• What is the first step of the scientific method?

• What is a hypothesis?

• What is a scientific law?

• How does induction differ from deduction?

Catalyst

ExperimentalDesign

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.

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.

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

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 light light

water water water

pot pot potsoil soil soil

type of plant type of plant type of plant

Light, water, pot, soil, and type of plant are all 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.

Read the paragraph aloud, then determine what is the independent variable, the dependent variable and what are the constants.

Independent Variable (The Cause): _________________________________

Dependent Variable (The Effect): ___________________________________

Constants (Kept the Same): _______________________________________

______________________________________________________________

Amount of light

Growth of plants

Same species, identical pots,

same type of soil, water, and fertilizer

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!

Read the paragraph aloud, then determine what is the independent variable, the dependent variable and what are the constants.

Remember:

Dependent variable = Data

INdependent variable = INput

If the data is given to you in the form of a table, then determining your variables is EASY!

The independent variable is always written in the

left column.

The dependent variable is always written in the

right column.

independent variable

dependent variable

Notice that this is the same rule that you

follow in math class!

X Y

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)? FertilizerWhat 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?

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.

Select the hypothesis that places the variables in the correct cause/effect relationship.

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 containerWhat 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?

The hypothesis can also be viewed as the question the experiment is trying to answer.

There are 2 very important facts about creating a valid hypothesis that you should be familiar with.

Emotions are NOT ____________.Opinions are NOT _________.

observabletestable

not testablevalid

not observablenot testable

valid

validnot observable

valid

not testablenot observable

validnot testable

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.

Notice that the constants are in every trial.This is what makes them constants!

light light light

water water water

pot pot potsoil soil soil

type of plant type of plant type of plant

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

What is the independent variable?Concentration of gases in the atmosphere

What are the constants?gas tank, bell jar, pot, soil, type of plant, temperature, water

Which trial contains all of the constants, but no change to the concentration of gases?

Trial 1 Trial 2 Trial 3

Control

Group

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 solutionG battery, electrical leads, light bulb, distilled waterH battery, gold wires, electrical leads, 30% salt solutionJ battery, electrical leads, light bulb, 25% salt solution

5% salt 10% salt 20% salt

electrical leads electrical leads electrical leadslightbulb lightbulb lightbulb

Remember: The control group is the trial that contains all of the constants, but NONE of the independent variables.

It is also important that you can determine what should be placed into a control group.

% Salt is the independent variable,

so there should be NO SALT in the control group!

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.

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.

Make sure that your conclusions and inferences are ALWAYS based on the results

of the experiment.

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.

Which group would have the most representative results?

Group 2This group has the largest sample size, so its

results will be the most accurate!

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.