# Biology Experiment Design and Variables
Research Summaries passages on the ACT describe biology experiments and ask you to understand their design: what was tested, how variables were controlled, and what conclusions can be drawn. Mastering this skill is essential for the ACT Science section.
1. Types of Variables
Independent Variable (IV)
- What the researcher deliberately changes
- Examples: temperature, drug dosage, light intensity, nutrient concentration
Dependent Variable (DV)
- What is measured or observed as a result
- Examples: growth rate, heart rate, cell count, enzyme activity
Controlled Variables (Constants)
- Factors kept the same across all groups to ensure a fair test
- Examples: same species, same volume of solution, same time period
Control Group
- A group with no treatment (or standard conditions) for comparison
- Example: Plants given water but no fertiliser
2. Evaluating Experimental Design
Key Questions to Ask
- What hypothesis is being tested?
- What is the IV and DV?
- What variables are controlled?
- Is there a proper control group?
- Is the sample size adequate?
- Was the experiment repeated (replicates)?
Common Design Features in ACT Passages
- Multiple trials/groups: Testing several concentrations, temperatures, etc.
- Sequential experiments: Experiment 1 establishes a baseline; Experiment 2 changes a variable
- Comparison studies: Different organisms, habitats, or treatments compared
3. Worked Example
Passage Summary: Scientists investigated the effect of pH on amylase activity. They prepared test tubes with starch solution at pH 2, 4, 6, 7, 8, and 10. They added equal amounts of amylase to each tube and measured the time for starch to be fully digested at 37°C.
Q: Identify the IV, DV, and two controlled variables.
A:
- IV: pH of the solution
- DV: Time for complete starch digestion
- Controlled variables: Temperature (37°C), amount of amylase, concentration of starch solution, volume of solution
Q: Why was 37°C chosen?
A: 37°C is human body temperature and the optimal temperature for most human enzymes including amylase. Using this temperature ensures the enzyme works near its optimum, so the effect of pH alone can be isolated.
4. Practice Questions
Q1. A student wants to test whether caffeine affects heart rate in Daphnia (water fleas). Describe an appropriate experimental design.
A1.
- IV: Caffeine concentration (e.g., 0%, 0.1%, 0.5%, 1%)
- DV: Heart rate (beats per minute)
- Control: 0% caffeine (plain water)
- Controlled variables: Temperature, Daphnia size/age, observation time, volume of solution
- Method: Place Daphnia in each concentration, count heartbeats under a microscope for 30 seconds, multiply by 2. Use at least 5 Daphnia per concentration and calculate the mean.
Q2. In an experiment, plants were grown in red, blue, green, and white light. Only white light was used in the control. A student says the experiment shows red light is best for growth. What's wrong with this conclusion?
A2. White light contains all wavelengths including red and blue, so it's not a true control for individual colours. Also, without knowing the light intensity of each colour, differences could be due to intensity rather than wavelength.
Want to check your answers and get step-by-step solutions?
Summary
- Identify the IV (changed), DV (measured), and controlled variables (kept constant)
- Look for proper controls and adequate sample sizes
- ACT passages often present 2-3 related experiments — note what changes between them
- Don't over-interpret: conclusions must be supported by the actual data