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Eliza on 25 Feb Vote 0. Commented: Image Analyst on 25 Feb Accepted Answer: Image Analyst. I have data set consist of features for 37 class. The data set is ordered according to the classes.

### Multiclass classification using scikit-learn

I trained and test the data using KNN classifier. However, the maximum accuracy that i have go is about Cancel Copy to Clipboard. I used classperf the see the performance of the classifier and here is the last result I have got. Label: ''. Description: ''.

## why knn classifier accuracy for multi class dataset is low?

ClassLabels: [37x1 double]. GroundTruth: [x1 double]. NumberOfObservations: ControlClasses: [36x1 double]. TargetClasses: 1.Scikit-Learn or sklearn library provides us with many tools that are required in almost every Machine Learning Model. We will work on a Multiclass dataset using various multiclass models provided by sklearn library. Let us start this tutorial with a brief introduction to Multi-Class Classification problems.

If a dataset contains 3 or more than 3 classes as labels, all are dependent on several features and we have to classify one of these labels as the output, then it is a multiclass classification problem.

We will take one of such a multiclass classification dataset named Iris. We will use several models on it. It includes 3 categorical Labels of the flower species and a total of samples. These are defined using four features. You can download the dataset here. You can also fund the iris dataset on the UCI website. The dataset we will work with is in the CSV format. We have imported the necessary libraries for the preprocessing part.

We also have separated the features as x and the labels which are the output as y. Let us see the components of data and visualize them by plotting each of the four features one by one in pairs and the species as the target using the seaborn library. Its time to split our data into the test set and the training set. Now that we have split our data its time to model our data. We will see several models on the same split dataset of different multiclass classifiers. Gaussian NB is based on the Naive Bayes theorem with the assumption of conditional independence between every pair of features given the label of the target class.

The Graph for the likelihood of the feature vectors is Gaussian. Let us apply Gaussian Naive Bayes on the iris dataset. We have import GaussianNB classifier from sklearn.

Next, because we are interested in checking out the accuracy of our model, We have predicted the model on the test set and compare the predictions with the actual value. In the end, we have imported the accuracy score metric from sklearn library and print the accuracy.

Logistic Regression is one of the basic and powerful classifiers used in the machine learning model used for binary as well as multiclass classification problems.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Cross Validated is a question and answer site for people interested in statistics, machine learning, data analysis, data mining, and data visualization. It only takes a minute to sign up.

I want to implement k-NN to use in a multi-class dataset. Then, for each test sample find it's nearest neighbours e. Then take majority class among these 5 neighbours as the class of the test sample so if 3 of the 5 nearest neighbour has the Classx, then we'll classify the test sample as Classx. It depends on what you mean by multi class--are you talking of a setting where a one item can have multiple classes or b an item can have one of many classes as opposed to binary classification?

In that case, your idea is right, except that there is no need to take the prior I think you mean prior instead of posterior probabilities in 1. That's typically not done, although I have no solid argument for that right now. For corret multi label classification, means several catergories in result you have tune you algorithm measuring method.

So you have to evaluate set examples to set predicted result comparison and also try several tresholds radius of sphere in which you select all categories. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. How can I implement multiclass k-NN? Ask Question.

Asked 8 years, 6 months ago. Active 8 years, 6 months ago. Viewed 7k times. Do you know any clear explanation of it? Would that be a correct implementation?So far, all of the methods for classificaiton that we have seen have been parametric.

For example, logistic regression had the form. This is a parameter which determines how the model is trained, instead of a parameter that is learned through training. Note that tuning parameters are not used exclusively with non-parametric methods. Later we will see examples of tuning parameters for parametric methods. Then, to create a classifier, as always, we simply classify to the class with the highest estimated probability. If more than one class is tied for the highest estimated probablity, simply assign a class at random to one of the classes tied for highest.

Again, if the probability for class 0 and 1 are equal, simply assign at random. We first load some necessary libraries. Unlike many of our previous methods, such as logistic regression, knn requires that all predictors be numeric, so we coerce student to be a 0 and 1 dummy variable instead of a factor. We can, and should, leave the response as a factor. Numeric predictors are required because of the distance calculations taking place. Like we saw with knn. Note that the y data should be a factor vector, not a data frame containing a factor vector.

Note that the FNN package also contains a knn function for classification. We choose knn from class as it seems to be much more popular.

However, you should be aware of which packages you have loaded and thus which functions you are using. They are very similar, but have some differences. Essentially the only training is to simply remember the inputs. Note that by deafult, knn uses Euclidean distance to determine neighbors.

Because of the lack of any need for training, the knn function immediately returns classifications. With logistic regression, we needed to use glm to fit the model, then predict to obtain probabilities we would use to make a classifier.

Here, the knn function directly returns classifications. We use the test data to evaluate. Often with knn we need to consider the scale of the predictors variables.

If one variable is contains much larger numbers because of the units or range of the variable, it will dominate other variables in the distance measurements.

It is common practice to scale the predictors to have a mean of zero and unit variance. Be sure to apply the scaling to both the train and test data. Here we see the scaling slightly improves the classification accuracy. This may not always be the case, and often, it is normal to attempt classification with and without scaling.

Try different values and see which works best. It often removes the need for an additional counter variable.Please cite us if you use the software. All classifiers in scikit-learn do multiclass classification out-of-the-box. The sklearn. Multiclass classification : classification task with more than two classes. Each sample can only be labelled as one class.

For example, classification using features extracted from a set of images of fruit, where each image may either be of an orange, an apple, or a pear. Each image is one sample and is labelled as one of the 3 possible classes.

Multiclass classification makes the assumption that each sample is assigned to one and only one label - one sample cannot, for example, be both a pear and an apple. An example of a vector y for 3 samples:. An example of a sparse binary matrix y for 3 samples, where the columns, in order, are orange, apple and pear:.

This can be thought of as predicting properties of a sample that are not mutually exclusive. Formally, a binary output is assigned to each class, for every sample. Positive classes are indicated with 1 and negative classes with 0 or This approach treats each label independently whereas multilabel classifiers may treat the multiple classes simultaneously, accounting for correlated behaviour amoung them.

For example, prediction of the topics relevant to a text document or video. Each column represents a class. An example of a dense matrix y for 3 samples:. An example of the same y in sparse matrix form:. Multioutput regression : predicts multiple numerical properties for each sample. Each property is a numerical variable and the number of properties to be predicted for each sample is greater than or equal to 2. For example, prediction of both wind speed and wind direction, in degrees, using data obtained at a certain location.

Each sample would be data obtained at one location and both wind speed and directtion would be output for each sample. A column wise concatenation of continuous variables. An example of y for 3 samples:. Multioutput-multiclass classification also known as multitask classification : classification task which labels each sample with a set of non-binary properties. Both the number of properties and the number of classes per property is greater than 2. A single estimator thus handles several joint classification tasks.

This is both a generalization of the multi label classification task, which only considers binary attributes, as well as a generalization of the multi class classification task, where only one property is considered.K Nearest Neighbor KNN is a very simple, easy to understand, versatile and one of the topmost machine learning algorithms.

KNN used in the variety of applications such as finance, healthcare, political science, handwriting detection, image recognition and video recognition. In Credit ratings, financial institutes will predict the credit rating of customers.

In loan disbursement, banking institutes will predict whether the loan is safe or risky. KNN algorithm used for both classification and regression problems.

KNN algorithm based on feature similarity approach. KNN is a non-parametric and lazy learning algorithm. Non-parametric means there is no assumption for underlying data distribution. In other words, the model structure determined from the dataset. This will be very helpful in practice where most of the real world datasets do not follow mathematical theoretical assumptions. Lazy algorithm means it does not need any training data points for model generation.

All training data used in the testing phase. This makes training faster and testing phase slower and costlier. Costly testing phase means time and memory.

In the worst case, KNN needs more time to scan all data points and scanning all data points will require more memory for storing training data.

Python Machine Learning Tutorial #7 - KNN p.3 - Implementation

In KNN, K is the number of nearest neighbors. The number of neighbors is the core deciding factor. K is generally an odd number if the number of classes is 2. This is the simplest case. Suppose P1 is the point, for which label needs to predict.Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions.