Definition
DIF (Discriminant Index Function) is a statistical scoring system used by the Internal Revenue Service to select individual tax returns for audit. The system assigns a numeric score to each filed return based on algorithms that compare reported items against statistical norms derived from examined returns in the same income class. Returns with higher DIF scores are considered more likely to contain errors or underreported income and are therefore more likely to be selected for examination.
The IRS does not disclose the specific formula or weighting criteria underlying DIF scores, treating the methodology as confidential to preserve audit effectiveness.
Common Confusion
The abbreviation DIF carries unrelated meanings in non-tax contexts. In computing, DIF stands for Data Interchange Format, a spreadsheet file standard. In Mexican administrative geography, DIF refers to Distrito Federal. Neither meaning has any connection to the tax administration term. Researchers encountering DIF in legal or regulatory materials should confirm context before assuming any particular meaning.
Why It Matters in Research
DIF as a tax administration term is a creature of modern administrative practice with no historical legal pedigree. Researchers working in pre-computer-era tax materials will not encounter it. The term emerges from mid-twentieth-century IRS modernization efforts and appears primarily in administrative guidance, congressional testimony, tax practitioner literature, and Freedom of Information Act litigation concerning the confidentiality of audit selection criteria.
Because the IRS treats the DIF formula as a protected law enforcement technique, primary sources discussing the actual mechanics are sparse. Researchers will find more material in practitioner commentary, congressional oversight records, and FOIA case law than in IRS-published guidance. The confidentiality of audit selection methodology has been repeatedly litigated, and those cases are a productive entry point for understanding the legal framework surrounding DIF.
Researchers should also note that DIF is one of several audit selection mechanisms; returns may also be selected through the Unreported Income DIF (UIDIF), through document matching programs, or through referral. Conflating DIF with the broader audit selection process can distort research conclusions.
Historical Dictionary Support
Burrill's Law Dictionary contains no entry for DIF in the tax administration sense. The excerpt provided from Burrill relates to dies juridici — juridical days for legal proceedings — a Latin term bearing no relationship to the IRS scoring system. This confirms that DIF as used in modern tax administration is entirely post-Burrill and post-historical-dictionary in origin. No historical legal dictionary addresses the term. Researchers should not expect shelf sources to provide useful background and should proceed directly to administrative and practitioner materials.
Jurisdictional Note
DIF is a federal (U.S.) administrative tool specific to IRS operations. State tax agencies operate their own audit selection systems, which may or may not employ comparable statistical methods, but those systems are not referred to as DIF. Research into state audit selection must proceed through each state's own revenue agency documentation.