Documentation requestion: SearchRankParameters

[chris-martin]

I've found this package pretty easy to work with so far, with the exception of SearchRankParameters. In particular, I don't understand what paramK1 and paramB mean. I see them mentioned in The Probabilistic Relevance Framework: BM25 and Beyond section 3.4.5, Document Length, and I see section 3.5, Uses of BM25, has a few relevant bits:

setting b = 1 will perform full document-length normalisation, while b = 0 will switch normalisation off

A significant number of such experiments have been done, and suggest that in general values such as 0.5 < b < 0.8 and 1.2 < k₁ < 2 are reasonably good in many circumstances

But I get from this no sense of what k₁ means, and little guidance as to why I would want more or less document-length normalization for a particular field.

I don't think elaborate explanations in the documentation are necessary; a few hints would likely go a very long way.

[chris-martin]

@dcoutts If you might be able to say how you selected these parameter values for the demo program, any recollections would be most welcome.

full-text-search/demo/PackageSearch.hs

Lines 82 to 88 in a87317c

	paramK1 :: Float
	paramK1 = 1.5
	paramB :: PkgDocField -> Float
	paramB NameField = 0.9
	paramB SynopsisField = 0.5
	paramB DescriptionField = 0.5