Hamilton 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.

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{{Video|src=https://www.youtube.com/watch?v=XeJs9sYAuMg}}

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| 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 |

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- **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.

- **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.

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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.

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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.

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- [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)

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