## What is stratified sampling and why is it important ?

Stratified sampling is a sampling method where population is divided into homogenous subgroups called strata and the right number of instances are sampled from each stratum. For further explanation visit here. This sampling is important to ensure that sampled dataset is representative of the entire population. To realise this point, consider an example of predicting…

## Suppose you build word vectors (embeddings) with each word vector having dimensions as the vocabulary size(V) and feature values as pPMI between corresponding words: What are the problems with this approach and how can you resolve them ?

Problems As the vocabulary size (V) is large, these vectors will be large in size. They will be sparse as a word may not have co-occurred with all possible words. Resolution Dimensionality Reduction using approaches like Singular Value Decomposition (SVD) of the term document matrix to get a K dimensional approximation. Other Matrix factorisation techniques…

## What is negative sampling when training the skip-gram model ?

Recap: Skip-Gram model tries to represent each word in a large text as a lower dimensional vector in a space of K dimensions such that similar words are closer to each other. This is achieved by training a feed-forward network where we try to predict the context words given a specific word, i.e.,     …

## What is PMI ?

PMI : Pointwise Mutual Information, is a measure of correlation between two events x and y.           As you can see from above expression, is directly proportional to the number of times both events occur together and inversely proportional to the individual counts which are in the denominator. This expression ensures high…

## Given the following two sentences, how do you determine if Teddy is a person or not? “Teddy bears are on sale!” and “Teddy Roosevelt was a great President!”

This is an example of Named Entity Recognition(NER) problem. One can build a sequence model such as an LSTM to perform this task. However, as shown in both the sentences above, forward only LSTM might fail here. Using forward only direction LSTM might result in a model which recognises Teddy as a product : “bear”, which is on…

## Say you’ve generated a language model using Bag of Words (BoW) with 1-hot encoding , and your training set has lot of sentences with the word “good” but none with the word “great”. Suppose I see sentence “Have a great day” p(great)=0.0 using this language model. How can you solve this problem leveraging the fact that good and great are similar words?

BoW with 1-hot encoding doesn’t capture the meaning of sentences, it only captures co-occurrence statistics. We need to build the language model using features which are representative of the meaning of the words. A simple solution could be to cluster the word embeddings and group synonyms into a unique token. Alternately, when a word has…

## What is the complexity of Viterbi algorithm ?

Viterbi algorithm is a dynamic programming approach to find the most probable sequence of hidden states given the observed data, as modeled  by a HMM.  Without dynamic programming, it becomes an exponential problem as there are exponential number of possible sequences for a given observation(How – explained in answer below). Let the transition probabilities(state transition)…

## Suppose you are modeling text with a HMM, What is the complexity of finding most the probable sequence of tags or states from a sequence of text using brute force algorithm?

Assume there are total states and let be the length of the largest sequence. Think how we generate text using an hMM. We first have a state sequence and from each state we emit an output. From each state, any word out of possible outcomes can be generated. Since there are states, at each possible…

## How do you find the most probable sequence of POS tags from a sequence of text?

This problem can be solved with a HMM. Using a HMM involves finding the transition probabilities (what is the probability of going from one POS tag to another and emission/output probabilities (what is the probability of observing a word given a POS tag) as explained in the question How do you train an hMM. Once…