Scientific Investigation Design
Master the principles of experimental design, including identifying variables, establishing controls, ensuring validity and reliability, and understanding ethical considerations in scientific research.
Pax says: "Good science starts with good design! A well-designed investigation produces data you can trust. Let's learn the principles that separate rigorous science from unreliable guesswork."
Experimental Design and Variables
A well-designed experiment tests a hypothesis by systematically changing one factor while keeping all others constant. This requires careful identification and management of three types of variables.
Types of Variables
Independent Variable
The factor deliberately changed by the researcher
e.g., concentration of fertiliser
Dependent Variable
The factor measured or observed as the outcome
e.g., plant height after 4 weeks
Controlled Variables
All other factors kept constant to ensure a fair test
e.g., light, water, soil type, temperature
Control Group
A group that does not receive the treatment or independent variable change. It provides a baseline for comparison to determine whether the independent variable had an effect.
Experimental Group
The group that receives the treatment or independent variable change. Results from this group are compared to the control to identify cause-and-effect relationships.
Validity, Reliability, and Accuracy
These three concepts are distinct and all essential for trustworthy scientific results. Understanding the difference is critical for designing strong investigations and evaluating the quality of scientific claims.
Validity vs. Reliability vs. Accuracy
Validity
Does the experiment actually test what it claims to test? Are confounding variables controlled?
Reliability
Can the results be consistently reproduced by repeating the experiment?
Accuracy
How close are the measurements to the true or accepted value?
Improving Reliability
Repeat trials: Perform multiple trials and calculate the mean to reduce the effect of random errors
Sample size: Use a larger sample to better represent the population and reduce sampling bias
Standardised procedure: Follow the same method precisely each time
Key Distinction: A measurement can be reliable (consistent) but not accurate (correct). For example, a miscalibrated thermometer might consistently read 2 degrees too high -- the results are reliable but not accurate. Good science requires both.
Ethics in Scientific Research
Ethical considerations are a fundamental part of experimental design. Scientists must balance the pursuit of knowledge with respect for living organisms, human participants, and the environment. In Australia, research ethics are governed by institutional review boards and national guidelines.
Human Research Ethics
- Informed consent: Participants must understand and agree
- Confidentiality: Personal data must be protected
- Right to withdraw: Participants can leave at any time
- Minimise harm: Physical and psychological risks must be minimised
Animal Research Ethics
- Replacement: Use alternatives where possible
- Reduction: Minimise the number of animals used
- Refinement: Minimise suffering and improve welfare
- These are known as the 3Rs principle
Scientific Integrity
Beyond participant welfare, ethical science requires: honest reporting of all results (including negative findings), proper attribution (avoiding plagiarism), peer review before publication, and transparency about methods and funding sources. Scientific fraud undermines public trust and can have serious real-world consequences.
Key Vocabulary
Validity
The extent to which an experiment actually measures what it claims to measure. Internal validity requires controlling confounding variables; external validity concerns generalisability.
Reliability
The consistency and reproducibility of results when an experiment is repeated under the same conditions. Improved by multiple trials and standardised methods.
Confounding Variable
An uncontrolled variable that changes alongside the independent variable and may affect the dependent variable, making it impossible to establish cause and effect.
Informed Consent
The ethical requirement that human research participants be fully informed about the study's purpose, procedures, risks, and their right to withdraw before agreeing to participate.
Worked Examples
A student investigates how temperature affects enzyme activity. Identify the independent, dependent, and two controlled variables.
Independent variable: Temperature (the factor deliberately changed, e.g., 20°C, 30°C, 40°C, 50°C)
Dependent variable: Rate of enzyme activity (measured by, e.g., volume of product formed per minute)
Controlled variables: (1) Enzyme concentration -- same amount of enzyme in each trial. (2) Substrate concentration -- same amount of substrate. Also: pH, volume, and timing.
A drug trial gives a new medication to patients in Group A and a sugar pill to patients in Group B. Neither the patients nor the doctors know who gets which. Explain this design.
Step 1: Group A is the experimental group; Group B receiving the sugar pill (placebo) is the control group.
Step 2: The sugar pill is a placebo, controlling for the psychological effect of believing one is being treated.
Answer: This is a double-blind trial -- neither patients nor doctors know who receives the real drug. This eliminates both placebo effect and observer bias, increasing validity.
A student measures the boiling point of water 5 times and gets: 99.8, 99.9, 100.1, 99.7, 100.0 (°C). Evaluate the reliability and accuracy of these results.
Reliability: The measurements are closely clustered (range = 0.4°C), indicating high reliability -- the results are consistent and reproducible.
Accuracy: The mean = (99.8 + 99.9 + 100.1 + 99.7 + 100.0) / 5 = 99.9°C. The accepted value is 100.0°C at standard pressure.
Answer: The results are both reliable (consistent) and accurate (mean very close to accepted value). The small deviation may be due to altitude or atmospheric pressure differences.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
In an experiment, the independent variable is the factor that:
Question 2
The purpose of a control group in an experiment is to:
Question 3
A set of measurements that are closely clustered together but far from the true value are:
Question 4
The "3Rs" principle in animal research ethics refers to:
Question 5
A confounding variable is problematic because it:
Key Concepts Summary
- ●Good experimental design identifies independent, dependent, and controlled variables and includes a control group for baseline comparison.
- ●Validity means the experiment tests what it claims to test; reliability means results are consistent and reproducible; accuracy means results are close to the true value.
- ●Confounding variables threaten validity by making it impossible to establish cause and effect.
- ●Reliability is improved through repeat trials, larger sample sizes, and standardised procedures.
- ●Ethical research requires informed consent, confidentiality, minimising harm, and applying the 3Rs (Replacement, Reduction, Refinement) for animal research.