Typically, raw data cannot be used directly in predictive modelling projects like classification or regression. For algorithms to function, data must be in numbers, and statistical noise and errors must be corrected, while some algorithms impose requirements on data. Raw data, clearly, is prohibited by all means. As a result, preprocessing of the raw data is necessary before fitting and assessing a machine learning model.
Data preparation is the process of converting raw data into a format that is computer-readable and understandable. It goes by many other names, such as “data wrangling,” “data cleaning,” “data pre-processing,” and “feature engineering.” Data scientists and analysts run transformed raw data through machine learning algorithms to disclose any insights or make predictions.
The idea is to begin with a problem in hand – it helps determine the kind of data required, how to ensure it links with the intended purpose and the transformation required in a format appropriate for the specific algorithm. With good data preparation comes more accurate and effective algorithms. This makes turning to new analytics problems and adaptation to model accuracy shifts a lot easier, saving data scientists an ample amount of time and effort.
Why is Data Preparation Important?
As we mentioned earlier, datasets typically need considerable preparation before they can produce meaningful insights because the majority of machine learning algorithms require data to be structured in a specific way. Some values contained in datasets may be missing, incorrect, or otherwise challenging for an algorithm to process. The algorithm cannot use missing data. Invalid data causes the algorithm to provide less accurate or even false results. Many datasets simply lack meaningful business context (e.g., poorly defined ID values), necessitating feature enrichment, while some datasets are relatively clean but need to be moulded (e.g., aggregated or rotated). Clean, well-curated data is generated through effective data preparation, and this delivers more accurate and useful model results.
Steps of Data Preparation in Machine Learning
Each machine learning project is different, since the specific data is different. The process may be quite difficult and time-intensive, yet equally necessary. Here are the key steps in preparing data for a machine learning project:
1. Define a Problem
The first step is to define and develop a detailed understanding of an underlying problem. It involves gathering sufficient project knowledge to choose the frame or frames for the prediction challenge. Is it, for instance, a classification problem, a regression problem, or another higher-order problem type? Begin by spending time with those who are active in the field and have a solid grasp of the problem domain. Synthesize what you learn from their interactions and draw on your own expertise to develop a set of hypotheses that characterise the dynamics and variables at play. This step can significantly affect the choice of data to be collected and can also offer helpful directions on how to prepare the data for the machine learning model and transform it.
2. Collect Data
The next step is to list prospective data sources from both internal and external third parties. Gathering data must take into account not just what the data seemingly represents, but also why it was gathered and what it might indicate, especially when used in another context. It is also crucial to consider variables that could have skewed the results. Apart from evaluating bias, it is also recommended to determine whether there is a reason to suspect that important missing data may result in an incomplete picture of the analysis being done. Sometimes, analytics teams employ data that is technically sound but yields inaccurate or partial results, and users of the resulting models base their decisions on these flawed discoveries without being aware of the error.
3. Analyse the Data
Data scientists must be thorough with the data they are working on to develop insights into its significance and usefulness. Examining the type and distribution of data in each variable, the relationships between them, and how they differ from the predicted or expected result are all examples of data exploration. This stage can reveal issues like collinearity, or variables that move in tandem, or instances where data set standardisation and other transformations are required. It can provide chances to enhance model performance, such as bringing down a data set's dimensionality. Data must be explored using summary statistics and visualisation tools like Tableau, Microsoft Power BI, D3.js, and Python libraries like Matplotlib, Bokeh, and the HoloViz stack.
4. Cleanse and Validate Data
Analytics teams can find and fix inconsistencies, outliers, anomalies, missing data, and other problems using a variety of data cleansing and validation tools. Imputation technologies, for instance, can frequently deal with missing data values by filling empty fields with statistically appropriate alternatives. Making a special category to record the importance of missing values or deliberately setting missing values as neutral is also helpful. To ensure high data quality and to clean and validate data for machine learning, a variety of open source tools, including Pandera and Great Expectations, are made to verify the validity of the data frames that are frequently used to arrange analytics data into two-dimensional tables. There are various tools available for validating workflows for data processing and programming, such as pytest.
5. Structure Data
Data regularisation techniques like data binning and smoothing continuous features lower the variance of the model by eliminating small statistical fluctuations. Either an equidistant approach (each bin has the same "width") or an equi-statistical method (each bin has about the same number of samples) can be used to bin data into several categories. It can also serve as a precondition in the local optimization of the data in each bin to help create low-bias models. Continuous feature smoothing can aid in "denoising" raw data. It is also possible to impose causal assumptions about the data-generating process by representing relationships in ordered data sets as monotonic functions that maintain the order of the data pieces. Creating separate data sets for training and testing models, data reduction through methods like attribute or record sampling and data aggregation, data normalisation through the use of dimensionality reduction, and data rescaling are other ways to structure data for machine learning.
6. Feature Engineering and Selection
The last step in data preparation before creating a machine learning model is feature engineering and selection. Feature engineering entails adding or developing additional variables to enhance a model's output. Examples include separating variables into distinct features, aggregating variables, and changing features according to probability distributions. Feature selection involves picking useful features to study and removing irrelevant ones. Many features that look promising initially may constrain a model's capacity to accurately evaluate fresh data by over fitting and extended model training. Techniques like lasso regression and algorithmic relevance assessment aid in feature selection, too.