Data Preprocessing Steps in Machine Learning

data preprocessing in machine learning

Data preprocessing is an essential step in any machine learning project. By cleaning and preparing your data, you can ensure that your machine learning model is as accurate as possible. In this blog post, we’ll cover some of the important and most common data preprocessing steps that every data scientist should know.

Replace/remove missing data

Before building a machine learning model, it is important to preprocess the data and remove or replace any missing values. Missing data can cause problems with the model, such as biased results or inaccurate predictions. There are a few different ways to handle missing data, but the best approach depends on the situation.

  • In some cases, it may be best to simply remove the rows or columns containing missing values. However, this can also lead to loss of information or skewed results. In general, it is generally preferable to remove data points with missing values rather than to replace them if we can’t fix the data source system or select the right data source. This is because replacing values can introduce bias into the data, which can reduce the accuracy of the model.
  • Another option is to replace the missing values with a placeholder, such as the mean or median value of the rest of the data. This is often a good choice when there is only a small amount of missing data. Check out my post on this – Replace missing values with mean, median or mode
  • Yet another option is to really select the right kind of data source where we can have all the data. Alternatively, fix the existing data source such that the missing data can be obtained. 

However, it is important to be careful not to introduce any bias when choosing a placeholder. Ultimately, the best way to deal with missing data will depend on the specific dataset and machine learning task.

Remove outliers

One of the key step in data preprocessing is to remove or rescale outliers. Outliers are data points that are far from the rest of the data. They can skew your results and make your machine learning model less accurate. There are a few different ways to detect outliers, but the most common method is to use standard deviation. Data points that are more than two standard deviations from the mean can be considered outliers.

Rescale your data

Another step in data preprocessing is to scale your data if . Scaling means to change the range of your data so that all the values are within a similar range. This is important because some machine learning algorithms use distance measures to calculate similarity between data points. If your data is not scaled, then these measures will be inaccurate. 

There are several techniques that can be used to rescale data, but the two most common are min-max scaling and standard scaling. Min-max scaling scales the data so that all values are between 0 and 1. This technique is simple to understand and implement, but it can sometimes distort the data. Standard scaling scales the data so that the mean is 0 and the standard deviation is 1. This technique is more complex to understand and implement, but it typically gives better results. It is also called as normalizing numeric features which is discussed in a later point. When rescaling data for machine learning, it is important to choose the right technique for the problem at hand.

Split Your Data

The most common method is to use a training set, which is used to train the machine learning algorithm, and a test set, which is used to evaluate the performance of the algorithm. Another popular method is to use a validation set, which is used to tune the parameters of the machine learning algorithm.

When we create machine learning models, we need to split our data into training, validation, and test sets. This is because we want to train our models on the training data, validate them on the validation data, and then test them on the test data. Splitting data is important because it allows us to assess how well our models generalize to new data. There are several different techniques that we can use to split data. For example, we can use stratified sampling to ensure that our training and test sets contain a representative sample of the population. We can also use cross-validation to split our data into multiple folds and train/test our models on each fold. 

The most common split is 70-30, where 70% of the data is used for training and 30% for testing. This provides a good balance between training and validation. However, there are other techniques that can be used to split the data, such as k-fold cross-validation. This technique can be useful for ensuring that the model is not overfitting the data. Ultimately, choosing the right split depends on the specific problem that you are trying to solve.

Encode categorical features

Encoding categorical features is an important part of data preprocessing for machine learning models. Encoding categorical features means to mapping the categorical values to numerical values. This is necessary because most machine learning algorithms require that the data be in numerical form. Encoding categorical features can improve the performance of machine learning models because it can help the algorithms to better understand the relationships between the features and the target variable. In addition, encoding categorical features can also help to reduce the amount of data that needs to be processed, which can save time and resources. 

There are many different techniques for categorical features encoding, and each has its own advantages and disadvantages. Some of the most popular techniques include one-hot encoding, target encoding, and leave-one-out encoding.

  • One-hot encoding is a simple technique that involves converting each categorical value into a separate binary column.
  • Target encoding is a more sophisticated technique that involves mapping each categorical value to a numerical target value.
  • Leave-one-out encoding is a similar technique that involves leaving out one value for each category in order to prevent overfitting.
  • Label encoding: This technique simply assigns a unique numerical value to each category. For example, if we have 3 categories (A, B, and C), label encoding would assign the values 0, 1, and 2 to each category respectively. 

Split / Stratify partitions based on target variable

In machine learning, it is important to stratify partitions when creating training and test data sets. This is because if the partitions are not stratified, the training and test data sets will not be representative of the entire population. Stratifying partitions ensures that each partition contains a representative sample of the population. This is important because it allows the machine learning algorithm to learn from a variety of different data points. Stratifying partitions also helps to prevent overfitting, which is when the machine learning algorithm learns too much from the training data and does not generalize well to new data. Overfitting can lead to poor performance on the test data set.

Resample / repartition partitions

Resampling partition is an important technique when building machine learning models. It is used to split the data into training and test sets, ensuring that the model is trained on a variety of data and preventing overfitting. Resampling partition also allows you to tune the model with different parameters, giving you the ability to find the best model for your data. Furthermore, resampling partition can be used to estimate the generalization error of a model, giving you an idea of how well the model will perform on unseen data. Finally, resampling partition is also useful for debugging machine learning models, as it can help you identify problems with the data or the model itself.

Normalize numeric features

When building machine learning models, it’s important to Normalize numeric features. Normalization is a technique that can be used to rescale data so that it falls within a given range. This is important because machine learning algorithms often work best when the data is in a consistent range. By normalizing the data, we can ensure that the algorithm performs as intended. There are a few different ways to normalize data, but the most common method is to rescale the data so that it falls between 0 and 1. This can be done by dividing each value by the maximum value in the dataset. Normalizing data is an important step in pre-processing data for machine learning, and it can help to improve the performance of the algorithm.

Log transforms are often used when the data is not normally distributed, as they can help to make the data more normal. In addition, log transforms can help to reduce the impact of outliers, as they make the data less sensitive to large values.

Conclusion

Data preprocessing is an essential step in any machine learning project. By cleaning and preparing your data, you can ensure that your machine learning model is as accurate as possible. In this blog post, we’ve covered essential data preprocessing steps such as removing outliers, re-scaling your data, encoding categorical variables, splitting / partitioning your data, etc. By following these steps, you can set yourself up for success in any machine learning project!

 

 

 

Ajitesh Kumar
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Ajitesh Kumar

I have been recently working in the area of Data analytics including Data Science and Machine Learning / Deep Learning. I am also passionate about different technologies including programming languages such as Java/JEE, Javascript, Python, R, Julia, etc, and technologies such as Blockchain, mobile computing, cloud-native technologies, application security, cloud computing platforms, big data, etc. For latest updates and blogs, follow us on Twitter. I would love to connect with you on Linkedin. Check out my latest book titled as First Principles Thinking: Building winning products using first principles thinking
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