :-)]
Just some thoughts.
Steve
--
Professor Stephen Duffull
Chair of Clinical Pharmacy
School of Pharmacy
University of Otago
PO Box 913 Dunedin
New Zealand
E: [EMAIL PROTECTED]
P: +64 3 479 5044
F: +64 3 479 7034
Design software: www.winpopt.com
Quoted reply history
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] On Behalf Of
> [EMAIL PROTECTED]
> Sent: Thursday, 23 August 2007 7:04 a.m.
> To: [email protected]
> Subject: [NMusers] Assigning occasions to sparse data
>
>
>
> Hi everyone;
>
> I'd like to get some feed-back from the group about how to
> assign occasions for the estimation of inter-occasional
> variability. The situation is this: I have a large Phase 1
> dataset (both single dose and multiple dose) where many
> samples were taken per subject, and "occasion" is readily
> assigned by using period. The maximum number of occasions in
> these data is equal to 4. However, I also have a dataset from
> a Phase 3 study where sparse samples were taken. As you would
> expect, the amount of data from patient to patient is quite
> variable, with some subjects having as few as 2 samples where
> others have as many as 8. Also, the times (relative to
> dosing) are quite variable. What I would like to do is to
> combine these two datasets, and keep the same number of
> occasions that are in the rich dataset by grouping these
> sparse samples by time relative to the first dose. For
> example (and this is arbitrary) I could define Occasion 1 as
> including any sample taken between 0-48 hours, Occasion 2 as
> including any sample between 48-72 hrs, etc. up to Occasion 4.
>
> Does anyone see a problem with this? Or, do you have a better idea?
>
> Many thanks for your time,
>
>
> Mike Fossler
> GSK
>