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Select Base year for Collated index #7
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Hi @RetoSchmucki, that's interesting, thanks! Could you let me know what would be the benefit of selecting a reference year instead of the mean of the time-series? I see what it would change on the collated indices graph but what would be the impact on the calculated trends? Thanks |
Hi @RetoSchmucki, linked to this question, in the European Grassland Butterfly Indicator, the log10 species collated indices were standardized to a value of 2 for the first year and not for the time-series average, and so the indicator is set to 100 for the first year. |
Hi @SarahVray, sorry for not replying to your requests earlier. You are correct that the code above standardises to the time series average. For indicators, however, it is convention to set the first year to 100, so it reads as change relative to that year. Although this is the convention, it is not always the best way to assess change (e.g. if your first year is poorly informed or biased). For the workshop code, I need to check the sequence of steps that Emily used and how the conversion was done, but the conversion from the scale 2 to 100 should keep the shape of the relationship (trend). "Would it be better to use the same year of reference for both computations?" - It should not change anything if the transformation between scales is done properly, but YES it would be easier to read and understand the code. Choosing a reference year is not trivial, and one must consider whether that year is representative and well-informed to be used as a baseline. In many cases (BMS), the first year of activity does not provide the most accurate baseline, although it does provide the longest time series. Happy New Year! |
Here is a solution for selecting the reference year to report trends, instead of using the mean of the series.
After bootstrap sampling in Vignette 2....
of transect monitored
set base year
merge the mean log index with the full bootstrap dataset
compute the log collated index for each bootstap samples
compute the metric used for the graph of the Collated Log-Index with base year (observed, bootstap sampel, credible confidence interval, linear trend)
output data
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