So just to illustrate how hierarchical clustering works I'm just going to simulate a very simple data set here. So I always set the seed so that the data are reproduced here. You can just run this code and simulate the data for yourself and take a look at it. And so I plotted a total of 12 points here and there are three clusters we can see very clearly from the plot, and I've labelled each of the points using the text function. So you can see, you know, which point is which. And so I'm going to run the hierarchical clustering algorithm to see how the points get merged together. So the first thing you need to do to run an hierarchical clustering algorithm is to calculate the distance between all of the different points. So, you need to calculate all the pairwise distances. Between all the points so you can figure out, you know, which two points are closest together. And so the easiest way to do this is with the dist function in R. The dist function takes a matrix or dataframe. here, this dataframe has two columns, the first column is the x-coordinate, the second column is the y-coordinate. And then and so it's basically a matrix or a data frame of points. And what the dist function does is it calculates the distance between all the different rows in the data frame, and it gives you what's called the distance matrix. Which gives you all the pairwise distances. And so if you call dist without any other arguments it defaults to the Euclidean distance metric but you can give it some other distance metrics if you want as options. And so here you. Down below here you can see the most of the distance matrix that is returned by dist. So you can see that it's a lower triangular matrix. And it gives you all the pair wise distances. So for example, the distances between point one, and point two is 0.34. Of course, the actual distance here is meaningless, I just. because I just simulated the data, so the numbers aren't particularly meaningful. But you can see that some distances are farther apart, some points are farther apart than others. So, for example, the distance between point 3 and point 1 is 0.574 and the distance between point 3 and point 2 is 0.24. So, so the, so point 3 is closer to point 2 than it is to point 1. And so you can kind of go down the line like this and see how far apart each of the various points are from each other. [BLANK_AUDIO] And so if you look, so the idea of the hierarchical clustering algorithm here is that we're going to take the two points that are closest to each other from the start. And so that happens to be points five and six, and so I colored in here in the orange and that idea is that the five points, five ans six, because of the closest together we're going to group them together. And merge them into a single cluster. And so the idea is that here I'm going to create a single point. And then the little plus sign in the middle is kind of like the new location of this merged set of points. And so now instead I've kind of taken these two points, taken these two points, five and six, and merged them into a single point. So now one of that, the next two points that are closes together are 10 and 11 down, down the lower right in the red. And so I'm going to take those two points and merge them together and create a new kind of super point. That that is kind, that will kind of filter out through the algorithm. And so we can keep go along like this. And eventually we will get a little picture here called the Dendrogram, and it shows us how the various points got clustered together. So you can see on the very right side we, there's, there's points five and six that are grouped together, and then in the middle there you got points ten and 11. And so the farther down the tree. In terms of the, the points that are kind of further down the tree are the ones that got clustered first. And the points that are farther up kind of got clustered later. And and so you can see that there are the the kind of points five and six when they got merged together, then they got kind of clustered with point seven. And then when points five, six and seven, they all merged together into a single super point. And they got merged with point 8 and etcetera. And so one of the things about the dendrogram that's produced by the clustering algorithm is that it doesn't actually tell you how many clusters there are, right? You'll notice that there's no specific label on the plot that tells you there are two clusters or three clusters or whatever. And so, what you have to do is you have to cut this tree at a certain point to determine how many clusters there are. And so, for example, if I were to cut it at the point that's labeled on the y-axis, 2.0. For example, if I were to draw a horizontal line. At, at the level of 2.0. The question is, you know, how many, how many branches would I come, would I run into? So if I were to draw a horizontal line at 2.0, I would run into about two branches. And that would indicate to me that there are roughly two clusters. However, if I were to draw a horizontal line at the, at hte level of 1.0 there, I'm sorry, 1.0, at the height of 1.0. Then you draw a horizontal line and you'll see you run into three branches there so that would tell you that there were roughly three pluses. So depending on where you want to draw this, you want to draw this horizontal line at the tree you'll get more or fewer clusters in your clustering. Of course, in the extreme case if you were to top the, the, the tree all the way at the bottom you just get 12 clusters, which equals the number of data points. And so you have to cut the tree at a place that's, that's convenient for you, convenient for you. At this point we don't really have a rule of where to cut it but then once you do cut it then you can get the cluster assignment. From hierarchical clustering.
