← Module 5: Scientific Investigations

NSWInvestigating ScienceSyllabus dot point

Inquiry Question 2: How does the design of a valid experimental investigation allow for the analysis of first-hand data?

Evaluate scientific investigations and findings in terms of reliability, validity, accuracy and precision of data

A focused answer to the HSC Investigating Science Module 5 dot point on reliability, validity, accuracy and precision. The four concepts every Investigating Science student must distinguish, with worked HSC past exam questions.

Generated by Claude OpusReviewed by Better Tuition Academy5 min answer

Have a quick question? Jump to the Q&A page

What this dot point is asking

NESA wants you to distinguish reliability, validity, accuracy and precision, apply each to a given investigation, and identify which property of an investigation is in question when results disagree or fail to replicate. These four terms are the most heavily tested vocabulary in Investigating Science.

The answer

Four properties describe the quality of a scientific investigation and its data. They are independent: an investigation can be high on some and low on others.

Validity

The extent to which an investigation tests what it claims to test. A valid investigation:

  • Uses appropriate variables and a clearly defined dependent variable.
  • Holds controlled variables constant so the result is attributable to the independent variable.
  • Includes a control group.
  • Uses appropriate sample sizes and randomisation.
  • Avoids confounders.

Threats to validity. Confounding variables, sampling bias, measurement instruments that do not measure what is claimed.

Reliability

The extent to which an investigation produces consistent results when repeated. A reliable investigation:

  • Yields similar measurements when repeated by the same researcher.
  • Yields similar measurements when repeated by different researchers (reproducibility).
  • Has tightly clustered repeat measurements.

Threats to reliability. Random error, inconsistent technique, unstable equipment.

Accuracy

How close a measurement is to the true or accepted value. A measurement that consistently disagrees with the true value by a fixed amount is inaccurate, even if it is precise.

Threats to accuracy. Systematic error, calibration drift, observer bias.

Precision

How close repeated measurements are to each other, regardless of whether they hit the true value. Precision describes the spread of a measurement set.

Threats to precision. Random error, low-resolution instruments, careless technique.

The four together

Think of a dart board.

  • High accuracy and high precision. All darts in the bullseye.
  • High precision, low accuracy. All darts tightly clustered, but off-centre.
  • High accuracy, low precision. Darts scattered around the bullseye on average.
  • Low accuracy, low precision. Darts scattered randomly.

How to improve each

Property How to improve
Validity Better experimental design; control more variables; add a control group
Reliability More replicates; standardise procedure; use trained operators
Accuracy Calibrate instruments; use a reference standard; remove systematic error
Precision Use higher-resolution instruments; refine technique; reduce random variation

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2023 HSC4 marksDistinguish between reliability and validity in a scientific investigation. Use an example to illustrate each.
Show worked answer →

A 4-mark answer needs clear definitions, the relationship between the two, and a worked example for each.

Reliability
The extent to which an investigation produces consistent results when repeated under the same conditions. A reliable investigation can be repeated by the same or different researchers and yield similar measurements.
Validity
The extent to which an investigation tests what it was intended to test. A valid investigation measures the dependent variable as a true response to the independent variable, with controlled variables held constant.
Worked example for reliability
A student measures the boiling point of water at sea level five times and gets 100.1, 100.0, 100.2, 100.1, 100.0 degrees Celsius. The investigation is reliable: the values are tightly clustered, so repeated measurements yield similar results.
Worked example for validity
A student wants to test whether sunlight affects plant growth. If different plant species are placed in sun and shade, the results are not valid because species variation is a confounder. The same species, soil and water schedule are required for the experiment to be valid.
Relationship
An investigation can be reliable without being valid (consistently wrong) but cannot be valid without being reliable. Markers reward both definitions and worked examples.
2022 HSC3 marksExplain the difference between accuracy and precision, with an example.
Show worked answer →

A 3-mark answer needs both definitions and a target-style example.

Accuracy
How close a measurement is to the true or accepted value.
Precision
How close repeated measurements are to each other, regardless of the true value.
Example
A student measures the density of water four times:
  • Set A: 1.00, 1.00, 1.01, 1.00 g/mL. Both accurate (true value is 1.00) and precise.
  • Set B: 0.85, 0.86, 0.85, 0.86 g/mL. Precise (tightly clustered) but not accurate (systematic error).
  • Set C: 0.90, 1.05, 0.95, 1.10 g/mL. On average accurate but not precise.

Relationship. Precision relates to random error; accuracy relates to systematic error (calibration, bias). A balance reading consistently 0.1 g too high is precise but not accurate. Improving precision requires more careful technique. Improving accuracy requires calibration.

Markers reward both definitions and an example that shows the two are independent properties of a measurement set.

Related dot points