[RsR] use of lmrob() on ecological time series

Dear Emily

This is a late answer to your message from July 15.

The first issue is the use of robust linear regression of log(abundance) on the year.

I think that this is a very reasonable way to summarize the time series -- as long as

a log-linear trend appears appropriate.

The trend is then measured by the slope. A great advantage of using the log is that

slopes are then on a common scale for rare and abundant species, as a certain slope

corresponds to a certain percent increase or decrease per year.

You then go on to question the classification into significantly increasing, and so on.

This classification is common but unreasonable.

A 5% increase per year can be significant for one species and insignificant for another,

just because the former shows less random fluctuations than the latter.

We should focus on estimation and supply confidence intervals for characterizing the

(im-)precision.

(You may have read about the controversy about "null hypothesis significance testing"

and p values.)

Since the slopes are used for further analysis, the classification is not needed nor helpful

at all.

In any case, I have not read in detail what is done with the slope. In one paper, it is used

as the target variable in further regression models.

I wonder if such regressions make sense when different species are used in the same

regression. I thought it was a basic paradigm of biology that species have different

ways to react to environments.

If one simply want to show that management is helpful, one might compare managed and

non-managed regions in terms of the number of species (within taxonomic groups?) that

have recovered -- or directly in terms of average slopes for individual species or taxonomic

groups.

Nevertheless, let me add a thought about (2).

I think the expression "non-significant change" is quite appropriate since a change of 0

does not exist in real life. It is likely small (unless fluctuations are big and/or the time series

short, which causes in-significance), but never precisely 0.

Again, a confidence interval says it all: It contains all plausible values of the true slope.

Are these thoughts helpful?

Werner Stahel
M +41 79 784 9330 | P +41 44 364 6424

[RsR] use of lmrob() on ecological time series

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