[Exercise taken from “Exploring Big Historical Data: The Historian’s Macroscope” by Shawn Graham, Ian Milligan, and Scott Weingart, London: Imperial College Press, 2015]
Here is the text of the Gettysburg Address, delivered by Abraham Lincoln on November 19, 1863.
“Four score and seven years ago our fathers brought forth on this continent a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal.
Now we are engaged in a great civil war, testing whether that nation, or any nation so conceived and so dedicated, can long endure. We are met on a great battlefield of that war. We have come to dedicate a portion of that field, as a final resting place for those who here gave their lives that that nation might live. It is altogether fitting and proper that we should do this.
But, in a larger sense, we can not dedicate, we can not consecrate, we can not hallow this ground. The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. The world will little note, nor long remember what we say here, but it can never forget what they did here. It is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. It is rather for us to be here dedicated to the great task remaining before us—that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth. “
Copy and paste this into a Google doc. How many topics are there in this short text and what are their most significant words?
Using different color highlighted text, color all words in RED that relate to “war” and BLUE those words that relate to “governance”. Some words may be double marked. As you are doing this, think about the process. How do you make your decisions? How different does your marked up text look from the person sitting next to you?
Now make two lists (in Google Sheets, if you want): one for “war” words and one for “governance” and a count next to each word for the number of times the word appears. Order by frequency.
The Gettysburg Address has 271 words , so work out the proportion of each word by dividing the frequency by the total number of words. If you make a histogram of this, you have created your own word cloud.
How does a computer “read” the text and decide how to classify words that could belong to both categories? And also words that have multiple meanings, like “lead”?
In computational topic modelling we are asking the computer to make a series of blind guesses as to which category/topic words belong. In a series of random attempts, the computer tries to reorganize this series of guesses into a set of high probability topic “bins of words”.