variable k values vs. optimal k value

Skip to first unread message

Jason Padgett

Jun 7, 2022, 4:12:32 AMJun 7
to OxCal

I am wondering if someone could help me understand the advantages and disadvantages of using variable k values over an optimal k value.

Isn't it possible to determine an optimal k value that accounts for the variable sedimentation rates within a core?

Can one determine the optimal k value by running multiple models each with different k values and comparing a set of indices, e.g., agreement index, mean confidence interval, the number of agreement indices <60%, and mean normalized age deviation?

Are there other methods to determine the optimal k value?

Cheers and thanks,

Christopher Ramsey

Jun 7, 2022, 4:38:06 AMJun 7
to OxCal group

Essentially what you suggest is what variable k does - but the output is then weighted to those values which are most probable rather than picking one value. The main reason for not using it would be if you have other ways to estimate k from the sedimentation rate (varve counts etc). The other (older) approach which is just to make k as high as you can with the Amodel > 60% will probably give higher precision and gives the most optimistic estimate. A possible advantage of this is that the inputs to the model are more explicit. However, the variable k method is more robust - and will generally have higher uncertainties.

Best wishes

> --
> You received this message because you are subscribed to the Google Groups "OxCal" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to
> To view this discussion on the web visit


Jun 7, 2022, 5:19:56 AMJun 7

If you want an optimal value, you could always run a variable k model and take the value with the maximum posterior density. But choosing an optimal value when the true value is unknown seems to me to be deliberately ignoring an aspect of uncertainty that we know is there. It’s equivalent to calibrating a radiocarbon date and just using the mode of the posterior distribution. For that reason I’d always go for a variable-k model over a fixed-k model unless there are strong reasons to think that we can estimate k a priori.


Best wishes



Dr. Andrew Millard

Associate Professor of Archaeology,

Durham University, UK


Personal page:

Scottish Soldiers Project:

Dunbar 1650 MOOC:



From: <> On Behalf Of Jason Padgett
Sent: 06 June 2022 21:13
To: OxCal <>
Subject: variable k values vs. optimal k value




Jason Padgett

Jun 8, 2022, 6:14:24 PMJun 8
to OxCal
Christopher and Andrew,

Right on, your answers are very helpful! Thank you very much!

Reply all
Reply to author
0 new messages