BCal help system: The Tutorial

Step A6:  The groups elicitation page

We should always give BCal information in relative chronological order - earliest first.  This means that we should start by providing information about the earliest group in our chronology.  At the fishpond used in this example, the earliest group is Layer III.  BCal requires a label for the group and labels for the boundary parameters.  Boundary parameters represent the calendar dates associated with the start and end of the group (or phase in this case).  By incorporating these into the calibration, we will not only compute chronological information about the ages of the organic samples, but also information about the boundary parameters for phases which contain them.  Following conventions developed in the literature on Bayesian radiocarbon calibration, we might (as in this tutorial) label early boundary parameters as "alpha" followed by a number (or word) and late boundaries as "beta" followed by a number (or word), so that they are easily identifiable as group boundary parameters.

Once the labels are defined for the group, BCal requires us to provide information about the determinations within that group.  At this point it is worth highlighting the difference between determinations and determination parameters.  Upon submitting an organic sample to a radiocarbon dating laboratory one or more determinations are returned.  A determination is an estimate of the radiocarbon age (in the form of a mean year and standard error) of that sample.  A determination parameter represents the unknown calendar age of that sample.  Usually a determination parameter will correspond to a single determination.  But a determination parameter can also correspond to more than one determination, if several determinations have been obtained from the same organic sample or are known for other reasons to date to exactly the same calendar year.  In BCal, we use the concept of a "pooled mean" to allow combination of such determinations as part of the calibration process. This is explained later in the tutorial.  For this group there is one determination corresponding to a single determination parameter.  Since there is only one determination parameter, the order is irrelevant.  But, by convention, it is a good idea to express this as unordered.

Once the information for Layer III has been supplied, we should enter the information for Layer II.  The main point of interest here is that this group has 4 determinations.  Each of these determinations corresponds to individual determination parameters.  In addition, because the chronological order of the organic samples in unknown, these determination parameters are described as unordered.