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Computing partial match scores

With length normalization to the side, we can concentrate on the main calculation of matching, summing the weight products of terms shared by query $q$ and document $d$:

But this mathematical characterization hides a number of messy details associated with actually computing it. We will need to make efficient use of two data structures in particular. The first is the inverted index (recall Figure (FOAref) .). The critical feature of this organization is that we can, for each term in the query, find the set of all doument postings associated with it. In particular, the freq statistic maintained with each posting allows us to compute the weight $w_{kd}$ we need for each. But even with these efficiencies, the need to consider every document posting for every query term can create a serious processing demand, and one that must be satisfied immediately - the users are waiting!

Since the elements of the sum can be re-ordered arbitrarily, we will consider a PARTIAL RANKING (a.k.a. filtering, or pruning) algorithm that attempts to include the dominant elements of the sum but ignore small, inconsequential ones [Salton89] [Buckley85] [Harman90] .

The fact that both $w_{kq}$ and $w_{kd}$ vary considerably suggests that, by ordering the inclusion of products into the sum, we may be able to truncate this process when they become smaller than we care to consider.

We therefore begin by sorting the terms in decreasing order of query weights $w_{kq}$. Considering terms in this order means we can expect to accumulate the match score beginning with its largest terms. Then, the fact that our list of postings was similarly (and not coincidentally:) ordered by decreasing frequency means that: \left( \forall j>i\right) \,w_{kd_{i}}>w_{kd_{j}}

Once these weights diminish below some threshold $$, we can quit walking down the postings list. (In fact, it may be that the weight associated with the very first posting is too small and we can ignore all weights associated with this term.)

The second important data structure is an ACCUMULATOR queue in which each document's match score is maintained. Since each query term may add an additional term to the match score for a particular document, these accumulators will keep a running total for each document. For moderate corpus sizes, it may not be unreasonable to allocate an accumulator for each document, but this can demand too much memory for very large corpora. Define $\mathname{NAccum}$ to be the number of accumulators we are willing to allocate. Then one obvious way to restrict this set is to only allocate an accumulator when a document's score becomes significant, again in comparison to some threshold $\tau_{insert}$. Since we will be processing query terms in decreasing $w_{kq}$ order and heuristically value the space associate with new accumulators more than the slight extra time to run down posting lists a bit farther, we can assume $\tau_{\mathname{insert}}>\tau _{\mathname{add}}$.

Picking appropriate values for these two thresholds is something of a black art, but Persin [Persin94] reports one especially careful experiment in which both are made proportional to the most highly matched document's accumulator $A^{*}$ (i.e., $A^{*}$ is the maximum match score in any document's accumulator): \tau_{\mathname{insert}} &=&\eta _{\mathname{insert}}\,\cdot \,A^{*} \\ \tau_{\mathname{add}} &=&\eta _{\mathname{add}}\,\cdot \,A^{*} Persin's experiments suggest that values of $\eta _{\mathname{insert} }=0.07,\,\eta _{\mathname{add}}=0.001$ give retrieval effectiveness near that of full matches (i.e., considering all query-document term products) while mininizing $\mathname{NAccum}$.

These two thresholds divide the range of possible query-document term products into three conditions: & w_{kd}\cdot w_{kq} > \tau_{\mathname{insert}} & \mathrm{Always\ add;\ create\ new\ accumulator\ }A_{d}\mathrm{\ if\ necessary} \\ \tau_{\mathname{add}} & < w_{kd}\cdot w_{kq} \leq \tau_{\mathname{insert}} & \mathrm{Add\ only\ if\ accumulator\ }A_{d}\mathrm{\ already\ exists} \\ & w_{kd}\cdot w_{kq} \leq \tau_{\mathname{add}} & \mathrm{Ignore;\ move\ on\ to\ next\ query\ term}

Since we want to remain flexible with respect to both long and short queries, we will assume that the query weights $w_{kq}$ are pre-computed and passed to our ranking procedure.

Using our definition for $w_{kd}$ and focusing first on the $_{\mathname{insert}}$ threshold: & w_{kd}\cdot w_{kq} > & \tau _{\mathname{insert}} \\ \Longleftrightarrow & \left( f_{kd} \cdot \mathname{idf}_{k}\,\right) \cdot w_{kq} > & \tau_{\mathname{insert}} \\ \Longleftrightarrow & f_{kd}& f_{kd}\ > & \frac{\tau_{\mathname{insert}}}{\mathname{idf}_{k}w_{kq}} \\ \Longleftrightarrow & f_{kd}\ > & \frac{\eta_{\mathname{insert}}\,\,A^{*}}{\mathname{idf}_{k}\,w_{kq}} \end{eqnarray} This finally becomes an operational question we can apply with respect to each posting's frequency $f_{kd}$. Note that this threshold must be updated every time we move to a new term of the query. And of course, the computation of $\tau _{\mathname{add}}$ proceeds similarly.

All the basic features of a partial ranking algorithm are now in place, and a pseudo-code sketch is shown in Figure (FOAref) . It also includes a few minor complications. First, a hashtable is required to find accumulators associated with a particular docno. Second, the set of accumulators $A_{d}$ is described as a queue, but must be slightly trickier than most - it must maintain them in order so that only the top $\mathname{NAccum}$ are maintained, and must support length() queries and a pop() function when it is full. Nondeterministic skip lists [Pugh90] are recommended for this purpose.[Cutting97] .

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