ST 17/12: Visitor statistics


Useful library statistics depend on standardization.

If two libraries define concepts like book, loan, visit and reference questions in different ways, they will be counting different entities. Their statistical data will not be comparable. The same applies to countries that use different definitions.

Two international standards are central for librarians. ISO standard 2789 – on international library statistics – defines the basic concepts involved in statistical observation, counting and measurement.

ISO standard 11620 – on library indicators – recommends indicators that could actually be used to compare libraries, library systems, regions and countries.

ISO standard 2789 was last revised in 2006. A new version will probably appear in late 2012 or early 2013. Voting on the final draft started in August and will continue till November. Some countries find it useful to involve people outside the national representatives in the drafting process.

My comments in this blog post are based on the draft which is being circulated for comments.

What should we mean by a library visit?

Personally and intuitively I would define a genuine library visitor as a person who enters the library premises in order to use the library’s professional facilities as a user. This concept excludes:

  • the comings and goings of the library staff
  • visits by janitors, sales representatives, postmen and others who enter for other (professional) purposes
  • visits by persons who enter for non-library purposes: coffee shopping, free toilets or convenient short-cuts through the building

When we set up a system to register visitors it may be impractical to distinguish customers from people who just happen to enter for other reasons.  But that is not a reason to be vague about the concepts involved,

The ISO standard looks ambiguous.

A visit  simply defined (2.2.40) is as a person (individual) entering the library premises. But the procedure  (6.2.10) adds: where necessary, the count should be adjusted to deduct entrances and exits of library staff, and of any persons visiting other institutions or departments situated within the library building. 

This seems unclear. I would rather define a visit as

a person who enters the library premises in order to use the library’s professional facilities as a user.

If the actual measurement is carried out by turnstile count or electronic counter,  there are various sources of errors. For instance:
  • persons who enter for other purposes
  • persons who walk closely together (so that two count as one)
  • children who are carried (two count as one)
  • persons who walk in and out several times, such as playful children
  • non-persons that break the electronic beam, such as dogs and suitcases
There may be additional errors due to the actual location of the turnstile or the electronic. With manual counts, such errors can mostly be avoided.
If the library suspects substantial errors of measurement (more than +/- 5%, say) , the counter should be calibrated (by running a manual count in parallell for a suitable period of time).
Sampling procedures
The draft standard suggests that
  • any of these methods, but particularly the manual count, may be used for one or more sample time periods and grossed up to give an annual estimate.
  • The method used should be reported.
The draft makes some general remarks about sampling:
  • this International Standard allows for the application of sampling methods, where data cannot be collected from automated systems, or where data collection over a reporting period would be too time-consuming.
  • For the calculation of a reliable sample size, handbooks of statistical procedures should be consulted.
  • Where statistics have been compiled by sample survey rather than from a full count, the method used should be indicated.
  • Care needs to be taken that samples are representative as regards time, place and selection methods, and that bias is not present in the responses. 

But in practice a sample size of one (!) or just a few “normal weeks” is recommended:

  • The annual total is to be established from a sample count. The sample should be taken in one or more normal weeks and grossed up.

Such time samples can hardly be called representative in a statistical sense.


The indicator standard (ISO 11620:2008) includes two indicators based on the number of visits:

  1. Library Visits per Capita. To assess the library’s success in attracting users of all its services (B.2.2.1)
  2. Cost per Library Visit. To assess the cost of the library’s service related to the number of library visits (B.3.1.4)

The first one is widely used. The second is seldom used (and not very informative, I would add)

The first one (A) can be improved by dividing the population (target group, size N) into users and non-users.
  1. The number of library visits per user (B) shows the (average) intenbsity of use among the actual users
  2. The percentage of users (P) and non-users (Q) shows the degree of penetration (P + Q = 100%)

The numbers are related by a simple formula:

  • A = B * P


  • The total number of visits = A * N = B * (P * N) + 0 * (Q * N)
  • Divide by N on both sides
  • A = B * P + 0 * Q = B * P



One Response to “ST 17/12: Visitor statistics”

  1. ST 60/12: Statistikken er et språk « Samstat Says:

    […] ST 17/12: Visitor statistics […]

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