Re: Computing Relevancy Differently

[Doug Cutting]

Terry Steichen wrote:

Can you give me an idea of what to replace the lengthNorm() method with to,
for example, remove any special weight given to shorter matching documents?

The goal of the default implementation is not to give any special weight to shorter documents, but rather to remove the advantage longer documents have. Longer documents are likely to have more matches simply because they contain more terms. Also, for the query "foo", a document containing just "foo" is a better match than a longer one containing "foo bar baz", since the match is more exact.

However, one problem with this approach can be that very short documents are in fact not very informative. Thus a bias against very short documents is sometimes useful.

I can certainly go through a bunch of trial-and-error efforts, but it would
help if I had some grasp of the logic initially.

For example, from DefaultSimilarity, here's the lengthNorm() method:

  public float lengthNorm(String fieldName, int numTerms) {
    return (float)(1.0 / Math.sqrt(numTerms));
  }

Should I (for the purpose of eliminating any size bias) override it to
always return a 1?

That's something to try, although, as mentioned above, I suspect your top hits will be dominated by long documents. Try it. It's really not a difficult experiment!

One trick I've used to keep very short documents from dominating results, that, while good matches, are not informative documents, is to override this with something like:

public float lengthNorm(String fieldName, int numTerms) {
super.lengthNorm(fieldName, Math.max(numTerms, 100));
}

This way all fields shorter than 100 terms are scored like fields containing 100 terms. Long documents are still normalized, but search is biased a bit against very short documents.

How would I boost the headline field here? Is that how you are supposed to
use the (presently unused) fieldName parameter?  If that's the case, I
assume I would logically (to do what I'm trying to do) make this factor
greater than 1 for the 'headline' field, and 1 for all other fields?

You could do that here too. So, for example, you could do something like:

public float lengthNorm(String fieldName, int numTerms) {
float n = super.lengthNorm(fieldName, Math.max(numTerms, 100));
if (fieldName.equals("headline"))
n *= 4.0f;
return n;
}

Equivalently, you could create your documents with something like:

Document d = new Document();
Field f = new Field.Text("headline", headline);
f.setBoost(4.0f);
...

But headlines tend to be short, and naturally benefit from the default lengthNorm implementation. So what you really might want is something like:

public float lengthNorm(String fieldName, int numTerms) {
if (fieldName.equals("headline"))
return 4.0f * super.lengthNorm(fieldName, numTerms);
else
return super.lengthNorm(fieldName, Math.max(numTerms, 100));
}

This is probably what I'd try first.

Doug


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