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Design references:

• SQL NULL on Wikipedia
• Missing Data Functionality in NumPy enhancement proposal, summary of discussions and wiki page (2011-2012)
• Working with missing data in the Pandas manual
• Pandas 2 Missing data consistency design document
• Missing Values in the SAS manual
• Missing Values in the EViews manual
• Missing Values in R, Stata, SAS and SPSS on UCLA's IDRE website
• Types of missing values (NullFlavor) in the Health Level 7 v3 standard, a subset of these used by OpenEHR and The HL7 Null Flavour Debate.
• "Handling of missing values in statistical software packages for Windows" , a survey from 1999