[SOUND] Now we examine Session 2: Distance on Numerical Data: Minkowski Distance. So we first introduced data matrix and dissimilarity matrix, or distance matrix. Data matrix is referenced in the typical matrix form is we have n data points, we use n rows. We have l dimensions, we use l columns to reference this data set. The distance matrix or dissimilar matrix usually is referenced in triangular matrix because you have n data points, we register only the distance between like objects one versus one or two versus one to look at their distance. But the distance function usually could be quite different, but different kinds of variables, like a real, boolean, categorical, ordinal, ratio, and vector variables. Their distance definition could be different. Moreover, sometimes they may have weights associated with those different variables based on the applications and data semantics. We are going to introduce this more later. Then, we look at an example. Suppose we have four points, four objects in two dimensional space. Then the data matrix is rampant in this typical form, okay. Then for dissimilarity matrix, or distance matrix, for Euclidean distance, you can see the matrix is in this way, like x1, x1, they are identical. Their distance is 0. x2, x1, their computation is based on the distance. This part is two, this distance is three, you take the sum of the square area. You take square root, you get this value. Then in general, we define the Minkowski distance of this formula. It means if we have area dimensions for object i and object j. Then their distance is defined by taking every dimension to look at their absolute value of their distance, then to the power of p, then you sum them up, get the root of p. Then we get the Minkowski distance. Such distance that P is the order we usually also call this distance this LP norm. The Minkowski distance in general have these properties. The first property is called positivity. It means, the distance be equal zero when they are identical otherwise they are greater in there. The second property called symmetry means the distance between I and J, distance between J and I should be identical. Then the third one called triangular inequality means for the distance between i and j. If you go through k, that means you go first to k, then from k to j. That distance should be no less than directly go from i to j. Should be greater than or equal to d(i,j). The distance measure that satisfies these three properties are metric distance. Notice not all the distance are metric for example set difference is not metric. Of course in this lecture we mainly discuss metric distance. Then we look at some special cases of Minkowski distance. If p = 1, we call L1 norm, they also call Manhattan or city block distance define this formula. In particular, if we are dealing with binary vectors we call these Hamming distance is the number of bits that are different. Then if p equals two, you can find also L2 norm. As Euclidean distance defined the typical way like this, and this vary for the middle one I think for all people, okay? Then if p goes to infinity means we're dealing with L infinity norm or L max norm, this is also caused supernum distance. Is that you find, is limit for p goes for infinity. In that case, actually the distance is really the maximum difference between any attribute of the vectors. For example you can see for F, from 1 to L. The maximum such absolute value of the distance, is the distance of L infinity norm or supremum distance. Let's look at some examples, for the same data sets, we get a four points. We get two dimensions. Then we look at the Manhattan distance is just a city block distance. For example from x2 to x1 you will go three blocks down then two blocks left. So the Manhattan distance is 3 plus 2, we get 5, okay. Of course x1 to x1 itself, is 0. And you Euclidean distance, as we discussed already, that's the measure. For Supremum distance, L infinity norm. You probably can see x2 to x1. So probably you can see the difference is you first go down 3 blocks for Manhattan distance, you can see you get a 3 plus 2. But for Supremum distance you could just see which attribute would give the maximum distance or maximum difference so you probably can see the maximum amount is 3 instead of 2 that's why x2 to x1 is 3. So it's pretty easy to compute, thank you. [MUSIC]
