Feature engineering
Feature engineering is one of the key components that contribute to the model's performance. A simple model with the right features can perform better than a complicated one with poor features. You can think of the feature engineering process as the most important step in determining your predictive model's success or failure. Feature engineering will be much easier if you understand the data.
Feature engineering is used extensively by anyone who uses machine learning to solve only one question, which is: how do you get the most out of your data samples for predictive modeling? This is the problem that the process and practice of feature engineering solves, and the success of your data science skills starts by knowing how to represent your data well.
Predictive modeling is a formula or rule that transforms a list of features or input variables (x1, x2,..., xn) into an output/target of interest (y). So, what is feature engineering? It's the process of creating new input variables or features (z1, z2, ..., zn) from existing input variables (x1, x2,..., xn). We don't just create any new features; the newly created features should contribute and be relevant to the model's output. Creating such features that will be relevant to the model's output will be an easy process with knowledge of the domain (such as marketing, medical, and so on). Even if machine learning practitioners interact with some domain experts during this process, the outcome of the feature engineering process will be much better.
An example where domain knowledge can be helpful is modeling the likelihood of rain, given a set of input variables/features (temperature, wind speed, and percentage of cloud cover). For this specific example, we can construct a new binary feature called overcast, where its value equals 1 or no whenever the percentage of cloud cover is less than 20%, and equals 0 or yes otherwise. In this example, domain knowledge was essential to specify the threshold or cut-off percentage. The more thoughtful and useful the inputs, the better the reliability and predictivity of your model.