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In archaeology, artefacts are commonly classified in categories according to certain criteria (those may include manufacturing technique, decoration, function, chronology, etc).

I am trying to estimate the probability that objects of a certain category $C$ are absent from the site, even after group $G$ has been collected and no category $C$ objects were found. Group $G$ is a “sample” from the group of all objects that were used at the site in a certain moment (actual examples are a waste dump from a house, and domestic tools "frozen" under a collapsed building). Archaeologists will recover all artefacts from the soil but it is an established fact that not all objects will survive, and that we will never recover all of them.

The data I have include some prior knowledge about the (relative and absolute) abundance of $C$: the amount and proportion of $C$ in all groups where it is present, and the total amount of artefacts in all groups, including ones where $C$ is not found (I have other related data but these variables seem the relevant ones). Groups aren't generally very large, around 300-500. Objects of category $C$ account for 1-3% of artefacts when they are found. I expect that the larger the “sample” the higher the probability of absence will be.

A professor of statistics suggested me that methods used by ecologists might be appropriate, e.g. the work of McBride & Johnstone found here: http://www.nzes.org.nz/nzje/contents.php?volume_issue=j35_2 (Credible Interval Value). Is is feasible to apply these methods to the above problem using R, and how? If not, what other methods are appropriate?

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  • $\begingroup$ steko, can you give us a little more info on exactly what you are trying to predict? The method described by McBride & Johnstone is using a Bayesian approach to estimate the likelihood that something existed despite not being measured. Using this method requires estimation of the sensitivity of the measurements being taken and assumes only part of a group was measured. In your question, it sounds like you have complete groups in your data and want to predict how likely a category of items is to be absent within any newly cataloged group. Is this correct? $\endgroup$
    – Dinre
    Commented Feb 4, 2013 at 13:50
  • $\begingroup$ Defining my groups complete is probably not correct: archaeological findings are always an incomplete "sample" of what was used at a certain past moment in a place/house, and a sample from an unknown population. I don't have a formal training in stats so take the terminology I use with some caution. $\endgroup$
    – steko
    Commented Feb 5, 2013 at 16:45
  • $\begingroup$ So the analogy with McBride & Johnstone is that object of category C may have been part of the original group (objects in use in the past) but not part of the objects preserved in the archaeological context. If one category is very abundant, it will be present anyway, but if it is not.. hence the question. $\endgroup$
    – steko
    Commented Feb 5, 2013 at 16:54
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    $\begingroup$ I think the key thing to clarify is whether you want to predict if objects of category C will be absent in the actual finite collection of group G (as your question is currently worded); or if you want an estimate of the probability that objects of category C are at the site, even after group G has been collected and no category C objects were found. McBride and Johnstone's approach is appropriate in the latter case but not in the former (and as you have currently worded your question). $\endgroup$ Commented Feb 6, 2013 at 9:58
  • $\begingroup$ @PeterEllis agreed, my wording is not entirely correct (probably goes hand in hand with my confusion about the subject) and my case is actually as you wrote: I need "an estimate of the probability that objects of category C are at the site, even after group G has been collected and no category C objects were found". I will try to reword my question. $\endgroup$
    – steko
    Commented Feb 6, 2013 at 15:26

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This sounds like a "Coverage" problem. It is a problem that interests Biologists, Software Engineers, Numismatists, Linguists and many others.

Typical solutions have an empirical-Bayes flavor. Lookup Turing-Good estimator, or start with with excellent review [1].

[1] Bunge, J., and M. Fitzpatrick. “Estimating the Number of Species: A Review.” Journal of the American Statistical Association 88, no. 421 (March 1993): 364–373.

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