Age and Usage Criteria
There are two factors that contribute to a MUSTIE evaluation, although these factors vary according to the type of material and subject matter. They are the age of the material and usage statistics. For example, the CREW handbook suggests that any book in medicine (the Dewey 610's) that is more than five years old should be considered for weeding. In history, (the Dewey 930's-990's) the age of materials does not become an issue until they are 15 years old. In most areas of non-fiction, materials that have not checked out in three years can be considered for weeding. In fiction, if a book has not gone out in two years, it can become a candidate for weeding. The CREW handbook gives these criteria in detail for all the Dewey classes and also covers audiovisual materials. It also supplies very useful comments that can help you make professional judgments about weeding.
Here is an example from the CREW handbook:
640 (Home economics) 5/3/MUSTIE
Be strict with old sewing and grooming materials in which styles change rapidly. Keep cookbooks unless little used; replace worn popular titles.
In the 5/3/MUSTIE, the five refers to the age of materials. Therefore, materials more than five years old can be considered for weeding. The three refers to the last circulation. Any item that has not been checked out in the last three years can be considered as a possible candidate for weeding. The MUSTIE refers to the MUSTIE criteria. The commentary gives a more detailed look at the category and gives some ideas about how to make professional judgments when weeding this subject area.
In some of the subject areas, one or both of the numbers may be replaced by an "X." This means that either age or circulation dates should not be considered an issue. For example in adult fiction, the formula reads X/2MUSTIE. This means that the age of the item is not an issue, but it must have circulated in the last two years or if it is MUSTIE, it might be considered for weeding.
It should be noted that while the criteria listed in the CREW Manual represent general guidelines for the field, there may be reasons to have different guidelines for your library. For example, a library that has a very small budget may want to extend the age of materials requirements by a few years. Or if a library has not been well-used, the criteria on circulation may also be extended. In some cases, a library may want to use more stringent criteria. For example, if a library is overcrowded and heavily used, it may want to shorten the age and use criteria because of the shortage of space. Any change in the criteria should be carefully thought through and justified. If the governing authority has stated that the CREW Manual will be the basis for the weeding policy, then changes should also be approved by that authority as a matter of policy. Notes of changes should be clearly written in the CREW Manual for each section where changes have been made.
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