BCal help system: The Tutorial

Step C6: Viewing HPD regions for individual parameters I

HPD regions (computed at a particular percentage) indicate the most likely calendar date range(s) for the parameter in question. BCal forms the interval (or intervals) by choosing the years with the highest posterior probabilities and summing their probabilities until the total probability included in the intervals corresponds to the prescribed percentage. In this manner, any year included in a reported HPD region has a higher posterior probability than any year outside the region.

BCal informs us that the HPD region for alpha 2, computed at 95%, is 455 to 13 cal BP. In this example, we have computed all values to the nearest year. In considering how to report such information, however, we must remember the limitations of the techniques we're using. The calibration curve is decadal and there are quite large errors associated with radiocarbon dating (certainly more than 10 years) as a result we normally round calibrated dates to the nearest ten years. In doing this we must respect any absolute prior information provided for the site. Our prior information for the pond tells us that all dates must be older than 9 BP. This means that we must be very careful when rounding any dates for Layer III to be sure that their early bounds are before 9 BP. As it happens this is not a problem and, unless you have good scientific reasons to do otherwise, we would suggest reporting the 95% HPD region for alpha 2 at this site as 460 to 10 cal BP.

We are not restricted to computing 95% HPD regions, suppose we wish to calculate an HPD region for alpha 2 at 68%. We would enter 68 into the "Percentage" field and select the "Submit" button.