1. Experimental
Design

2. Understanding how experiments are designed is an important aspect of your science education. Today, we will look at a few experiments and discuss the key aspects of experimental design.

3. What two things changed between the trials?
Experiments are designed to show the cause and effect relationship between two different variables. We call these two variables the independent variable (the cause) and the dependent variable (the effect).
What two things changed between the trials?
The fertilizer and the height of the plant.

4. What was the independent variable (the cause)?
The fertilizer.
What was the dependent variable (the effect)?
The height of the plant

5. What are the constants in the experiment below?
Everything else in the experiment is held constant. This means that nothing else is changed. This is done so that nothing will interfere with how the independent and dependent variables interact.
What are the constants in the experiment below?
Light, water, pot, soil, and type of plant are all constants.
light
light
light
water
water
water
type of plant
type of plant
type of plant
soil
soil
soil
pot
pot
pot

6. Amount of light Growth of plants
Read the paragraph aloud, then determine what is the independent variable, the dependent variable and what are the constants.
A student decided to conduct an experiment to investigate one of the factors involved in plant growth. She randomly selected twenty plants of the same species from the local plant nursery and placed them all in identical pots with the same type of soil. She gave them all the same amount of water and fertilizer, but she placed ten of the plants by a window and ten of the plants in a dark closet. She observed the plants and measured their growth daily for three weeks.
Amount of light
Independent Variable (The Cause): _________________________________
Growth of plants
Dependent Variable (The Effect): ___________________________________
Same species, identical pots,
same type of soil, water, and fertilizer
Constants (Kept the Same): _______________________________________
______________________________________________________________

7. Read the paragraph aloud, then determine what is the independent variable, the dependent variable and what are the constants.
A student decided to conduct an experiment to investigate one of the factors involved in plant growth. She randomly selected twenty plants of the same species from the local plant nursery and placed them all in identical pots with the same type of soil. She gave them all the same amount of water and fertilizer, but she placed ten of the plants by a window and ten of the plants in a dark closet. She observed the plants and measured their growth daily for three weeks.
Notice that the dependent variable is what gets measured and observed by the scientist at the end of the experiment. Whatever the scientist measures or observes at the end of the experiment is the dependent variable. The scientist is checking to see how this variable was affected!

8. If the data is given to you in the form of a table, then determining your variables is EASY!X Y
independent variable
dependent variable
The independent variable is always written in the left column.
The dependent variable is always written in the right column.
Notice that this is the same rule that you follow in math class!

9. Once you have determined what the variables are, determining the hypothesis of the experiment is as simple as placing the variables into a sentence.
What was the independent variable (the cause)?
Fertilizer
What was the dependent variable (the effect)?
Height of the plant
So how might you place these into a sentence that predicts this cause and effect relationship?

10. Select the hypothesis that places the variables in the correct cause/effect relationship.
What of the following would be the best hypothesis for this experiment?
A The amount of sunlight a plant receives decreases the growth of the plant.
B Plant growth determines the amount of fertilizer found in the soil.
C The size of the pots will affect the growth of the plants.
D The addition of fertilizer will increase the growth of the plant.

11. The hypothesis can also be viewed as the question the experiment is trying to answer.
The containers in the picture were filled with the same amount of hydrogen gas. The pressure inside of each container is observed after they all reach room temperature (22oC).
What is the independent variable (the cause)?
Volume of container
What is the dependent variable (the effect)?
Gas pressure
Which of these questions can best be answered with this experimental setup?
F Does the temperature of the gas affect the volume of the container?
G Does the volume of a container affect the pressure of the gas inside?
H How is the volume of a gas affected by the temperature of the gases?
J How does the pressure inside the container affect the volume of the gas?

12. Emotions are NOT ____________. Opinions are NOT _________.
There are 2 very important facts about creating a valid hypothesis that you should be familiar with.
Emotions are NOT ____________.
observable
Opinions are NOT _________.
testable
not testable
valid
not observable
not testable
valid
valid
not observable
valid
not testable
not observable
valid
not testable

13. Every valid experiment should also include a CONTROL GROUP.
The control group is the trial that contains
all of the constants,
but NONE of the independent variable.

14. Control Group Notice that the constants are in every trial.
This is what makes them constants!
Which trial contains none of the independent variable?
Notice that the entire trial is considered the control group, not any one part of the trial.
Control Group
light
light
light
water
water
water
type of plant
type of plant
type of plant
soil
soil
soil
pot
pot
pot

15. Control Group What is the independent variable?
Trial 1
Trial 2
Trial 3
Control Group
What is the independent variable?
Concentration of gases in the atmosphere
What are the constants?
gas tank, bell jar, pot, soil, type of plant, water (not shown)
Which trial contains all of the constants, but no change to the concentration of gases?

16. It is also important that you can determine what should be placed into a control group.
Remember: The control group is the trial that contains all of the constants, but NONE of the independent variable.
electrical leads
lightbulb
electrical leads
lightbulb
electrical leads
lightbulb
5% salt
10% salt
20% salt
A control group for this experiment would contain electrodes, a beaker and which of the following sets of materials?
F battery, electrical leads, light bulb, 1% salt solution
G battery, electrical leads, light bulb, distilled water
H battery, gold wires, electrical leads, 30% salt solution
J battery, electrical leads, light bulb, 25% salt solution
% Salt is the independent variable, so there should be NO SALT in the control group!

17. Another word for conclusion is inference.
Once an experiment has been finished, the scientist can draw a conclusion.
Another word for conclusion is inference.
Which conclusion can be based on the results of the experiment above?
A The growth of the plant is unaffected by the presence of fertilizer.
B Fertilizer B was the most beneficial fertilizer to the plant’s growth.
C Fertilizer A provided the most complete source of vitamins and minerals.
D The results are invalid because no fertilizer was added to trial 1.

18. Make sure that your conclusions and inferences are ALWAYS based on the results of the experiment.
Which inference can be based on the results of the experiment above?
A The growth of plants can be stunted by the presence of carbon dioxide.
B Placing a plant in air will result in the largest number of leaves.
C Plants thrive when surrounded by an atmosphere full of ozone.
D Carbon dioxide gas increases the health and growth of plants.

19. When samples are selected randomly there is no visible pattern.
Important Fact: In order for an experiment to be valid, it is important that your samples are selected __________.
randomly
When samples are selected randomly there is no visible pattern.

20. Which group would have the most representative results?
Three groups of students are doing an experiment to determine how a name brand fertilizer affects the growth of a plant. Their experimental and control groups are shown below.
Which group would have the most representative results?
Group 2
This group has the largest sample size, so its results will be the most accurate!

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