> **DRAFT** — under teacher review. # Context Diagram vs Data Flow Diagram — What Goes Where The Hamilton and Alexandra College · Year 12 · 2026 One of the most common errors in C2-2 is treating these two diagrams as interchangeable — or producing a second context diagram and calling it a DFD. They are different levels of the same model, and each has strict rules about what can appear on it. > [!TIP] > **Quick test:** count the circles. *One* circle = context diagram (Level 0). *Multiple* numbered circles = DFD (Level 1). If you have two circles on a "context diagram", you've drifted into DFD territory. --- ## Watch first (≈8 min) {{Video|src=https://www.youtube.com/watch?v=XeJs9sYAuMg}} *NSW Software Engineering — covers context diagrams and dataflow diagrams side by side using the same worked example. The notation conventions align closely with VCE.* --- ## Side-by-side: the two diagrams compared | Feature | Context Diagram (Level 0) | Level-1 DFD | |---|---|---| | **How many system shapes?** | Exactly **one circle** — the whole system | **3–6 circles** — one per major process | | **System shapes labelled as** | Your system's name (e.g. "Study Planner") | Numbered verb-noun phrases (e.g. "1. Verify Login") | | **Internal processes shown?** | **No** — this is the most common mistake | **Yes** — that is the whole point of Level 1 | | **Data stores shown?** | **No** | **Yes** — two parallel lines, named | | **External entities** | Yes — rectangles outside the system circle | Yes — same entities as the context diagram | | **Data flow labels** | Specific (e.g. "login credentials") | Specific — must be consistent with context diagram | | **Entity ↔ Entity flows?** | Never | Never | | **Entity ↔ Data store flows?** | N/A | Never — all flows go through a process | --- ## The most common mistakes ::: danger **On the context diagram** - **Showing internal processes** (e.g. drawing *"Verify User"* or *"Save Data"* as separate circles). If you have more than one circle, you have drawn a DFD, not a context diagram. - **Showing a data store** (the double-line rectangle). Data stores do not exist at context-diagram level — the whole system is one black box. - **Connecting entities to each other** with arrows. Every arrow must touch the system circle. ::: ::: danger **On the Level-1 DFD** - **Drawing only one process circle** — that is a context diagram in disguise. Decompose the system into its major functions (3–6 processes). - **Missing data stores** — if your software saves anything (user accounts, records, preferences), you need at least one data store. - **Inconsistent entities** — your DFD entities must be identical names and roles to your context diagram entities. Divergence is a rubric error. - **A process with only inputs and no outputs** (or only outputs) — a process that transforms nothing is not a process. ::: --- ## A mental check before you submit ::: success Ask yourself for each diagram: - **Context diagram:** *"If I covered the label on the system circle, would a reader know this is one system, not a set of functions?"* — If you have multiple circles, it fails this check. - **DFD:** *"Can I trace a path — from an external entity, through at least two processes and one data store, to an output?"* — If not, your decomposition is incomplete. ::: --- ::: info **Why VCAA cares** The C2-2 rubric at **3–4** requires *"illustrates the features of the context diagram/s"*. The marker is checking: one system circle, correct entities, labelled flows. Drawing internal processes on the context diagram is a notation error that costs marks at every level above 3–4. At **9–10**, the rubric requires *"no errors, inconsistencies or omissions"* across **all three diagrams**. Inconsistent entities between context and DFD is an inconsistency — even if both diagrams are otherwise correct. ::: --- ## See also - [Roadmap View vs Gantt Chart](/sd/C01/Roadmap%20View%20vs%20Gantt%20Chart) — same "common mistake" structure for a C01 diagram - C02 Resources: [external reading and videos](/sd/Resources/C02-Resources) --- ← Back to [VCE Software Development Hub](/sd/VCE%20Software%20Development%20Hub)