# Information Retrieval Lab: Advanced Retrieval¶

In this exercise, we will implement a more advanced retrieval model. In contrast to the last exercise, where we explored vector space retrieval using TFIDF-vectors, in this notebook, we will take a look at a generative (language) model for retrieval.

## Lecture Recall¶

First, recall some important concepts from the lecture:

###### Document representations¶

The representation $\pmb{d}$ of a document $d$ is a probability distribution over the index terms $T$, where the probability of the $i$-th term in $T$ indicates how likely it occurs in documents generated by the topics’ language model that also generated $\pmb{d}$.

###### Query representations¶

A query $q$ is represented as a sequence $\pmb{q}$ of index terms.

###### Relevance Function $\rho$¶

The relevance function formulated as a query likelihood model: $\rho(d,q) =P(\pmb{d}|\pmb{q})$

## Setup¶

To implement a scoring model, we first import existing code from the previous exercises. We will make use of the Shakespeare document collection, the preprocessing component and the index. We supply a reference solution you can import, but feel free to continue using your own code!

Exercise: using the code from previous exercises, construct an index on top of the Shakespeare document collection.

In [ ]:
from modules.shakespeare import corpus
from modules.preprocessing import PreprocessorSpacy as Preprocessor
from modules.indexing import Index



## Deriving the relevance function $\rho$¶

Let $d$ denote a language model for document, and $q$ the sequence of query terms from query. Then the query likelihood model is derived as follows, utilizing Bayes' rule:

$$\rho = P(\pmb{d} | \pmb{q}) = \frac{P(\pmb{q}|\pmb{d})\cdot P(\pmb{d})}{P(\pmb{q})}$$

Here, $P(\pmb{q})$ can be omitted without affecting the relative ranking of all documents (i.e. omitting it influences all scores equally), and $P(\pmb{d})$ is assumed uniform, cancelling its influence.

The query representation $\pmb{q}$ can be written as sequence of $t_1, ..., t_{|q|}$ of index terms, which (under the assumption that terms are independent of each other) allows us to rewrite the equation as follows:

$$\rho = P(q | \pmb{d}) = P(t_1, ..., t_{|q|} | \pmb{d}) = \prod_{i=1}^{|q|} P(t_i | \pmb{d})$$

This means that we can divide our implementation into four steps:

1. Write a function to calculate the probability $P(t | d)$ for a single term
2. Apply this function to all query terms
3. Multiply the results to get the relevance score of a document
4. Repeate 1-3 for all documents to rank them w.r.t the query

We can use maximum likelihood estimation to calculate the term probability:

$$P(t | \pmb{d}) = \frac{\text{tf}(t,d)}{|d|}$$

However, this creates a problem: if a term $t$ is contained in the query, but does not occur in a document (i.e. $\text{tf}(t,d) = 0$), $P(t|\pmb{d}) = 0$. Since the individual term probabilities are multiplied, this results in zero relevance for all documents that only partially contain the query!

Solution: we also calculate the probability of the term based on the language model of the corpus, $D$, and combine both probabilities.

## Document Probability¶

As first part to our relevance function, we implement the probability of a term based on a document language model $P(t|\pmb{d})$.

$$P(t | \pmb{d}) = \frac{\text{tf}(t,d)}{|d|}$$

Exercise: implement a function to calculate the term probability give a document.

In [ ]:
def document_probability(index, term, doc_id):
"""
Calculates the conditional probability of a term give a document.
:param index: index to get frequency data from
:param term: term to calculate the probability for
:param doc_id: document to calculate the probability for
"""



## Collection Probability¶

As first part to our relevance function, we implement the probability of a term based on a document language model $P(t|\pmb{D})$.

$$P(t | \pmb{D}) = \frac{\sum_{d \in D}\text{tf}(t,d)}{\sum_{d \in D}|d|}$$

Exercise: implement a function to calculate the term probability give a document collection.

In [ ]:
def collection_probability(index, term):
"""
Calculates the conditional probability of a term give a document collection.
:param index: index to get frequency data from
:param term: term to calculate the probability for
"""



## Combining Probabilities¶

#### Jelinek-Mercer-Smoothing¶

Now that we can calculate both the document probability and the collection probability of a term $t$, we need a way of combining them into a single value, indicating the overall probability of the term. To start out, we implement Jelinek-Mercer smoothing, which is a linear interpolation between both probabilities:

$$P(t|\pmb{d}) = (1- \omega) \cdot P(t | \pmb{d}) + \omega\cdot P(t|\pmb{D})$$

where $\omega$ is a paramter that controls the relative influence of both terms on the result.

Note: we call the weigthing term $\omega$ in this exercise. In the lecture slides, this is denoted as $\lambda$, but since lambda is a reserved keyword in the Python language, we opt to rename it to avoid confusion.

Exercise: implement the overall term probability with Jelinek-Mercer smoothing.

In [ ]:
def jm_term_probability(index, term, doc_id, omega):
"""
Calculates the conditional probability of a term give a document using Jelinek-Mercer smoothing.
:param index: index to get frequency data from
:param term: term to calculate the probability for
:param doc_id: document to calculate the probability for
:param omega: weigthing factor for the linear interpolation
"""



#### Dirichlet-Smoothing¶

Instead of interpolating linearly between both probabilities, we can take a more fine-grained approach and base the interpolation factor on the length of the document: the longer it is, the more confidence is placed on the document probability, assuming more data = better prediction.

This substitutes the term $\omega$ with the following term:

$$\omega = \frac{\alpha}{|d| + \alpha}$$

where $\alpha$ is a parameter that indicates the boundary at which more weight is put towards the document probability or the collection probability.

Note: this method is called Dirichlet smoothing, since from a Bayesian point of view, this corresponds to the expected value of the posterior distribution, using a symmetric Dirichlet distribution with parameter $\alpha$ as a prior.

Exercise: implement a weight function to calculate $\omega$ for a given document and $\alpha$ value.

In [ ]:
def weight(index, doc_id, alpha):
"""
Calculates the dirichlet smoothing weighting factor for a given document and alpha value
:param index: index to get frequency data from
:param doc_id: document to calculate the weight factor for
:param alpha: alpha-prior for the dirichlet smoothing
"""



Exercise: implement the overall term probability with Dirichlet smoothing.

In [ ]:
def dirichlet_term_probability(index, term, doc_id, alpha):
"""
Calculates the conditional probability of a term give a document using Dirichlet smoothing.
:param index: index to get frequency data from
:param term: term to calculate the probability for
:param doc_id: document to calculate the probability for
:param alpha: alpha-prior for the dirichlet interpolation
"""



## Implementing Relevance Functions¶

#### Jelinek-Mercer Smoothing¶

Given the probability function for a single term $t$ using Jelinek-Mercer-Smoothing, we can now implement the full relevance scoring function. We will however utilize a slightly different formulation of the total probability by taking its logarithm:

$$\rho = \log\prod_{i=1}^{|q|} P(t_i | \pmb{d}) = \sum_{i=1}^{|q|} \log P(t_i | \pmb{d})$$

The reason behind this is that the individual term probabilities can get extremely small. If we multiply them, we might run into an underflow: the number being too small to be represented by a 64-bit float. Therefore, utilizing the logarithm product rule, we can instead calculate the sum of the logarithm of the individual probabilities. Note that logarithmization yields negative relevance scores; recall that only the relative ranking among documents is important, not the scores themselves.

Exercise: implement the relevance function $\rho$ with Jelinek-Mercer smoothing. Use the sum of log probabilities as defined above.

In [ ]:
from math import log

def jm_score(index, query, doc_id, omega):
"""
Calculates the relevance of a document given a query using Jelinek-Mercer smoothing.
:param index: index to get relevance data from
:param query: query to calculate the relevance for
:param doc_id: document to calculate the relevance for
:param omega: omega paramter for Jelinek-Mercer smoothing
"""



#### Dirichlet Smoothing¶

Similarly, we can implement $\rho$ with Dirichlet smoothing, once again applying the logarithm.

Exercise: implement the relevance function $\rho$ with Dirichlet smoothing. Use the sum of log probabilities as defined above.

In [ ]:
from math import log

def dirichlet_score(index, query, doc_id, alpha):
"""
Calculates the relevance of a document given a query using Dirichlet smoothing.
:param index: index to get relevance data from
:param query: query to calculate the relevance for
:param doc_id: document to calculate the relevance for
:param alpha: alpha paramter for Dirichlet smoothing
"""



## Querying the index¶

We now have all the components needed to answer queries. Write a wrapper function that takes a text and returns the (top $k$) results for the query. It construct a query representation and calculate the pairwise probability between the query and all documents, returning an ordered list of (doc_id, score) tuples, descending by score.

#### Jelinek-Mercer Smoothing¶

Exercise: implement a query function for Jelinek-Mercer smoothing. Successful parameters are $\omega= 0.1$ and $\omega$= 0.7 for short and long queries, respectively.

In [ ]:
def jm_query(index, preprocessor, text, omega=0.1, topK=-1):
"""
Queries a given text against the given index using a Jelinek-Mercer smoothed language model
:param preprocessor: preprocessor instance to process the query with
:param index: the index data to query against
:param text: query text
:param omega: weight parameter for Jelinek-Mercer smoothing
:param topK: number of top results to return
:return: list of (doc_id, score) tuples descending by score for all documents in the vector space
"""


In [ ]:
jm_query(index, preprocessor, "proceed", topK=5)


#### Dirichlet Smoothing¶

Exercise: implement a query function for Dirichlet smoothing. Typical parameters are $1000 < \alpha < 2000$.

In [ ]:
def dirichlet_query(index, preprocessor, text, alpha=1000, topK=-1):
"""
Queries a given text against the given index using a Dirichlet smoothed language model
:param preprocessor: preprocessor instance to process the query with
:param index: the index data to query against
:param text: query text
:param alpha: alpha-parameter for Dirichlet smoothing
:param topK: number of top results to return
:return: list of (doc_id, score) tuples descending by score for all documents in the vector space
"""


dirichlet_query(index, preprocessor, "proceed", topK=5)