# Longitudinal data collection

In a **longitudinal** project, the same subject — a household, a farmer, a patient, an animal, a plot of land — is visited and surveyed repeatedly over time. The dataset is not "one row per household" but "many rows per household, linked together so you can study them as a single trajectory".

FormShare supports this pattern natively with the concept of **cases**. A case is the long-lived identity of a subject (the household ID, the farmer ID, etc.). Each case is created once with a **case creator form**, and is then updated through **follow-up forms** at every subsequent visit. FormShare maintains a live list of the cases enumerators can visit — the **real-time CSV file** — and exposes it to ODK Collect so the field team always picks the right case at the right time.

## When you need it

You probably want a longitudinal project if any of these are true:

* You revisit the same subjects over time (baseline + endline; quarterly check-ins; weekly observations).
* The forms you collect are split by *what* you're measuring (registration, vitals, livelihoods, compliance) but always against the same subject.
* You need to enforce that a subject is registered before any follow-up data can be collected for it.
* You need to mark some subjects as "no longer in the panel" without losing their history.

If your project is a one-off cross-sectional survey — every submission is a distinct unit of analysis, you never revisit — you don't need longitudinal mode. Keep things simple with a standard project.

## The shape of a longitudinal project

A longitudinal project has three moving parts:

* **One case creator form**. The very first form you upload becomes the case creator: it registers new subjects into the project. The case creator form designates which variable is the **case ID** (the long-lived identifier), which is the **case label** (the human-readable display name), and which is the **case date** (when the case was registered). See "[Case creator forms](/data-management/longitudinal/case-creator-forms.md)".
* **Any number of follow-up forms**. Every subsequent form added to the project attaches data to existing cases rather than creating new ones. Each follow-up form has a **case selector** field that the enumerator uses in ODK Collect to pick which case they are visiting. See "[Follow-up forms](/data-management/longitudinal/follow-up-forms.md)".
* **The real-time CSV file**. FormShare automatically maintains a CSV with the currently active cases. ODK Collect downloads this file along with each follow-up form, so enumerators always see an up-to-date list. The file is regenerated whenever cases change. See "[The real-time CSV file](/data-management/longitudinal/real-time-csv.md)".

In addition, follow-up forms can carry an **activation effect**: the same submission that adds data to a case can also mark it inactive (so it disappears from the picker) or reactivate a previously inactive case. See "[Activating / de-activating a case](/data-management/longitudinal/activate-deactivate-case.md)".

## Turning the feature on

A longitudinal project is set up at creation time by ticking **Use a case/longitudinal workflow** on the "Add new project" page. Once any form has been added to the project, the setting becomes effectively read-only: you cannot convert a regular project to a longitudinal one or the other way around without starting over.

> Add screenshot of the "Add new project" form with the case/longitudinal checkbox highlighted.

{% hint style="warning" %}
**Plan this up front.** Whether a project is longitudinal is a structural decision, not a configuration knob. Once you upload the case creator form, the project's primary key shifts from "submission ID" to "case ID + visit date" semantics, and FormShare cannot retrofit a non-longitudinal project into a longitudinal one. If in doubt, start a new test project to try both modes before committing to your real project.
{% endhint %}

## What's next

* "[Case creator forms](/data-management/longitudinal/case-creator-forms.md)" walks through uploading the first form and configuring the case fields.
* "[The real-time CSV file](/data-management/longitudinal/real-time-csv.md)" explains how ODK Collect picks up the case list and how to choose which fields appear in the picker.
* "[Follow-up forms](/data-management/longitudinal/follow-up-forms.md)" covers adding subsequent forms and wiring them to the case picker.
* "[Activating / de-activating a case](/data-management/longitudinal/activate-deactivate-case.md)" describes how cases get marked active or inactive and what that means for follow-up data collection.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.formshare.app/data-management/longitudinal.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.