search - how do I normalise a solr/lucene score?

Question

I am trying to work out how to improve the scoring of solr search results. My application needs to take the score from the solr results and display a number of “stars” depending on how good the result(s) are to the query. 5 Stars = almost/exact down to 0 stars meaning not matching the search very well, e.g. only one element hits. However I am getting scores from 1.4 to 0.8660254 both are returning results that I would give 5 stars to. What I need to do is somehow turn these results in to a percentage so that I can mark these results, with the correct number of stars.

The query that I run that gives me the 1.4 score is:

euallowed:true AND(grade:"2:1")

The query that gives me the 0.8660254 score is:

euallowed:true AND(grade:"2:1" OR grade:"1st")

I've already updated the Similarity so that the tf and idf return 1.0 as I am only interested if a document has a term, not the number of that term in the document. This is what my similarity code looks like:

import org.apache.lucene.search.Similarity;

public class StudentSearchSimilarity extends Similarity {

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

    @Override
    public float queryNorm(float sumOfSquaredWeights) {

        return (float) (1.0 / Math.sqrt(sumOfSquaredWeights));

    }

    @Override
    public float sloppyFreq(int distance) {
        return 1.0f / (distance + 1);
    }

    @Override
    public float tf(float freq) {
        return (float) 1.0;
    }

    @Override
    public float idf(int docFreq, int numDocs) {

        //return (float) (Math.log(numDocs / (double) (docFreq + 1)) + 1.0);
        return (float)1.0;

    }

    @Override
    public float coord(int overlap, int maxOverlap) {
        return overlap / (float) maxOverlap;
    }
}

So I suppose my questions are:

1. How is the best way of normalising the score so that I can work out how many “stars” to give?

2. Is there another way of scoring the results?

Thanks

Grant

Answer 1

To quote http://wiki.apache.org/lucene-java/ScoresAsPercentages:

People frequently want to compute a "Percentage" from Lucene scores to determine what is a "100% perfect" match vs a "50%" match. This is also somethings called a "normalized score"

Don't do this.

Seriously. Stop trying to think about your problem this way, it's not going to end well.

That page does give an example of how you could in theory do this, but it's very hard.