Sequence without bounaries

[Kate]

Hello!  

I am looking over an oxcal model (paleoseismic study) made by a student that uses the Sequence command but no Boundaries.  I understand that a typical Sequence model with boundaries (e.g., below) assumes dates uniformly distributed ages.  But I have been unable to figure out the underlying assumption when no Boundary commands are used in a sequence. Can you clarify?  

Thanks!

Plot()
 {
  Sequence("WW")
  {
   Boundary("base");

etc....

Boundary("top");
  };
 };

[Erik Marsh]

HI Kate – yes, all models need boundaries. The sequence in your example assumes a sequence of events. It correctly starts and ends the list of events with boundaries.

Your example doesn't assume uniformly distributed ages. To do that, you need a uniform phase, which would look like this:

Sequence("WW")
  {
   Boundary("base");

   Phase("Uniform Phase")

   {

   etc....

    };

   Boundary("top");
  };

This is a very standard setup that often works well with few assumptions.

Hope this helps, Erik

[MILLARD, ANDREW R.]

Dear Kate,

A sequence specifies that the dates have the order shown, and adding boundaries that they are randomly distributed in a between unknown start and end dates. With only a sequence the posterior distributions of dates tend to spread wider than their true range because the ordering assumption pushes the probability distribution of one date away from the next. Adding boundaries allows for the spread of dates being composed of two components - duration of the deposition (which is usually of interest) and the uncertainty of measurement and calibration. Boundaries make the most difference to the outcome when the duration is short compared to uncertainty in the dates, and they tend to have more effect on the posterior distribution of the dates near the start and end of the sequence.

So omitting boundaries is a modelling choice, and like all modelling choices it has consequences for the results. As it is non-standard, and not the recommended approach in the papers that developed these types of models, I would expect to see a justification for its use.

Best wishes 

Andrew 

-- 

Dr. Andrew Millard 

Associate Professor of Archaeology,

Durham University, UK 

Email: A.R.M...@durham.ac.uk  

Personal page: https://www.durham.ac.uk/staff/a-r-millard/ 

Dunbar 1650 MOOC: https://www.futurelearn.com/courses/battle-of-dunbar-1650 

 

---

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Sent: Wednesday, October 2, 2024 14:30
To: OxCal <ox...@googlegroups.com>
Subject: Re: Sequence without bounaries

 

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[Kate]

Thanks - this is helpful.  (I was surprised the model ran without them!) 

Our sequences are long (thousand+ years) thus reason for similar outcomes with and without boundaries.   Cheers. kate