• Quick retrieval from earlier learning (Repeat readings, means and anomalies; Converting units). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A group measured the temperature rise in a reaction three times: 9, 6 and 6 °C. What is the mean? Give your answer to 1 decimal place. → 7 °CPrompt: Add the three readings.
2. Convert 12 L into mL. → 12000 mLPrompt: How many mL are in 1 L?
3. Four repeat readings of the time for a ball to roll down a ramp (s): 25, 26, 25, 13. Which reading is anomalous (does not fit the pattern)? (13 / 26 / 25) → 13Prompt: Most of the readings are close together.

• Show a simple experiment with one thing changed and ask what makes it a fair test.
• Share the objective: Accuracy, precision, errors and significant figures. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A balance shows 0.3 g when nothing is on it. What kind of error is this?
• Step 1: A scale that does not start at zero adds the same error every time: a systematic (zero) error.
• Step 2: So the answer is: a zero error (a systematic error)
• Answer: a zero error (a systematic error)
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. A balance shows 0.3 g when nothing is on it. What kind of error is this? (a random error / no error / a zero error (a systematic error) / an anomalous result) → a zero error (a systematic error)Prompt: It shifts every reading by the same amount.
2. A balance reads 0.5 g with nothing on it. With a rock on it, it reads 30.2 g. What is the true mass of the rock, in g? → 29.7 gPrompt: The zero error is included in every reading.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Readings of the same length: 12.1, 12.1, 12.2, 12.1 cm. The true length is 13.0 cm. The readings are… (neither repeatable nor close / accurate but not precise / accurate and precise / precise but not accurate) → precise but not accuratePrompt: Precise means close to each other. Accurate means close to the true value.
2. A balance reads 0.4 g with nothing on it. With a rock on it, it reads 12.2 g. What is the true mass of the rock, in g? → 11.8 gPrompt: The zero error is included in every reading.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.