Controlled Experiments: Variables, Groups, and Experimental Design

Controlled Experiments

Controlled experiments are a fundamental method in biology for testing the effect of a single variable (factor) on an outcome. By comparing an experimental group to a control group, scientists can determine whether a specific factor causes a particular effect.

Key Concepts in Experimental Design

• Independent Variable: The factor that is deliberately changed or manipulated by the experimenter.

• Dependent Variable: The factor that is measured or observed; it is expected to change in response to the independent variable.

• Controlled Variables (Constants): All other factors that are kept the same between experimental and control groups to ensure that only the independent variable affects the outcome.

• Control Group: The group that does not receive the experimental treatment; used as a baseline for comparison.

• Experimental Group: The group that receives the experimental treatment (the independent variable is applied).

Example Scenarios and Analysis

Below are several scenarios illustrating how to identify variables and groups in controlled experiments.

1. Effect of Amount of Reading Material on Test Performance

• Scenario: Students are split into two groups. One group receives $10 of reading material to review at the end of the course, while the other group does not.

• Independent Variable: Amount of reading material offered

• Dependent Variable: Performance on test

• Controlled Variables: Reading performance before test, age, what they are reading, etc.

• Control Group: Students not receiving the reading material

• Experimental Group: Students receiving the reading material

2. Effect of Fertilizer on Growth of Daisies

• Scenario: Two large planters are set up, each with the same number of daisy plants. One receives fertilizer, the other does not.

• Independent Variable: Amount of fertilizer added

• Dependent Variable: Growth of daisies

• Controlled Variables: Amount of water, amount of sun, size of planter

• Control Group: Daisies without fertilizer

• Experimental Group: Daisies with fertilizer

3. Effect of Shape/Position on Model Plane Speed

• Scenario: The shape or position of model planes is altered to test the effect on speed.

• Independent Variable: Shape/position of model planes

• Dependent Variable: Speed of planes

• Controlled Variables: Type of plane used, motor, weight, flying conditions

• Control Group: Planes with no altered wing shape/position

• Experimental Group: Planes with new shape/position

• Additional info: This could be considered two separate experiments if both shape and position are tested independently.

4. Effect of College Degree on Perceived Respectfulness

• Scenario: Comparing how respectful people are perceived to be, based on whether they have a college degree.

• Independent Variable: College degree status

• Dependent Variable: How respectful they are perceived

• Controlled Variables: Where they're from, age range, gender

• Control Group: People without a college degree

• Experimental Group: People with a college degree

Summary Table: Variables and Groups in Example Experiments

Scenario	Independent Variable	Dependent Variable	Control Group	Experimental Group
Reading material & test performance	Amount of reading material	Test performance	No reading material	With reading material
Fertilizer & daisy growth	Amount of fertilizer	Growth of daisies	No fertilizer	With fertilizer
Model plane shape/position & speed	Shape/position of plane	Speed of plane	No change	Altered shape/position
College degree & respectfulness	College degree	Perceived respectfulness	No degree	With degree

Key Takeaways

• Always identify and control variables to ensure valid experimental results.

• Use control and experimental groups to isolate the effect of the independent variable.

• Careful experimental design is essential for drawing reliable scientific conclusions.