Yes, it definately seems to be the main problem for us. I did some simple tests of the encoding and decoding calculations in DefaultSimilarity, and my findings are:
* For input between 1.0 and 0.5, a difference of 0.01 in the input causes the output to change by a value of 0 or 0.125 depending if it is an edge case or not * For input between 0.5 and 0.25, a difference of 0.01 in the input causes the output to change by a value of 0 or 0.0625 * For input between 0.25 and 0.125, a difference of 0.01 in the input causes the output to change by a value of 0 or 0.015625 * And so on, with smaller and smaller differences in the output value for edge cases I would say that the main problem is for input values between 1.0 and 0.5. So if one could tweak the SweetSpotSimilarity to start it's "raw" (ie not encoded) lengthNorm values at 0.5 instead of 1.0, it would solve my problem for the title field. This would of course worsen the precision for longer text values, but since this is a title field that is not a problem. So, is there a way to configure SweetSpotSimilarity to use 0.5 as it's highest lengthNorm value, instead of 1.0? /Jimi ________________________________________ From: Ahmet Arslan <iori...@yahoo.com.INVALID> Sent: Thursday, April 21, 2016 2:24 AM To: solr-user@lucene.apache.org Subject: Re: Is it possible to configure a minimum field length for the fieldNorm value? Hi Jim, fieldNorm encode/decode thing cause some precision loss. This may be a problem when dealing with very short documents. You can find many discussions on this topic. ahmet On Thursday, April 21, 2016 3:10 AM, "jimi.hulleg...@svensktnaringsliv.se" <jimi.hulleg...@svensktnaringsliv.se> wrote: Ok sure, I can try and give some examples :) Lets say that we have the following documents: Id: 1 Title: John Doe Id: 2 Title: John Doe Jr. Id: 3 Title: John Lennon: The Life Id: 4 Title: John Thompson's Modern Course for the Piano: First Grade Book Id: 5 Title: I Rode With Stonewall: Being Chiefly The War Experiences of the Youngest Member of Jackson's Staff from John Brown's Raid to the Hanging of Mrs. Surratt And in general, when a search word matches the title, I would like to have the length of the title field influence the score, so that matching documents with shorter title get a higher score than documents with longer title, all else considered equal. So, when a user searches for "John", I would like the results to be pretty much in the order presented above. Though, it is not crucial that for example document 1 comes before document 2. But I would surely want document 1-3 to come before document 4 and 5. In my mind, the fieldNorm is a perfect solution for this. At least in theory. In practice, the encoding of the fieldNorm seems to make this function much less useful for this use case. Unless I have missed something. Is there another way to achive something like this? Note that I don't want a general boost on documents with short titles, I only want to boost them if the title field actually matched the query. /Jimi ________________________________________ From: Jack Krupansky <jack.krupan...@gmail.com> Sent: Thursday, April 21, 2016 1:28 AM To: solr-user@lucene.apache.org Subject: Re: Is it possible to configure a minimum field length for the fieldNorm value? I'm not sure I fully follow what distinction you're trying to focus on. I mean, traditionally length normalization has simply tried to distinguish a title field (rarely more than a dozen words) from a full body of text, or maybe an abstract, not things like exactly how many words were in a title. Or, as another example, a short newswire article of a few paragraphs vs. a feature-length article, paper, or even book. IOW, traditionally it was more of a boolean than a broad range of values. Sure, yes, you absolutely can define a custom similarity with a custom norm that supports a wide range of lengths, but you'll have to decide what you really want to achieve to tune it. Maybe you could give a couple examples of field values that you feel should be scored differently based on length. -- Jack Krupansky On Wed, Apr 20, 2016 at 7:17 PM, <jimi.hulleg...@svensktnaringsliv.se> wrote: > I am talking about the title field. And for the title field, a sweetspot > interval of 1 to 50 makes very little sense. I want to have a fieldNorm > value that differentiates between for example 2, 3, 4 and 5 terms in the > title, but only very little. > > The 20% number I got by simply calculating the difference in the title > fieldNorm of two documents, where one title was one word longer than the > other title. And one fieldNorm value was 20% larger then the other as a > result of that. And since we use multiplicative scoring calculation, a 20% > increase in the fieldNorm results in a 20% increase in the final score. > > I'm not talking about "scores as percentages". I'm simply noting that this > minor change in the text data (adding or removing one single word) causes > the score to change by a almost 20%. I noted this when I renamed a > document, removing a word from the title, and that single change caused the > document to move up several positions in the result list. We don't want > such minor modifications to have such big impact of the resulting score. > > I'm not sure I can agree with you that "the effect of document length > normalization factor is minimal". Then why does it inpact our result in > such a big way? And as I said, we don't want to disable it completely, we > just want it to have a much lesser effect, even on really short texts. > > /Jimi > > ________________________________________ > From: Ahmet Arslan <iori...@yahoo.com.INVALID> > Sent: Thursday, April 21, 2016 12:10 AM > To: solr-user@lucene.apache.org > Subject: Re: Is it possible to configure a minimum field length for the > fieldNorm value? > > Hi Jimi, > > Please define a meaningful document-lenght range like min=1 max=50. > By the way you need to reindex every time you change something. > > Regarding 20% score change, I am not sure how you calculated that number > and I assume it is correct. > What really matters is the relative order of documents. It doesn't mean > anything addition of a word decreases the initial score by x%. Please see : > https://wiki.apache.org/lucene-java/ScoresAsPercentages > > There is an information retrieval heuristic which says that addition of a > non-query term should decrease the score. > > Lucene's default document length normalization may favor short document > too much. But folks blend score with other structural fields (popularity), > even completely bypass relevancy score and order by price, production date > etc. I mean there are many use cases, the effect of document length > normalization factor is minimal. > > Lucene/Solr is highly pluggable, very easy to customize. > > Ahmet > > > On Wednesday, April 20, 2016 11:05 PM, " > jimi.hulleg...@svensktnaringsliv.se" <jimi.hulleg...@svensktnaringsliv.se> > wrote: > Hi Ahmet, > > SweetSpotSimilarity seems quite nice. Some simple testing by throwing some > different values at the class gives quite good results. Setting ln_min=1, > ln_max=2, steepness=0.1 and discountOverlaps=true should give me more or > less what I want. At least for the title field. I'm not sure what the > actual effect of those settings would be on longer text fields, so maybe I > will use the SweetSpotSimilarity only for the title field to start with. > > Of course I understand that there are many things that can be considered > domain specific requirements, like if to favor/punish short/medium/long > texts, and how. I was just wondering how many actual use cases there are > where one want's a ~20% difference in score between two documents, where > the only difference is that one of the documents has one extra word in one > field. (And now I'm talking about an extra word that doesn't affect > anything else except the fieldNorm value). I for one find it hard to find > such a use case, and would consider it a very special use case, and would > consider a more lenient calculation a better fit for most use cases (and > therefore most domains). :) > > /Jimi > > > -----Original Message----- > From: Ahmet Arslan [mailto:iori...@yahoo.com.INVALID] > Sent: Wednesday, April 20, 2016 8:14 PM > To: solr-user@lucene.apache.org > Subject: Re: Is it possible to configure a minimum field length for the > fieldNorm value? > > Hi Jimi, > > SweetSpotSimilarity allows you define a document length range, so that all > documents in that range will get same fieldNorm value. > In your case, you can say that from 1 word up to 100 words do not employ > document length punishment. If a document is longer than 100 do some > punishment. > > By the way; favoring/punishing short, middle, or long documents is domain > specific thing. You are free to decide what to do. > > Ahmet > > > > On Wednesday, April 20, 2016 7:46 PM, "jimi.hulleg...@svensktnaringsliv.se" > <jimi.hulleg...@svensktnaringsliv.se> wrote: > OK. Well, still, the fact that the score increases almost 20% because of > just one extra term in the field, is not really reasonable if you ask me. > But you seem to say that this is expected, reasonable and wanted behavior > for most use case? > > I'm not sure that I feel comfortable replacing the default Similarity > implementation with a custom one. That would just increase the complexity > of our setup and would make future upgrades harder (we would for example > have to remember to check if the default similarity configuration or > implementation changes). > > No, if it really is the case that most people like and want this, and > there is no way to configure Solr/Lucene to calculate fieldNorm in a more > reasonable way (in my book) for short field values, then I just think we > are forced to set omitNorms="true", maybe in combination with a simple > field boost for shorter fields. > > /Jimi > > > > -----Original Message----- > From: Jack Krupansky [mailto:jack.krupan...@gmail.com] > Sent: Wednesday, April 20, 2016 5:18 PM > To: solr-user@lucene.apache.org > Subject: Re: Is it possible to configure a minimum field length for the > fieldNorm value? > > FWIW, length for normalization is measured in terms (tokens), not > characters. > > With TDIFS similarity (the default before 6.0), the normalization is based > on the inverse square root of the number of terms in the field: > > return state.getBoost() * ((float) (1.0 / Math.sqrt(numTerms))); > > That code is in ClassicSimilarity: > > https://github.com/apache/lucene-solr/blob/releases/lucene-solr/5.5.0/lucene/core/src/java/org/apache/lucene/search/similarities/ClassicSimilarity.java#L115 > > You can always write your own custom Similarity class to override that > calculation. > > -- Jack Krupansky > > On Wed, Apr 20, 2016 at 10:43 AM, <jimi.hulleg...@svensktnaringsliv.se> > wrote: > > > Hi, > > > > In general I think that the fieldNorm factor in the score calculation > > is quite good. But when the text is short I think that the effect is two > big. > > > > Ie with two documents that have a short text in the same field, just a > > few characters extra in of the documents lower the fieldNorm factor too > much. > > In one test the text in document 1 is 30 characters long and has > > fieldNorm 0.4375, and in document 2 the text is 37 characters long and > > has fieldNorm 0.375. That means that the first document gets almost a > > 20% higher score simply because of the 7 character difference. > > > > What are my options if I want to change this behavior? Can I set a > > lower character limit, meaning that all fields with a length below > > this limit gets the same fieldNorm value? > > > > I know I can force fieldNorm to be 1 by setting omitNorms="true" for > > that field, but I would prefer to still have it, just limit its effect > > on short texts. > > > > Regards > > /Jimi > > > > > > >
