RE: NDCG

[Stephen Robertson <ser@microsoft.com>]

> From: ireval@nist.gov [mailto:ireval@nist.gov] On Behalf Of Kalervo
> Jarvelin

> This is a good summary. A point to add is that if one wants to
> consistently discount the gains, the log(rank+1) variant runs the risk
> of boosting (instead of discounting) at ranks below the log base. With
> base=2 this does not matter.

Just a small comment on this.  It is true that the absolute gain values are boosted in those conditions -- however, the relativities still work (the "discounted" gain for rank n+1 is always less than the "discounted" gain for the same relevance value at rank n).  Furthermore, the issue disappears as soon as you normalize.

Stephen