Map of Italy example – choosing the value of k

In our data, we are given some points (about 1 percent) from the map of Italy and its surroundings. The blue points represent water, and the green points represent land; the white points are unknown. From the partial information that we have been given, we would like to predict whether there is water or land in the white areas.

Drawing only 1% of the map data in the picture would make it almost invisible. If we were given about 33 times more data from the map of Italy and its surroundings, and drew it in the picture instead, it would look as follows:

Analysis

For this problem, we will use the k-NN algorithm—k here means that we will look at k-closest neighbors. Given a white point, it will be classified as an area of water if the majority of its k-closest neighbors are in an area of water and classified as land if the majority of its k-closest neighbors are an area of land. We will use the Euclidean metric for the distance: given two points, X=[x₀,x₁] and Y=[y₀,y₁], their Euclidean distance is defined as d_Euclidean = sqrt((x₀-y₀)²+(x₁-y₁)²).

The Euclidean distance is the most common metric. Given two points on a piece of paper, their Euclidean distance is just the length between the two points, as measured by a ruler, as shown in the following diagram:

To apply the k-NN algorithm to an incomplete map, we have to choose the value of k. Since the resulting class of a point is the class of the majority of the k-closest neighbors of that point, k should be odd. Let's apply this algorithm to the values of k=1,3,5,7,9.

Applying this algorithm to every white point on the incomplete map will result in the following complete maps:

k=1

k=3

k=5

k=7

 

k=9

As you may have noticed, the highest value of k results in a complete map with smoother boundaries. An actual complete map of Italy is shown here:

We can use this real, complete map to calculate the percentage of incorrectly classified points for the various values of k to determine the accuracy of the k-NN algorithm for different values of k:

k	Precentage of incorrectly classified points
1	2.97
3	3.24
5	3.29
7	3.40
9	3.57

 

Thus, for this particular type of classification problem, the k-NN algorithm achieves the highest accuracy (least error rate) for k=1.

However, in real life, the problem is that we wouldn't usually have complete data or a solution. In such scenarios, we need to choose a value of k that is appropriate to the data that is partially available. For this, consult Problem 4 at the end of this chapter.