This entails the data that's available but we missed during its extraction from a source. It deals with engineering tasks such as the following:

• Scraping from a website
• Querying from a database
• Extracting from flat files

There can be many sources of missing values, some of which are as follows:

• Regular expressions resulting in the wrong or non-unique results
• Wrong query
• A different data type storage
• Incomplete download
• Incomplete processing