Actually not only the numerator in this calculation is very broad (just observed yes or no) with no indication of numbers present but the denominator
(the total number of sheets submitted for the reporting period) is a very clunky measure. It is all we have on these data. It would help to know what the general range of
the number of data sheets / year is and how stable that is over the years and whether the proportion of data sheets out of the total, taken from suitable habitat and time of year the birds are around is sort of stable over time. That would be a far more difficult
question and involve some arbitrary choices. Actually given how loose the raw material is on this, it is really rather good to see that the variation from year to year is as small as it is. I still would want to know why the first two years are high before
seeing the rest as a decline. Was it higher earlier than 1991?
From: Paul Fennell [
Sent: Friday, 8 April, 2016 1:19 PM
To: David Rees
Cc: Jenny Bounds; Canberrabirds
Subject: Re: [canberrabirds] Scarlet Robin and Thompson
The graph is from the general surveys that COG collects since 1991. I interpret it as a very gradual, but inexorable decline over the period. The reporting rate is the percentage of survey sheets which report Scarlet Robins, divided by
the total number of sheets submitted for the reporting period, converted to a percentage. The area covered is the COG area of interest, although the majority of sheets submitted will be within the ACT.
