Every year BEST Ljubljana organizes BEST Days of Technology and Sciences, an event hosting a broad variety of workshops, hackathons and lectures for the students of natural sciences and technology. Introduction to Data Science, organized by our own Laboratory for Bioinformatics, was this year one of them.
The task was to teach and explain basic data mining concepts and techniques in four hours. To complete beginners. Not daunting at all…
Luckily, we had Orange at hand. First, we showed how the program works and how to easily import data into the software. We created a poll using Google Forms on the fly and imported the results from Google Sheets into Orange.
To get the first impression of our data, we used Distributions and Scatter Plot. This was just to show how to approach the construction and simple visual exploration on any new data set. Then we delved deep into the workings of classification with Classification Tree and Tree Viewer and showed how easy it is to fall into the trap of overfitting (and how to avoid it). Another topic was clustering and how to relate similar data instances to one another. Finally, we had some fun with ImageAnalytics add-on and observed whether we can detect wrongly labelled microscopy images with machine learning.
One of the key techniques of exploratory data mining is clustering – separating instances into distinct groups based on some measure of similarity. We can estimate the similarity between two data instances through euclidean (pythagorean), manhattan (sum of absolute differences between coordinates) and mahalanobis distance (distance from the mean by standard deviation), or, say, through Pearson correlation or Spearman correlation.
Our main goal when clustering data is to get groups of data instances where:
each group (Ci) is a a subset of the training data (U): Ci ⊂ U
an intersection of all the sets is an empty set: Ci ∩ Cj = 0
a union of all groups equals the train data: Ci ∪ Cj = U
This would be ideal. But we rarely get the data, where separation is so clear. One of the easiest techniques to cluster the data is hierarchical clustering. First, we take an instance from, say, 2D plot. Now we want to find its nearest neighbor. Nearest neighbor of course depends on the measure of distance we choose, but let’s go with euclidean for now as it is the easiest to visualize.
Euclidean distance is calculated as:
Naturally, the shorter the distance the more similar the two instances are. In the beginning, all instances are in their own particular clusters. Then we seek for the closest instances of every instance in the plot. We pin down the closest instance and make a cluster of the original and the closest instance. Now we repeat the process again. What is the closest instances to our new cluster –> add it to the cluster –> find the closest instance. We repeat this procedure until all the instances are grouped in one single cluster.
We can write this down also in a form of a pseudocode:
every instance is in its own cluster
repeat until instances are all in one group:
find the closest instances to the group (distance has to be minimum)
join closest instances with the group
Visualization of this procedure is called a dendrogram, which is what Hierarchical clustering widget displays in Orange.
Another thing to consider is the distance between instances when we have already two or more instances in a cluster. Do we go with the closest instance in a cluster or to the furthest one?
Picture A shows the distances to the closest instance – single linkage.
Picture B shows the distance to the furthest instance – complete linkage.
Picture C shows the average of all distances in a cluster to the instance – average linkage.
The downside of single linkage is, even by intuition, creating elongated, stretched clusters. Instances at the top part of the red C are in fact quite different from the lower part of the red C. Complete linkage does much better here as it centers clustering nicely. However, the downside of complete linkage is taking outliers too much into consideration. Naturally, each approach has its own pros and cons and it’s good to know how they work in order to use them correctly. One extra hint: single linkage works great for image recognition, exactly because it can follow the curve.
There’s a lot more we could say about hierarchical clustering, but to sum it up, let’s state pros and cons of this method:
pros: sums up the data, good for small data sets
cons: computationally demanding, fails on larger sets
Paint Data widget might initially look like a kids’ game, but in combination with other Orange widgets it becomes a very simple and useful tool for conveying statistical concepts, such as k-means, hierarchical clustering and prediction models (like SVM, logistical regression, etc.).
The widget enables you to draw your data on a 2-D plane. You can name the x and y axes, select the number of classes (which are represented by different colors) and then position the points on a graph.
The data will be represented in a data table with two attributes, where their instances correspond to coordinates in the system. Such data set is great for demonstrating k-means and hierarchical clustering methods. Just like we do below. In the screenshot we see that k-means, with our particular settings, recognizes clusters way better than hierarchical clustering. It returns a score rank, where the best score (the one with the highest value) means the most likely number of clusters. Hierarchical clustering, however, doesn’t even group the right classes together.
Another way to use Paint Data is to observe the performance of classification methods, where we can alter the graph to demonstrate improvement or deterioration of prediction models. By painting the data points we can try to construct the data set, which would be difficult for one but easy for another classifier. Say, why does linear SVM fail on the data set below?
I am lately having fun with Image Viewer. The widget has been recently updated and can display images stored locally or on the internet. But wait, what images? How on earth can Orange now display images if it can handle mere tabular or basket-based data?
Here’s an example. I have considered a subset of animals from the [download id=”864″] data set (comes with Orange installation), and for demonstration purposes selected only a handful of attributes. I have added a new string attribute (“images”) and declared that this is a meta attribute of the type “image”. The values of this attribute are links to images on the web:
Here is the resulting data set, [download id=”859″]. I have used this data set in a schema with hierarchical clustering, where upon selection of the part of the clustering tree I can display the associated images:
Typically and just like above, you would use a string meta attribute to store the link to images. Images can be referred to using a HTTP address, or, if stored locally, using a relative path from the data file location to the image files.
Here is another example, where all the images were local and we have associated them with a famous digits data set ([download id=”868″] is a data set in the Orange format with the image files). The task for this data set is to classify handwritten digits based on their bitmap representation. In the schema below we wanted to find out which are the most frequent errors some classification algorithm would make, and how do the images of the misclassified digits look like. Turns out that SVM with RBF kernel most often misclassify the digit 9 and confuses it with a digit 3: