Taxonomies and Categorisation - The implications for safety statistics
Taken from a PowerPoint presentation.
Everything is taxonomical
- Any categorisation of anything is part of a taxonomy.
- For example: Red, Orange, Yellow, Blue, Indigo and Violet are a taxonomic categorisation of colours.
- Notice the categories have to be Exhaustive and mutually Exclusive or the taxonomy would be useless.
Databases and Reliability
- Any method of categorising data is to put it into a database (be it electronic or otherwise). E.g. a library.
- The key question is: can two or more people classify the same event/object/situation under the same taxonomic categorisation (e.g. colours)?
- The current taxonomy was developed in a Scottish police force and was tested via a reliability trial.
- There were six classifiers given the SAME event to code/classify: one code per event.
- Reliability was calculated by comparing each classifier with each other classifier.
- 1\2 41.6
- 1\3 45.2
- 1\4 41.9
- 1\5 58.1
- 1\6 48.4
- 2\3 61.3
- 2\4 41.9
- 2\5 45.2
- 2\6 45.8
- 3\4 41.9
- 3\5 35.5
- 3\6 32
- 4\5 32.2
- 4\6 25.8
- 5\6 32.3
Total Reliability or Consensus
- Examples of lack of consensus
- Coder X: Unsafe Right Turn
- Coder Y: Visibility
- Coder X: Disobeying Sign
- Coder Y: Side Road Etc etc. etc. etc. etc……
- Noise in database will rise and rise until database is effectively worthless.
- NOW: important to remember these were experienced classifiers/coders
- This is NOT a training problem.
- What is the problem then?
- The problem is the taxonomy itself.
- Look at the codes: you may well ask yourself: I don't see a taxonomy here! This is just a list of 'causation codes'!
- Well, exactly.
- Taxonomies must be MEE
- Mutually Exclusive and Exhaustive.
- In the list shown, there are 'confounds'.
- For example: to 'change lane without ascertaining that the road is clear' (10) can clearly be the same as 'overtaking improperly' (11)
- 'Mechanical Fault' (37)
- 'Tyre Defective' (38)
- But a defective tyre IS a mechanical fault surely…..
- And so on.
Ideal taxonomies are nested hierarchies
Classifiers move 'through' them via AND/OR gates.
A Taxonomy from Nature.