CS6200: Information Retrieval

Homework 2

Return to basic course information.

Assigned: Thursday, 19 September 2013
Due: Thursday, 3 October 2013, 6pm

Instructions

  1. If you collaborated with others, you must write down with whom you worked on the assignment. If this changes from problem to problem, then you should write down this information separately with each problem.
  2. Submit the requested written answers, code, and instructions to the TAs on how to (compile and) run the code.

Problems

  1. [10 points] In the lectures on text acquisition, we discussed duplicate detection techniques such as fingerprinting and locality sensitive hashing (simhash). Plagiarism detection is similar but often involves finding passages in documents that are duplicates, or near duplicates, of passages in other documents. (For instance, your roommate may be copying your answer to this question but may think his own answer to question #3 is much better.) Assuming you have a large database of original documents available, describe in detail a design for a system that detects plagiarized passages in a target document.
  2. [15 points] [Courtesy James Allan]
    1. In this homework problem, you will write a quick program to explore Zipf's Law.

      Go to the Project Gutenberg website and download Alice in Wonderland by Lewis Carol (Plain Text UTF-8 format). Strip off the header, and thus consider only the text starting at "ALICE'S ADVENTURES IN WONDERLAND", just preceding "CHAPTER 1"; also, strip off the footer, eliminating the license agreement and other extraneous text, and thus consider only the text up through, and including, "THE END". Use this perl script to strip the text of punctuation obtaining the original text as a list of words. For example, on a unix based systems, you should run a command like 
      parse.pl alice30.txt > output

      Write a quick program or script that counts word frequencies. For the most frequent 25 words and for the most frequent 25 additional words that start with the letter f (a total of 50 words), print the word, the number of times it occurs, its rank in the overall list of words, the probability of occurrence, and the product of the rank and the probability. Also indicate the total number of words and the total number of unique words that you found. Discuss whether this text satisfies Zipf's Law. Feel free to use other parts of the ranked list of terms.

    2. Suppose that while you were building retrieval index, you decided to omit all words that occur fewer than five times (i.e., one to four times). According to Zipf's Law, what proportion of the total words in the collection would you omit? (Justify your answer.) What proportion would actually be omitted in the Alice in Wonderland text above?
  3. [15 points]
    1. According to Heaps' Law, what proportion of a collection of text must be read before 90% of its vocabulary has been encountered? You may assume that beta=0.5. Hint: to solve this problem you don't need to know the value of K
    2. [This part is 15 additional points of extra credit.] Verify Heap's Law on the Alice in Wonderland text. Process each word of the text, in order, and compute the following pairs of numbers: (number of words processed, number of unique words seen). These pairs of numbers, treated as (x,y) values, should satisfy Heaps Law. Appropriately transform the data and use least squares to determine the model parameters K and beta. As explained in class, there are better ways of estimating a power-law distribution, but this quick-and-dirty hack is often effective.
  4. [10 points] Use the result numbers associated with a web search engine query to do the following:
    1. Think up two 3-word queries and submit them to a search engine. Tell us which search engine you used. Estimate the number of results for these two queries using numbers from single words and pairs of words contained in the query. Compare them to the numbers returned by the search engine. Discuss results.
    2. Estimate the size of the search engine's indexed corpus. Compare the size estimates from the two queries and discuss the consistency (or lack of it) of these estimates.