Suggestion: Create a new complex type for 'count' data / address how EML handles counts

[amoeba]

Count data is quite common ("Count of birds in a plot", "Number of windows in a house") and there are often questions about how to write out the appropriate EML metadata for an attribute. We often use a unit of 'number' though sometimes 'dimensionless' gets used.

@mpsaloha suggested revisiting the idea of having another measurement scale of 'count', to address attributes for counts.

I'm probably not explaining enough detail but I'm writing this issue down so we can at least discuss the treatment of count data in EML when EML undergoes future revision/improvement.

[mbjones]

This has been discussed extensively in the EML community (many, many hours of conversation and emails). The solution that was proposed and that most people adopt via convention is:

• use Dimensionless->number as the UnitType and Unit to indicate that its a count

Alternatives that I have also seen used are:

• use Amount->dimensionless
• create a custom unit named count that has Dimensionless as its UnitType

They are all reasonably equivalent, and most humans understand it. Overall, unsatisfactory, but ultimately it works fine if one can read the attribute definition and understand that the count is of human blood cells, versus another count of antelope.

This issue of how SI does not provide adequate representation of counts is being debated for the next release of the SI Units standards. See Section 5 of Mohr and Williams for an excellent treatment of the issues and the complexity. It boils down to: counts are in fact physical quantities that are closely related to the SI unit mol for amount of substance, and one must provide the type of particle counted to have any understanding of what was measured. Examples are given in terms of particle physics, but they apply equally well to other disciplines.

Peter J Mohr and William D Phillips. 2014. Dimensionless units in the SI. Metrologia, Volume 52, Number 1. https://doi.org/10.1088/0026-1394/52/1/40

@article{0026-1394-52-1-40,
  author={Peter J Mohr and William D Phillips},
  title={Dimensionless units in the SI},
  journal={Metrologia},
  volume={52},
  number={1},
  pages={40},
  url={http://stacks.iop.org/0026-1394/52/i=1/a=40},
  year={2015},
  doi={https://doi.org/10.1088/0026-1394/52/1/40},
  abstract={The International System of Units (SI) is supposed to be coherent. That is, when a combination of units is replaced by an equivalent unit, there is no additional numerical factor. Here we consider dimensionless units as defined in the SI, e.g. angular units like radians or steradians and counting units like radioactive decays or molecules. We show that an incoherence may arise when different units of this type are replaced by a single dimensionless unit, the unit ‘one’, and suggest how to properly include such units into the SI in order to remove the incoherence. In particular, we argue that the radian is the appropriate coherent unit for angles and that hertz is not a coherent unit in the SI. We also discuss how including angular and counting units affects the fundamental constants.}
}