--- title: '28 Ways to Be Missing in SAS' description: A SAS numeric missing is not a lone wolf - there are 28 of them, and Data Controller has supported all of them since v4. How they work, and what the validation rules do with them. date: '2026-09-22 09:00:00' author: 'Allan Bowe' authorLink: https://www.linkedin.com/in/allanbowe/ tags: - Special Missings - Data Quality previewImg: './cover.jpeg' --- # 28 Ways to Be Missing in SAS A SAS numeric missing is not a lone wolf: the ordinary missing (`.`) is one of **28** distinct missing values a numeric variable can hold. The other 27 are written with a single character - the letters `A` to `Z`, or an underscore (`._`). They exist because "missing" is usually not the whole story. A survey question that was never reached, a reading that was illegible, a value the respondent refused to give - in a well-run process those are different facts, and a lone `.` throws the difference away. Special missings record *why* the value is missing. ## They are numbers, not text - Arithmetic on them yields missing - `.A + 1` is `.` - `PROC MEANS`, `PROC SUMMARY` and the other summarisation procedures exclude them, exactly as they exclude `.` - They sort below every non-missing number, in the order `._`, then `.`, then `.A` to `.Z` - `NMISS()` and `CMISS()` count them as missing - Converting one to text drops the period - `CATS(.A)` is the string `A` One display detail is worth knowing: a regular missing prints as `.`, unless `OPTIONS MISSING=` changes that character - set it to blank and a regular missing renders as an empty cell. The option affects only the regular missing; `._` and `.A`-`.Z` always print as their own letter. So a blank cell in a SAS listing is still unambiguously a regular missing, and a lone letter is still a special one. Data Controller itself is unaffected either way - it sends a regular missing to the browser as `null` and a special missing as its letter. In a SAS dataset they are written with a leading period (`.A`, `.B` ... `._`). In Data Controller you type the letter or the underscore, with or without that period - `.a` and `a` are the same missing - and the letter is not case sensitive. Two letters, or a letter mixed with a number, are refused rather than guessed. There is one cell where the letter cannot be typed at all. A numeric column that carries a date, datetime or time format is edited through a date picker rather than the numeric editor, and a picker accepts only a date. ## Carrying them between the browser and SAS The Data Controller frontend and the SAS backend exchange data as JSON, and JSON has no way to express a letter as a numeric value - `A` is a string. The conversion is handled in the open source [SASjs Adapter](https://github.com/sasjs/adapter#variable-types): - The adapter infers each column's SAS type from the values it is given. All numeric values mean a numeric column, all strings mean a character column, and a column holding a single character (`a`-`z`, `_` or `.`) alongside numeric values is numeric, with the lone characters written to SAS as special missings. - `null` becomes `.` or an empty string, according to the type derived for that column. - Two cases cannot be inferred from the values alone: a numeric column containing *only* special missings looks like a single character column, and a character column containing only nulls looks numeric. For those, the adapter accepts an explicit format for the column - and Data Controller sends one automatically, because it already knows each column's SAS format from the metadata returned by the backend. - A value that is neither a number nor a single valid character is refused rather than guessed - `aaaa`, or `!` in a numeric column. - A lone `.` is accepted as another way of typing the regular missing. There is nothing to configure. Special missings are available by default, for numeric cells - a date, datetime or time formatted column aside, since those edit through a date picker. Once in SAS they are ordinary values, so they are what the approval DIFF screen compares, and the DIFF's formatted / unformatted switch shows either the formatted representation or the raw value - useful for confirming exactly which missing was set. ## What the Data Controller validation rules do with them These are Data Controller's own rules, configured per column in the `MPE_VALIDATIONS` table and applied in the browser as you edit and submit. - `NOTNULL` - rejects one. A special missing is a missing value, so it fails the rule, and a physical NOT NULL constraint on the target table rejects it as well. A primary key column is treated as NOT NULL whether or not a rule is configured for it - `HARDREGEX` - checked against the pattern like any other value; unlike blanks and the plain `.`, special missings are **not** exempt, so a numeric column that carries them needs a pattern which allows for a single letter - `SOFTREGEX` - the same check, but a failure is only a warning rather than a block, and it is ignored entirely if the column also has a `HARDREGEX` rule - `SOFTSELECT` / `HARDSELECT` - both support them. The dropdown lists a special missing as a bare letter alongside the ordinary values, and a hard rule then accepts it like any other listed value - it still rejects a value that is not in the list - `ROUND` - no effect. It only rounds values that are numbers, so a special missing is left as it was typed The range rules compare in the order SAS itself uses, which is what lets a range be written in special missings. SAS puts every missing below every non-missing value, and orders the missing values among themselves: `._` is the lowest, then the regular missing, then `.A` through `.Z`. A range therefore means exactly what SAS would mean by it: - `MINVAL .A` with `MAXVAL .C` accepts `.B` and rejects `.D`, and a blank - the regular missing - fails that floor because it sorts below `.A` - a number sits above every missing, so it passes a floor of `.A` and fails a ceiling of `.C` - against a numeric bound the ordering does the obvious thing: a missing sorts below every number, so it fails a `MINVAL` of 1 and passes a `MAXVAL` of 100. Use `NOTNULL` if the column has to be populated - a rule value that is neither a number nor a special missing - a typo such as `..` or `AB` - satisfies nothing, so the column fails until the rule is corrected A formula is the one case where a special missing genuinely does not work: - `HARDFORMULA` / `SOFTFORMULA` - a formula that reads a special-missing cell does not compute; the grid's spreadsheet engine returns `#VALUE!` for that row, where the plain `.` contributes 0 `CASE` (`UPCASE` / `LOWCASE`) is a character rule, so it does not apply: SAS hands the browser a special missing as an uppercase letter, so there is no case to enforce, and a `CASE` rule on a numeric column would reject the column's numbers rather than the missing. Worth knowing about the regex rules: although the pattern is written in SAS PRX syntax, the check itself runs in the browser - SAS only parses the pattern (`PRXPARSE`) when the rule is saved - so the two engines can disagree on exotic patterns, and SAS pads a numeric-to-character conversion, so a pattern re-used in SAS needs `strip()` for an anchored match. In short, a special missing counts as a value for the pattern and dropdown rules, and takes its own place in the order for the range rules: it sits below every number, so a numeric minimum rejects it and a numeric maximum accepts it, while a range written in special missings is decided among the missing values themselves. ## See it in action The recording below runs the whole cycle on one table: entering special missings, the rules that reject them, submitting the changes, approving them, and reviewing the DIFF - including a change from one special missing to another, and the formatted / unformatted switch on a date column. It also shows the range rules doing what the section above describes. A special missing is refused by a numeric `MINVAL` and accepted by a numeric `MAXVAL`, and on a column carrying `MINVAL .A` with `MAXVAL .C`, `.B` is taken while `.D` is refused.