Investigation Lab
Learn
This lesson focuses on experimental design and investigation analysis, a critical skill for the ACT Science section. You will learn to identify variables, evaluate experimental procedures, and predict outcomes based on methodology.
Key Concepts
- Independent Variable: The factor you deliberately change or manipulate in an experiment
- Dependent Variable: The factor you measure or observe as a result of changing the independent variable
- Control Variables: Factors kept constant to ensure a fair test
- Control Group: A baseline group that receives no treatment for comparison
- Experimental Group: The group that receives the treatment being tested
Experimental Design Principles
Strong experiments follow these principles:
- Single Variable Testing: Change only one variable at a time
- Replication: Repeat trials to ensure reliability
- Randomization: Assign subjects randomly to reduce bias
- Sample Size: Use enough subjects for statistically meaningful results
Common ACT Question Types
- Identifying the purpose of an experiment
- Determining which variable is independent vs. dependent
- Evaluating whether a conclusion is supported by the data
- Predicting results of modified experiments
- Identifying flaws in experimental design
Examples
Example 1: Identifying Variables
A scientist investigates how fertilizer concentration affects plant height. She grows 20 plants in identical conditions, varying only the amount of fertilizer (0g, 5g, 10g, 15g per pot). After 4 weeks, she measures plant height.
Question: What is the independent variable?
Solution: The independent variable is fertilizer concentration (what the scientist deliberately changes). The dependent variable is plant height (what she measures as a result).
Example 2: Evaluating Experimental Design
A student wants to test if caffeine improves reaction time. He has 10 participants drink coffee, then tests their reaction times using a computer program.
Question: What is a major flaw in this experimental design?
Solution: There is no control group. The student should have some participants drink decaffeinated coffee (or water) to compare reaction times with and without caffeine.
Example 3: Predicting Modified Experiments
Experiment 1 tested enzyme activity at temperatures of 20C, 30C, 40C, and 50C. Results showed activity peaked at 40C and declined at 50C.
Question: If researchers repeated the experiment at 60C, what would most likely happen to enzyme activity?
Solution: Based on the trend (declining after 40C), enzyme activity would likely continue to decrease at 60C, possibly approaching zero as enzymes denature at high temperatures.
Practice
Complete these practice problems to reinforce your understanding of experimental investigation.
1. A researcher studies how light intensity affects photosynthesis rate in aquatic plants. She places plants at distances of 10cm, 20cm, 30cm, and 40cm from a light source and measures oxygen bubble production per minute. Identify the: (a) independent variable, (b) dependent variable, (c) one control variable that should be kept constant.
2. An experiment tests whether a new antibiotic kills bacteria. The researcher applies the antibiotic to bacterial cultures and measures the zone of inhibition (area with no bacterial growth). What would serve as an appropriate control in this experiment?
3. A study claims that listening to classical music improves test scores. Thirty students listened to Mozart for 10 minutes before a math test and scored an average of 85%. Is this conclusion valid? Explain why or why not.
4. Experiment A tested reaction rates at pH levels of 4, 5, 6, and 7. The fastest rate occurred at pH 6. If the experiment were modified to test pH 8 and pH 9, would you expect the reaction rate to increase, decrease, or remain the same? Justify your prediction.
5. A scientist wants to determine if a new fertilizer increases crop yield. She applies the fertilizer to one field and compares the harvest to a field without fertilizer. The fertilized field produced 20% more corn. What additional information would strengthen her conclusion?
6. Two students design experiments to test how salt concentration affects seed germination. Student A uses 5 seeds per concentration. Student B uses 50 seeds per concentration. Which experimental design is more reliable and why?
7. A pharmaceutical company tests a new headache medication. Group A receives the medication, Group B receives a sugar pill. Neither the patients nor the doctors know who received which treatment. What is the purpose of this double-blind design?
8. An ecologist studies bird populations in two forests. Forest A has been affected by pollution; Forest B has not. She counts 45 species in Forest A and 78 species in Forest B. Can she conclude that pollution reduces bird diversity? What confounding variables might affect this comparison?
9. A student tests how different surfaces affect the distance a toy car travels. She releases the car from the same height on carpet, tile, wood, and concrete. She conducts 3 trials on each surface. What is the purpose of conducting multiple trials?
10. Researchers want to study if a protein supplement helps athletes build muscle. They recruit 100 athletes and randomly assign 50 to receive the supplement and 50 to receive a placebo. After 8 weeks, they measure muscle mass changes. Why is random assignment important in this study?
Check Your Understanding
Answer these review questions to assess your mastery of investigation and experimental design concepts.
1. In an experiment testing how temperature affects enzyme activity, what must remain constant?
Show Answer
pH level, enzyme concentration, substrate concentration, reaction time, and type of enzyme should all remain constant (control variables).
2. Why is having a control group essential in most experiments?
Show Answer
A control group provides a baseline for comparison. Without it, you cannot determine whether observed changes are due to the treatment or other factors.
3. What is the difference between correlation and causation?
Show Answer
Correlation means two variables are related or change together. Causation means one variable directly causes the other to change. Correlation does not prove causation.
Next Steps
- Review the difference between independent, dependent, and control variables
- Practice identifying experimental flaws in sample ACT passages
- Move on to the next lesson: Data and Graphs