[LTER-im] measurmentScale/precision - what definition? how tohandle?

[Matt Jones]

A short note on Barbara's two-fish-scales problem.  This really begs the 
question "what is an attribute?".  I have always felt that an attribute 
within a table should contain a set of values that can be analyzed 
uniformly because they were measured using the same methods and 
conditions.  Barbara's example shows that some of her attributes 
represent measurements that were taken using different methods, and even 
different instruments.  Thus, one might expect that the precision (and 
other attribute properties) for her measurements is not constant, and 
that some analyses may be sensitive to her different methods.

Personally, I think the right thing in this case is to represent this 
data as two separate attributes, each of which uses a homogeneous set of 
methods and instruments.  If someone wants to combine the two attributes 
together for a particular analysis, they can inspect the attribute 
metadata (e.g., precision), and determine if it would be acceptable for 
their particular analysis.  Combining the measurements using different 
methods together a priori gives the impression that they are homogeneous 
and appropriate for all analytical purposes, which is clearly not the 
case here.

Of course, this is probably inconvenient for the investigators who 
collected the data and know it well.  They probably understand the 
limitations of the data and have a sense of the differences in precision 
among the values.  But, when representing the data for external 
consumption (e.g., through EML and the web), it would probably be better 
to be explicit about these details.

Matt


Tim Bergsma wrote:
> P.S.  This doesn't solve Barbara's two-fish-scales problem.  Alas!  I
> fear it is unsolvable.  Perhaps she should just report the worst of the
> two relevant precisions, or split the table.  

-- 
-------------------------------------------------------------------
Matt Jones                                     jones at nceas.ucsb.edu
http://www.nceas.ucsb.edu/    Fax: 425-920-2439    Ph: 907-789-0496
National Center for Ecological Analysis and Synthesis (NCEAS)
University of California Santa Barbara
Interested in ecological informatics? http://www.ecoinformatics.org
-------------------------------------------------------------------