Form Logic

ODK Collect supports a wide range of dynamic form behavior. This document covers how to specify this behavior in your XLSForm definition.

See also

XLSForm

Form logic building blocks

Variables

Variables reference the value of previously answered questions. To use a variable, put the question's name in curly brackets preceded by a dollar sign:

${question-name}

Variables can be used in label, hint, and repeat_count columns, as well as any column that accepts an expression.

XLSForm

survey

type	name	label
text	your_name	What is your name?
note	hello_name	Hello, ${your_name}.

Expressions

An expression, sometimes called a formula, is evaluated dynamically as a form is filled out. It can include XPath functions, operators, values from previous responses, and (in some cases) the value of the current response.

Example expressions

${bill_amount} * 0.18

Multiplies the previous value bill_amount by 18%, to calculate a suitable tip.

concat(${first_name}, ' ', ${last_name})

Concatenates two previous responses with a space between them into a single string.

${age} >= 18

Evaluates to True or False, depending on the value of age.

round(${bill_amount} * ${tip_percent} * 0.01, 2)

Calculates a tip amount based on two previously entered values, and then rounds the result to two decimal places.

Expressions are used in:

• Calculations
• Validating and restricting responses
• Conditionally showing questions

Calculations

To evaluate complex expressions, use a calculate row. Put the expression to be evaluated in the calculation column. Then, you can refer to the calculated value using the calculate row's name.

Expressions cannot be used in label and hint columns, so if you want to display calculated values to the user, you must first use a calculate row and then a variable.

XLSForm

survey

type	name	label	calculation
decimal	bill_amount	Bill amount:
calculate	tip_18		round((${bill_amount} * 0.18),2)
calculate	tip_18_total		${bill_amount} + ${tip_18}
note	tip_18_note	Bill: $${bill_amount} Tip (18%): $${tip_18} Total: $${tip_18_total}

Form logic gotchas

When expressions are evaluated

Every expression is constantly re-evaluated as an enumerator progresses through a form. This is an important mental model to have and can explain sometimes unexpected behavior. More specifically, expressions are re-evaluated when:

• a form is opened
• the value of any question in the form changes
• a repeat group is added or deleted
• a form is saved or finalized

A common misconception is that expressions are only evaluated when a question that uses it is reached. This faulty mental model leads form designers to include functions such as random() or now() and to expect them to be evaluated exactly once. In fact, they will be re-evaluated over and over again until the form is finalized for the last time. For example, the following calculate will keep track of the last time the form was saved:

survey

type	name	label	calculation
calculate	datetime_last_saved		now()

The once() function prevents multiple evaluation by only evaluating the expression passed into it if the node has no value. That means the expression will be evaluated once either on form open or when any values the expression depends on are set.

Every call on now() in the form will have the same value unless the once() function is used. For example, the following calculate will keep track of the first time the form was opened:

survey

type	name	label	calculation
calculate	datetime_first_opened		once(now())

The following calculate will keep track of the first time the enumerator set a value for the age question:

survey

type	name	label	calculation
integer	age	What is your age?
calculate	age_timestamp		if(age = '', '', once(now()))

Empty values in math

Unanswered number questions are nil. That is, they have no value. When a variable referencing an empty value is used in a math operator or function, it is treated as Not a Number (NaN). The empty value will not be converted to zero. The result of a calculation including NaN will also be NaN, which may not be the behavior you want or expect.

To convert empty values to zero, use either the coalesce() function or the if() function.

coalesce(${potentially_empty_value}, 0)

if(${potentially_empty_value}="", 0, ${potentially_empty_value})

Requiring responses

By default, users are able to skip questions in a form. To make a question required, put yes in the required column.

Required questions are marked with a small asterisk to the left of the question label. You can optionally include a required_message which will be displayed to the user who tries to advance the form without answering the question.

XLSForm

survey

type	name	label	required	required_message
text	name	What is your name?	yes	Please answer the question.

Setting default responses

To provide a default response to a question, put the response value in the default column.

Default values must be static values, not expressions or variables.

Note

The content of the default row in a question is taken literally as the default value. Quotes should not be used to wrap string values, unless you actually want those quote marks to appear in the default response value.

XLSForm

survey

type	name	label	default
select_one contacts	contact_method	How should we contact you?	phone_call

choices

list_name	name	label
contacts	phone_call	Phone call
contacts	text_message	Text message
contacts	email	Email

Tip

You may want to use a previously entered value as a default, but the default column does not accept dynamic values.

To work around this, use the calculation column instead, and wrap your default value expression in a once() function.

XLSForm

survey

type	name	label	calculation
text	name	Child's name
integer	current_age	Child's age
select_one gndr	gender	Gender
integer	malaria_age	Age at malaria diagnosis	once(${current_age})

This solution has some limitations, though.

• The value of the calculated default will get set to the first value that the earlier question receives, even if it is changed before viewing the later question.

  Example: In the above form, if you enter 8 on current_age, then advance to gender, then back up and change current_age to 10, when you get to malaria_age, the default value will be 8.

• If the first earlier question has a value, the dependent question will also have a value — once() will evaluate anytime the question's value is blank.

  Example: In the above form, if you enter 8 on current_age and then delete the value 8 when you get to malaria_age (intending to leave it blank) the 8 value will come back as the answer when you advance. (In this case, using a blank value to indicate "child does not have malaria" would fail.)

Validating and restricting responses

To validate or restrict response values, use the constraint column. The constraint expression will be evaluated when the user advances to the next screen. If the expression evaluates to True, the form advances as usual. If False, the form does not advance and the constraint_message is displayed.

The entered value of the response is represented in the expression with a single dot (.).

Constraint expressions often use comparison operators and regular expressions. For example:

. >= 18

True if response is greater than or equal to 18.

. < 20 and . > 200

True if the response is between 20 and 200.

regex(.,'p{L}+')

True if the response only contains letters, without spaces, separators, or numbers.

not(contains(., 'prohibited'))

True if the substring prohibited does not appear in the response.

Note

Constraints are not evaluated if the response is left blank. To restrict empty responses, make the question required.

See also

Using regular expressions

XLSForm

survey

type	name	label	constraint	constraint_message
text	middle_initial	What is your middle initial?	regex(., 'p{L}')	Just the first letter.

Read-only questions

To completely restrict user-entry, use the read_only column with a value of yes. This is usually combined with a default response, which is often calculated based on previous responses.

XLSForm

survey

type	name	label	read_only	default	calculation
decimal	salary_income	Income from salary
decimal	self_income	Income from self-employment
decimal	other_income	Other income
calculate	income_sum				${salary_income} + ${self_income} + ${other_income}
decimal	total_income	Total income	yes	${income_sum}

Conditionally showing questions

The relevant column can be used to show or hide questions and groups of questions based on previous responses.

If the expression in the relevant column evaluates to True, the question or group is shown. If False, the question is skipped.

Often, comparison operators are used in relevance expressions. For example:

${age} <= 5

True if age is five or less.

${has_children} = 'yes'

True if the answer to has_children was yes.

Relevance expressions can also use functions. For example:

selected(${allergies}, 'peanut')

True if peanut was selected in the Multi select widget named allergies.

contains(${haystack}, 'needle')

True if the exact string needle is contained anywhere inside the response to haystack.

count-selected(${toppings}) > 5

True if more than five options were selected in the Multi select widget named toppings.

Simple example

XLSForm

survey

type	name	label	relevant
select_one yes_no	watch_sports	Do you watch sports?
text	favorite_team	What is your favorite team?	${watch_sports} = 'yes'

choices

list_name	name	label
yes_no	yes	Yes
yes_no	no	No

Complex example

XLSForm

survey

type	name	label	hint	relevant	constraint
select_multiple medical_issues	what_issues	Have you experienced any of the following?	Select all that apply.
select_multiple cancer_types	what_cancer	What type of cancer have you experienced?	Select all that apply.	selected(${what_issues}, 'cancer')
select_multiple diabetes_types	what_diabetes	What type of diabetes do you have?	Select all that apply.	selected(${what_issues}, 'diabetes')
begin_group	blood_pressure	Blood pressure reading	selected(${what_issues}, 'hypertension')
integer	systolic_bp	Systolic			. > 40 and . < 400
integer	diastolic_bp	Diastolic			. >= 20 and . <= 200
end_group
text	other_health	List other issues.		selected(${what_issues}, 'other')
note	after_health_note	This note is after all health questions.

choices

list_name	name	label
medical_issues	cancer	Cancer
medical_issues	diabetes	Diabetes
medical_issues	hypertension	Hypertension
medical_issues	other	Other
cancer_types	lung	Lung cancer
cancer_types	skin	Skin cancer
cancer_types	prostate	Prostate cancer
cancer_types	breast	Breast cancer
cancer_types	other	Other
diabetes_types	type_1	Type 1 (Insulin dependent)
diabetes_types	type_2	Type 2 (Insulin resistant)

Warning

Calculations are evaluated regardless of their relevance.

For example, if you have a calculate widget that adds together two previous responses, you cannot use relevant to skip in the case of missing values. (Missing values will cause an error.)

Instead, use the if() function to check for the existence of a value, and put your calculation inside the then argument.

For example, when adding together fields a and b:

if(${a} != '' and ${b} != '', ${a} + ${b}, '')

In context:

type	name	label	calculation
integer	a	a =
integer	b	b =
calculate	a_plus_b		if(${a} != '' and ${b} != '', ${a} + ${b}, '')
note	display_sum	a + b = ${a_plus_b}

Repeating questions and groups of questions

Note

Using repetition in a form is very powerful but can also make training and data analysis more time-consuming. Aggregate does not export repeats so Briefcase or one of the data publishers will be needed to transfer data from Aggregate. Repeats will be in their own documents and will need to be joined with their parent records for analysis.

Before adding repeats to your form, consider other options:

• if the number of repetitions is small and known ahead of time, consider "unrolling" the repeat by copying the same questions several times.
• if the number of repetitions is large and includes many questions, consider building a separate form that enumerators fill out multiple times and link the forms with some parent key (e.g., a household ID).

If repeats are needed, consider adding some summary calculations at the end so that analysis will not require joining the repeats with their parent records. For example, if you are gathering household information and would like to compute the total number of households visited across all enumerators, add a calculation after the repeats that counts the repetitions in each submission.

To repeat questions or groups of questions use the begin_repeat…end_repeat syntax.

XLSForm (Single question repeat)

survey

type	name	label
begin_repeat	my_repeat_group	Repeat group label
text	repeated_question	This question will be repeated.
end_repeat

XLSForm (Multi-question repeat)

survey

type	name	label
begin_repeat	my_repeat	Repeat group label
note	repeated_note	These questions will be repeated as an entire group.
text	name	What is your name?
text	quest	What is your quest?
text	fave_color	What is your favorite color?
end_repeat

Controlling the number of repetitions

User-controlled repeats

By default, the user controls how many times the questions are repeated.

Before each repetition, the user is asked if they want to add another repeat group.

Note

The label in the begin_repeat row is shown in the Add New Group? message.

A meaningful label will help enumerators and participants navigate the form as intended.

The user is given the option to add each iteration.

XLSForm

survey

type	name	label
begin_repeat	repeat_example	repeat group label
text	repeat_test	Question label
end_repeat

Note

This interaction may be confusing to users the first time they see it. If enumerators know the number of repetitions ahead of time, consider using dynamically defined repeats.

Statically defined repeats

Use the repeat_count column to define the number of times a group will repeat.

XLSForm

survey

type	name	label	repeat_count
begin_repeat	my_repeat	Repeat group label	3
note	repeated_note	These questions will be repeated as an entire group.
text	name	What is your name?
text	quest	What is your quest?
text	fave_color	What is your favorite color?
end_repeat

Dynamically defined repeats

The repeat_count column can reference previous responses and calculations.

XLSForm

survey

type	name	label	repeat_count
integer	number_of_children	How many children do you have?
begin_repeat	child_questions	Questions about child	${number_of_children}
text	child_name	Child's name
integer	child_age	Child's age
end_repeat

See also

Repeat Recipes and Tips

Filtering options in select questions

To limit the options in a select question based on the answer to a previous question, use a choice_filter row in the survey sheet, and filter key columns in the choices sheet.

For example, you might ask the user to select a state first, and then only display cities within that state. This is called a cascading select, and can be extended to any depth. This example form shows a three-tiered cascade: state, county, city.

XLSForm

survey

type	name	label	choice_filter
select_one job_categories	job_category	Job category
select_one job_titles	job_title	Job title	job_category=${job_category}

choices

list_name	name	label	job_category
job_categories	finance	Finance
job_categories	hr	Human Resources
job_categories	admin	Administration/Office
job_categories	marketing	Marketing
job_titles	ar	Accounts Receivable	finance
job_titles	ap	Account Payable	finance
job_titles	bk	Bookkeeping	finance
job_titles	pay	Payroll	finance
job_titles	recruiting	Recruiting	hr
job_titles	training	Training	hr
job_titles	retention	Retention	hr
job_titles	asst	Office Assistant	admin
job_titles	mngr	Office Manager	admin
job_titles	scheduler	Scheduler	admin
job_titles	reception	Receptionist	admin
job_titles	creative_dir	Creative Director	marketing
job_titles	print_design	Print Designer	marketing
job_titles	ad_buyer	Ad Buyer	marketing
job_titles	copywriter	Copywriter	marketing