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Stupid catalog tricks: Subject Headings and the Long Tail

Library of Congress Subject Headings (LCSH) in particular.

I’ve always been down on LCSH because I don’t understand them. They kinda look like a hierarchy, but they’re not really. Things get modifiers. Geography is inline and …weird.

And, of course, in our faceting catalog when you click on a linked LCSH to do an automatic search, you often get nothing but the record you started from. Which is super-annoying.

So, just for kicks, I ran some numbers.

The process

I extracted all the field 650, indicator2=”0″ from our catalog, threw away the subfield 6’s, and threw away any trailing punctuation in any of the subfields. I called the concatenation of what was left a unique LCSH.

Then I printed them out and put them all onto index cards, using tick-marks to indicate…

No, of course not. I used sort, uniq -c, and wc -l. Here’s what I found.

Counts of LCSH

…in round numbers.

In our catalog, there are:

  • 8.50M subject headings (using the definition above)
  • 1.87M unique subject headings
  • …66% of which (1.23M) appear exactly once

We only have to go out to 30K subjects to account for half of all subject entries. The top 1000 most-used subjects account for 14.5% of all 8.5M subject entries.

The top ten subjects by count are:

  • 6029 $$aSermons, American
  • 6131 $$aPhilosophy
  • 7224 $$aFeature films
  • 7591 $$aPiano music
  • 7968 $$aSocialism
  • 8796 $$aEconomics
  • 9185 $$aCommunism
  • 12440 $$aSermons, English$$y17th century
  • 13539 $$aBills, Private$$zUnited States
  • 58823 $$aEconomics$$xHistory$$vSources

From a record’s point of view

Our catalog has:

  • 7M records
  • 4.4M records with at least one subject (as defined above)
  • 2.4M records with more than one subject
  • 2.0M records with exactly one subject
  • 2.6M records with zero subjects

The records with the most subject headings tend to be collections of stuff (theses, photos, etc). Our local standout is the Dept. of Medicine and Surgery (University of Michigan) theses, 1851-1878 with 208 subject entries. 14 records have at least 30 subject entries.

What it means

Gee, lady, I don’t know.

One way to look at it: suppose you’re considering defining subjects in this way, and making them “hot” in the catalog interface. For our data, 2/3 of records would have either no subjects or a subject that found only the record you’re at. So…think again.

In real life, we index lots of possible subject fields, and we additionally index the $$a as well as the whole string, so ours are a little bit more useful. A little.