Machine learning algorithms generally do not work well with missing values. Rare exceptions include decision trees, Naive Bayes classifier, and some rule-based learners. It is very important to understand why a value is missing. It can be missing due to many reasons, such as random error, systematic error, and sensor noise. Once we identify the reason, there are multiple ways to deal with the missing values, as shown in the following list:

• Remove the instance: If there is enough data, and only a couple of non-relevant instances have some missing values, then it is safe to remove these instances.
• Remove the attribute: Removing an attribute makes sense when most of the values are missing, values are constant, or an attribute is strongly correlated with another attribute.
• Assign a special value (N/A): Sometimes a value is missing due to valid reasons, such as the value is out of scope, the discrete attribute value is not defined, or it is not possible to obtain or measure the value. For example, if a person never rates a movie, their rating on this movie is nonexistent.
• Take the average attribute value: If we have a limited number of instances, we might not be able to afford removing instances or attributes. In that case, we can estimate the missing values by assigning the average attribute value.
• Predict the value from other attributes: Predict the value from previous entries if the attribute possesses time dependencies.

As we have seen, the value can be missing for many reasons, and hence, it is important to understand why the value is missing, absent, or corrupted.