Australian Curriculum v9 / ACiQ Year 9 Science - Unit 3 - Valid, safe and reproducible investigations

Valid, safe and reproducible investigations

Plan controlled methods, precision, sample size, risk and ethical or intercultural considerations.

Updated 2026-07-26 - 11 min read

Valid, safe and reproducible investigations requires students to connect an observable phenomenon with a scientific model, mechanism or evidence chain. A dependable Year 9 explanation names the relevant system, traces what changes, supports the claim with evidence and recognises the limits of the model or investigation.

This note is designed to work with the guided lessons, curated practice, flashcards, Tutor context, Review and Rapid Revision for the same canonical target. The same three evidence checks are used throughout, so feedback can route a learner back to the precise idea that needs repair.

Plan a valid controlled method

Controls should address plausible alternative causes, not freeze every condition regardless of relevance.

A dependable reasoning routine

  1. Define the system or phenomenon and identify the change being explained.
  2. Trace the mechanism for plan a valid controlled method in causal order rather than listing disconnected terms.
  3. Link each claim to an observation, measurement, model or accepted scientific relationship.
  4. State the boundary of the conclusion: what was tested, what remains uncertain and what evidence would strengthen it.

Repair: Change the independent variable deliberately while controlling relevant alternatives.

The repair matters because the shortcut may appear to work in one familiar example while failing when the system boundary, causal mechanism, variable or evidence limit changes. Use the routine above to make the reasoning visible enough for another learner to verify.

Example 1.1

Testing light level on plant growth: key control?

Step 1 - identify the governing idea: Validity improves when the method isolates the intended relationship and measures the dependent variable appropriately.

Step 2 - apply it to this evidence: These could otherwise change growth.

Result: Plant species and starting size

The evidence-to-mechanism link is: These could otherwise change growth. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Light level — It conflicts with the stated mechanism or evidence: These could otherwise change growth.
  • Final growth result — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • The research question — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 1.2

Why randomise sample assignment?

Step 1 - identify the governing idea: Validity improves when the method isolates the intended relationship and measures the dependent variable appropriately.

Step 2 - apply it to this evidence: Randomisation supports fair comparison.

Result: To reduce systematic pre-existing differences between groups

The evidence-to-mechanism link is: Randomisation supports fair comparison. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • To guarantee identical individuals — It conflicts with the stated mechanism or evidence: Randomisation supports fair comparison.
  • To remove all uncertainty — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • To change the dependent variable — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 1.3

A dissolving test changes temperature and cube size together. Main issue?

Step 1 - identify the governing idea: Validity improves when the method isolates the intended relationship and measures the dependent variable appropriately.

Step 2 - apply it to this evidence: Two plausible causes vary.

Result: Confounding prevents attributing change to temperature

The evidence-to-mechanism link is: Two plausible causes vary. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Too many repeats — It conflicts with the stated mechanism or evidence: Two plausible causes vary.
  • No independent variable — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • Temperature has no units — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Precision, sample size and replicability

Repeated measurements assess within-method consistency; independent replication tests whether the method produces compatible evidence elsewhere.

A dependable reasoning routine

  1. Define the system or phenomenon and identify the change being explained.
  2. Trace the mechanism for precision, sample size and replicability in causal order rather than listing disconnected terms.
  3. Link each claim to an observation, measurement, model or accepted scientific relationship.
  4. State the boundary of the conclusion: what was tested, what remains uncertain and what evidence would strengthen it.

Repair: Precision must be supported by suitable calibrated equipment and a reproducible method.

The repair matters because the shortcut may appear to work in one familiar example while failing when the system boundary, causal mechanism, variable or evidence limit changes. Use the routine above to make the reasoning visible enough for another learner to verify.

Example 2.1

Best tool for 0.1 g mass differences?

Step 1 - identify the governing idea: Instrument resolution, calibration, repeat count and sample size should match the expected effect and variation.

Step 2 - apply it to this evidence: Resolution must detect the expected difference.

Result: A calibrated balance resolving at least 0.1 g

The evidence-to-mechanism link is: Resolution must detect the expected difference. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • A metre ruler — It conflicts with the stated mechanism or evidence: Resolution must detect the expected difference.
  • A whole-kilogram scale — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • Visual guessing — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 2.2

Why repeat trials?

Step 1 - identify the governing idea: Instrument resolution, calibration, repeat count and sample size should match the expected effect and variation.

Step 2 - apply it to this evidence: Repeats reveal consistency and anomalies.

Result: To assess variability and reduce reliance on one measurement

The evidence-to-mechanism link is: Repeats reveal consistency and anomalies. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • To make the hypothesis true — It conflicts with the stated mechanism or evidence: Repeats reveal consistency and anomalies.
  • To remove all systematic error — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • To change the independent variable — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 2.3

What supports independent replication?

Step 1 - identify the governing idea: Instrument resolution, calibration, repeat count and sample size should match the expected effect and variation.

Step 2 - apply it to this evidence: Another team needs an executable method.

Result: Detailed materials, steps, settings and analysis rules

The evidence-to-mechanism link is: Another team needs an executable method. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Only the final graph — It conflicts with the stated mechanism or evidence: Another team needs an executable method.
  • A secret adjustment — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • No units — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Risk, ethics and cultural protocols

Ethical planning considers consent, privacy, harm, data ownership and cultural protocols, including permissions for Country/Place, heritage sites and artefacts.

A dependable reasoning routine

  1. Define the system or phenomenon and identify the change being explained.
  2. Trace the mechanism for risk, ethics and cultural protocols in causal order rather than listing disconnected terms.
  3. Link each claim to an observation, measurement, model or accepted scientific relationship.
  4. State the boundary of the conclusion: what was tested, what remains uncertain and what evidence would strengthen it.

Repair: Controls and permissions must be specific to the activity, people, place and data.

The repair matters because the shortcut may appear to work in one familiar example while failing when the system boundary, causal mechanism, variable or evidence limit changes. Use the routine above to make the reasoning visible enough for another learner to verify.

Example 3.1

Heating glassware: useful control?

Step 1 - identify the governing idea: A risk assessment links each hazard to likelihood, consequence and a specific control.

Step 2 - apply it to this evidence: Controls address burns and breakage.

Result: Use heatproof equipment, eye protection and safe handling procedure

The evidence-to-mechanism link is: Controls address burns and breakage. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Be careful — It conflicts with the stated mechanism or evidence: Controls address burns and breakage.
  • Ignore cracks — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • Touch to test heat — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 3.2

Collecting student health data requires?

Step 1 - identify the governing idea: A risk assessment links each hazard to likelihood, consequence and a specific control.

Step 2 - apply it to this evidence: People and sensitive information require safeguards.

Result: Informed consent, privacy protection and limited necessary data

The evidence-to-mechanism link is: People and sensitive information require safeguards. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Public names by default — It conflicts with the stated mechanism or evidence: People and sensitive information require safeguards.
  • No explanation — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • Permanent unrestricted sharing — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Example 3.3

Sampling at a culturally significant site should begin with?

Step 1 - identify the governing idea: A risk assessment links each hazard to likelihood, consequence and a specific control.

Step 2 - apply it to this evidence: Access and knowledge protocols must be respected.

Result: Appropriate permission and guidance from relevant custodians or authorities

The evidence-to-mechanism link is: Access and knowledge protocols must be respected. Notice that the conclusion does not extend beyond the stated system or evidence. A scientific explanation must trace cause and effect, not only name the relevant vocabulary.

Why the alternatives fail:

  • Taking samples secretly — It conflicts with the stated mechanism or evidence: Access and knowledge protocols must be respected.
  • Assuming public access permits all research — It changes the system boundary or claims a cause that the supplied observations do not establish.
  • Publishing locations immediately — It extends the conclusion beyond the evidence. A valid answer must preserve the variables, sequence and uncertainty in the prompt.

Retrieval check

Try these without looking back at the examples.

  1. Why randomise sample assignment?
  2. Why repeat trials?
  3. Collecting student health data requires?

Answers

  1. To reduce systematic pre-existing differences between groups — Randomisation supports fair comparison.
  2. To assess variability and reduce reliance on one measurement — Repeats reveal consistency and anomalies.
  3. Informed consent, privacy protection and limited necessary data — People and sensitive information require safeguards.

Transfer task

Find an unfamiliar example from school, daily life, a credible news source or another subject. Explain which of the three evidence checks applies. Complete the task, then audit your own response: identify the evidence used, the relationship applied, one plausible misconception and the final reasonableness check. If a peer could not reproduce your reasoning, add the missing step.

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