Hamilton College · Year 12 · 2026

On context diagrams and data flow diagrams, students frequently draw only the humans they can think of — "Student", "Teacher" — and miss every external system their software talks to. Missing an entity is a rubric error at every level above 3–4, because the rubric rewards *"relationships between the existing system, entities and data flows"*.

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Two words matter: **users** (human roles) and **external systems** (non-human). Both go on your context diagram as rectangles.

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| Type | What it is | VCE SAT examples |

|---|---|---|

| **Human user (primary)** | The person the software is primarily built for | Student, Teacher, Administrator |

| **Human user (secondary)** | A person affected by the system's outputs | Parent, School Principal |

| **External software system** | An API, platform, or service your system sends data to or receives data from | Google Calendar API, Payment Gateway, Email Server |

| **External data source** | A data feed or file system outside your application boundary | School database, CSV import, Government open data |

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Ask three questions:

1. Does it send data **into** my system, or receive data **out** of it?

2. Is it **outside** the boundary of the software I am building?

3. Is it a **distinct role or system** (not an internal component of my software)?

If all three answers are "yes" — it is an entity. Add it to your diagram.

If it is *inside* your system (e.g. a function you are writing, a class in your code, a database table your code owns) — it is **not** an entity. It belongs on the DFD as a process or data store.

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These are the entities students most often miss, sorted by project type:

| Project type | Commonly missed entity | Why it is missed |

|---|---|---|

| Any project with login | Authentication provider (e.g. Google OAuth) | Students think login is "inside" their app |

| Any project that emails users | Email server / SMTP service | Students don't model outgoing system calls |

| School management tool | Admin user or IT department | Students only think about the primary user |

| App using external data | External database or API (e.g. weather API) | Students model the API as a feature, not an entity |

| App with notifications | Push notification service | Treated as internal, but it is external |

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Your `data-collection-plan.md` user stories are the best source of entities. For each story:

- **"As a [role]"** → that role is probably an entity (if they interact with your system directly).

- **"I want [goal] so that [benefit]"** → what external systems does achieving this goal require? Each one is a candidate entity.

Work through every user story before finalising your context diagram.

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The C2-2 rubric at **5–6** requires *"illustrates the relationships between the existing system, entities and data flows"*. "Some errors or omissions exist" is still acceptable at 5–6. But at **9–10**, "no errors, inconsistencies or omissions exist" — a missing entity is an omission, and omissions cost you the top band.

The most common C2-2 omission flagged in past years: the email server or notification service that every project needs but few students draw.

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- [Context Diagram vs Data Flow Diagram](/sd/C02/Context%20Diagram%20vs%20Data%20Flow%20Diagram)

- C02 Resources: [external reading and videos](/sd/Resources/C02-Resources)

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