Troubleshooting Expressions

Common mistakes when writing validation, skip logic, calculations, and default values — with symptoms, causes, and fixes.

Wrong variable reference

Symptom: the expression evaluates to blank / never matches.

Cause: the expression is missing the ${...} reference braces.

Incorrect:

age(dob, visit_date)

Correct:

age(${dob}, ${visit_date})

Using the question label instead of the variable name

Symptom: the expression never matches what the respondent enters.

Cause: expressions must use the question's variable name (name), not its displayed label.

Incorrect:

What is your date of birth? = '2020-01-01'

Correct:

${dob} = '2020-01-01'

Wrong argument order

Symptom: ages, durations, or date differences come out negative or wrong.

Cause: argument order is not symmetric across all functions. Verify against the Expression Reference.

• date-diff(unit, start, end) — start first, end second.
• time-diff(unit, start, end) — start first, end second.
• duration(end, start, [unit]) — end first (legacy!).
• days-between(d1, d2) — arg order differs by platform; prefer date-diff('days', ...).

Incorrect BMI height unit

Symptom: BMI values are absurdly large.

Cause: height passed in metres instead of centimetres.

Incorrect:

bmi(70, 1.75)

Correct (height in cm):

bmi(70, 175)

Invalid date format

Symptom: a date calculation returns blank.

Cause: the date isn't a supported format. MQUAD accepts YYYY-MM-DD (best), day-first DD/MM/YYYY / DD-MM-YYYY, and ISO datetimes. Ambiguous formats like 03/08/2026 are read day-first (3 August).

Fix: write dates as YYYY-MM-DD in your expressions.

Comparing a number with text

Symptom: numeric comparisons behave strangely (e.g. . >= 18 passes for "9" because "9" > "18" lexically in some contexts).

Cause: comparing a number to a quoted string.

Incorrect:

. >= '18'

Correct:

. >= 18

Missing quotes around a string

Symptom: an equality never matches.

Cause: string values must be quoted in the internal format.

Incorrect:

${sex} = female

Correct:

${sex} = 'female'

(In the skip-logic Raw Expression tab, unquoted values are accepted and converted for you.)

Unknown function

Symptom: the designer flags "Unknown function" when saving.

Cause: the function name is not in the whitelist, or is a typo.

Fix: check the Expression Reference. Remember that some functions are platform-specific (floor, ceiling, normalize-space are backend-only; pulldata is mobile-only) — avoid them in survey logic you need everywhere.

Unknown unit

Symptom: a date/time calculation returns blank.

Cause: the unit string isn't recognized.

Incorrect:

date-diff('fortnights', ${start}, ${end})

Correct units:

• date-diff / date-add / date-subtract: 'days', 'weeks', 'months', 'years'
• time-diff / duration: 'seconds', 'minutes', 'hours', 'days'

There is no shorthand d/w/m/y for the date functions (to avoid minutes-vs-months confusion).

Circular calculations

Symptom: the designer blocks saving with a circular-dependency error.

Cause: two expressions reference each other, e.g. question A's calculation uses B and B's uses A.

Fix: break the cycle — one of the two values should be an input, not a calculation.

Calculation is blank / null

Symptom: a calculated field shows nothing.

Cause: one of the referenced questions is unanswered, or an argument is malformed. Any missing/malformed argument makes a function return null, and the field keeps its previous (blank) value.

Fix: see Blank and Invalid Values. Make dependency questions Required, and validate inputs.

Skip condition never becomes true

Symptom: a question never appears.

Cause: typically a reference/typo problem — a multi-select compared with = instead of selected(), or a comparison against the label rather than the variable name.

Fix: test the condition in Preview. For multi-select use selected(${symptoms}, 'fever'), not ${symptoms} = 'fever'.

Validation always fails

Symptom: a question can never be submitted.

Cause: the constraint can never be true — e.g. comparing . to the wrong question, and/or confusion, or a date-order mistake (${end_date} > ${start_date} with a wrong comparison direction).

Fix: validate with example values in Preview. For date ranges, ensure the ordering matches (end >= start, not start >= end).

Different default-value reference grammar

Symptom: a default value with ${...} doesn't work, or a calculation with #... doesn't work.

Cause: the two grammars are different by design:

See Default Values and Auto-Populate and Calculations.

Using <> for not-equals

Symptom: the expression doesn't parse or never matches.

Cause: <> is not supported.

Fix: use !=.

Using ? : ternary in a calculation

Symptom: a calculation that worked in one place doesn't parse on the web runtime.

Cause: the ? : ternary is only supported by the Flutter expression engine.

Fix: use if(condition, then, else).

Still stuck?

1. Check the reference — Expression Reference, Date and Time Reference.
2. Test in Preview — the designer's Preview mode runs the real runtime.
3. Check validation messages — a constraint failure shows the constraint_message (or a generated explanation); a blank value is usually a null-result, not an error.
4. Ask with the exact expression — include the question names and the expression you wrote; that is usually enough to spot the issue.