There is more to understanding a collection age beyond average and thanks to Emma, who made a comment on my last collection analysis post, I thought it would also help to discuss median age in a collection. My experience has been that often “average” and “median” are used interchangeably (which is so very wrong!). Median age of a collection really has some serious power in helping librarians talk about collection age.
First, let us get clear on the difference is between median age and average age of a collection. (Again, as I have done in previous posts, the best way to get a handle on the process is to use a small set of numbers until you feel comfortable.) The average is the sum of all the dates in the set divided by the number of items in the set. (If you are using Excel, it will be the @average function)
Here is the example of some publication dates:
Calculating the average is done by adding all the years published is: 19,925. Then dividing by the 10 books in the set, the average age is 1992.5.
The Median age is similar to average in that it can indicate an overall age of the collection, but it actually gives us more information by considering the distribution of that range. Taking the same 10 books, the oldest in the collection is 1975 and the newest is 2013. Median is calculated by examining the middle point of the range. In our example, the median age is 1982. Of course, the wonderful Excel will also help you calculate this with the @median function. You can find this under formulas tab in the statistical functions group.
In our little example, you can already see the dramatic difference in the resulting statistics. Average indicates an age around 1993 and the median is 1982. So what does this statistic mean? It means that most of the books are older and not just slightly older, but REALLY old. Remember 1982 is the middle of the age range.
The difference between median and average is pretty significant in this small set. Over a large set of data this can be very helpful in getting sense of the age. As with average, it is unfair to include items that are in special collections such as a geneaology or local history collection. Archival material should be excluded in such a context. Like average, using this statistic on the overall library collection. Distinct collections or subjects benefit from this statistic, especially where currency is an issue. (Think legal, medical etc.) I like using it with the teen fiction collection, where I really want the latest and greatest. Practice and compare the median and average ages of your particular collection.
Statistics and analysis of your collection should be a regular part of your management and decisions. Boards, directors and other assorted folks in the money part of the collection equation will appreciate this analysis. It shows consideration and care in your collection.
Originally published at http://www.practicallibrarian.net/collection-analysis-median-vs-average/ on April 8, 2013.