offset on priors BEAST

[Gabriela Cruz Luna]

Hi everybody 

I have a very quick question

On BEAUti I'm trying to make my priors for calibrating my tree

I'm following a paper where they suggest using uniform distribution as following:

The root of our tree was calibrated with the crown group pleurodont iguanian Saichangurvel from the late Campanian (70.6 +- 0.6 Mya) (...).We assigned this fossil point calibration to the root of our tree using a uniform distribution prior (70–72 mya)

For the uniform distribution they ask for "lower" "upper" and "offset". I'm guessing lower and upper is the range of years (in this case 70-72) but I don't know what "offset" means? Is this like the range where it says "+-0.6)??

Thanks for your help in advance

           

       

      

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[Jordan Douglas]

Hi Gabriela,

Long answer: All parameteric distributions here have an offset d, where the probability density of x is equal to p(x-d). This is useful for lognormal/gamma distributions for instance, where the minimum value is nornally 0, but the offset can increase the minimum to d. In the special case of uniform distributions, the offset is a redundant parameter because the same can be achieved using 'lower'. 

Short answer: Leave offset at 0,  and set lower to 70 and upper to 71.2.

Jordan

[Gabriela Cruz Luna]

Thank you very much Jordan for your help

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