Blank and Invalid Values - MQUAD User Manual

This page documents what MQUAD actually does when a value is missing, blank, malformed, or unexpected. The rules below are frozen for the calculation functions and verified across web, mobile, and backend.

The core rule: missing input → null

For every calculation function, any missing / null / empty / malformed argument, wrong argument count, or unknown unit returns null. Functions never throw, never abort the enclosing expression, and never produce 0 or an empty string just because an input was bad.

When a calculation returns null, the question's stored value is left unchanged (it keeps its previous value). A later valid result overwrites it.

What happens in each situation

Situation	Result
Referenced question has not been answered yet	The reference resolves to blank/null → functions that use it return `null`
Date is invalid or malformed (`age('not-a-date', '2026-08-14')`)	`null`
Calculation argument is null (`bmi(null, 175)`)	`null` — not `0`
Unit is unknown (`date-diff('fortnights', ...)`)	`null`
Height is zero or negative (`bmi(70, 0)`)	`null`
Weight is non-numeric (`bmi('abc', 175)`)	`null`
Wrong argument count (`age('2000-01-01')`)	`null`
End date before start date (`date-diff('days', ${end}, ${start})`)	a negative number (date-diff is signed)
LMP after reference date (`gestational-weeks(${future_lmp}, ${visit})`)	`0` (gestational functions clamp to 0)
DOB after reference date (`age(${future_dob}, ${visit})`)	`0` (age functions clamp to 0)
Mixed plain-time + datetime (`time-diff('hours', '09:00', '2026-08-14T10:00:00Z')`)	`null`
Question is hidden by skip logic	its stored answer is retained; if it was never answered, references resolve to blank
Non-numeric amount in date-add (`date-add('2026-08-14', 'x', 'days')`)	`null`

Worked examples

bmi(null, 175)                    → null
date-diff('fortnights', '2026-08-01', '2026-08-14')  → null
date-diff('months', '', '2026-08-14')                → null
age('not-a-date', '2026-08-14')   → null
date-add('2026-08-14', 'x', 'days') → null

MQUAD does not show a visible error message when a function returns null — the result is simply blank. If blank results are a problem, add a Validation rule to prevent bad input in the first place.

Validation vs null

Validation (constraints) is skipped for empty values — an unanswered question only fails if it is marked Required. This means:

An empty symptoms answer does not fail count-selected(${symptoms}) >= 2.
It fails only if the field is Required.

If a field must be validated only when answered, that is the default behaviour. If it must be required under a condition, use:

if(${has_children} = 'yes', . != '', .)

Hidden questions

When a question is hidden by Skip Logic, its previously entered answer is retained, not erased. References to a hidden-but-answered question still resolve. If a question was never shown and never answered, references to it resolve to blank.

If you need a hidden question's stale value cleared, add a calculation that clears it (e.g. an if() that yields blank), or clear it manually.

Preventing problems

Because null results are silent, the practical guidance is:

Validate inputs — numeric range checks (bmi, age inputs), date order checks (end ≥ start), required checks.
Use the signed rule — date-diff returns negatives for reversed dates; validate with ${end_date} >= ${start_date} instead of guessing.
Reference dates explicitly — age(${dob}, ${interview_date}) fails to null if either date is blank; make both questions Required.
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