Effort doubling in dht2

[Emilija Reuter]

Hello,

I am currently working on some data to develop abundance estimates for penguins and seals. We have data taken from ship surveys and aerial surveys. 

I have fitted two detection functions, one for each transect type, and want to combine these into one overall density and abundance estimate. I have used dht2 to specifiy the dataframes with observations and transect information, with ddf_id specifiying which df applies. There is no stratification necessary. However, when I run it, the Effort and therefore, Covered Area double. The number of sightings remains unchanged, and sothe outcomes are roughly half of what I expect. I have checked my dataframes for repeat observations and that the correct lengths are provided to dht2. 

Does anyone know where I may be going wrong?

Any advice would be very welcome. 

Thank you,

Emilija Reuter

> sum(cbe.effort$Effort) [1] 9544.67


cbe.geo.Strat <- data.frame(Region.Label = 'All' ,Area = 1500000)

cbe.dht <- dht2(
  observations = cbe.obs,
  transects = cbe.effort,
  geo_strat = cbe.geo.Strat,
  ddf = cbe.obs.ddf,
  binomial_var = FALSE,

  strat_formula  = ~1)

CoveredArea: 30542.94 Effort: 19089.34

[Eric Rexstad]

Emilija

Welcome to the list. It may be the case that you've uncovered a bug; as this "double detection function" feature is relatively new and has not been used in many analyses.

If it is a bug, we ask that you report it using the means described here

https://distancesampling.org/software/bug_reporting.html

and provide a bit more detail as I describe below.

It may be the case that the bug comes in the reporting of effort and covered area, which might not be the same values used in the calculations of density. Did this come from the summary table output?

One thing to test is to perform a simple analysis from only one of the platforms to gauge the approximate density estimate for the survey. Does the combined analysis produce a similar density estimate; if so, perhaps the bug is with the reporting of effort and covered area, rather than the estimate of density and abundance.

There may be others on the list that have experience with dht2 and the double detection function analysis; they may have insights to offer.

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[Emilija Reuter]

Hi Eric, 

Thank you for getting back to me quickly! I do think it may be a bug, and I'll report it tomorrow when I get a moment to write some reproducable code. 

I ran the two detection functions for abundance estimates individually using summary(), and they came back with densities of 0.61 and 0.64. The dht2 result with both detection functions is 0.28. These results & the ones I initally provided came from the summary table output. 

As one last check, I've just run a dht2 with just one of the detection functions and that has the correct Effort value (not doubled). 

Thank you for your help.

Emilija