Designing an experiment 7 TH /8 TH GRADE SCIENCE

Designing an experiment 7 TH /8 TH GRADE SCIENCE

Scientific inquiry 1. Make an observation 2. Ask a question 3. Create a hypothesis 4. Design an experiment 5. Gather and analyze data 6. Draw conclusions 7. Share results 8. Repeat?

Designing an experiment Scientific ideas must be tested by gathering evidence through experimentation. Experiments are carefully designed to create the data needed to evaluate your hypothesis. You need to consider several things Variables Control and experimental groups Data collection Procedure

Variables Variables are factors of an experiment that can change. Ex. Temperature, amount of light, time, concentration of chemicals, or species of animal. Some variables the scientist will change, but others change on their own. In a well designed experiment, the scientist only changes one variable at a time. If multiple variables are changed at once, you might have trouble figuring out which one is effecting your results.

Variables The independent variable (or manipulated variable) is the variable that is directly changed by the scientist. Ex: different types of fertilizer, amount of water given to the plants, the amount of sunlight a plant receives. Scientists only change (manipulate) ONE variable at a time.

Variables The dependent variable(s) (or responding variable) are the changes that happen because the scientist manipulated the independent variable. Ex: The height of the corn plants, the size of the ear of corn, the number ears of corn grown. The changes observed in the responding variables are the data collected during the experiment.

Variables The constants are independent (manipulated) variables that could be changed, but are kept the same. Remember, scientists only manipulate one variable at a time.

Writing a hypothesis Hypotheses also generally follow a standard format. If (the independent variable is changed), then (the dependent variable will change). For example If the amount of fertilizer is increased, then the height of the corn will increase.

Control and experimental groups Within an experiment there are two groups; the control group and the experimental group. The two groups are identical except for one factor. The independent variable.

Control and experimental groups The control group is the normal group in an experiment where everything, including the variable being tested, is kept at what is considered normal. This group s purpose is to serve as a comparison for the experimental group.

Control and experimental groups The experimental group is the group where the manipulated variable has been changed away from normal. This is the group that is being tested.

Example In order to test the effectiveness of a new medicine, 50 volunteers are selected and divided into two groups. One group will be the control group and the other will be the experimental group. Both groups are given a pill to take that is identical in size, shape, color and texture. Describe the experimental group. Describe the control group. What variables are kept constant? What variable is being changed?

Data Collection It is important to decide what data you want to collect before starting an experiment. Ask yourself three questions What am I trying to find out with this experiment? What can I measure to show that? How do I need to design this experiment to allow me to collect that data?

Procedure A procedure is a plan you follow when doing an experiment. Before starting an experiment, it is important to have a procedure.

Procedure A procedure tells you what materials you will need, what steps you need to take, and what data needs to be collected. It is very important to follow your procedure exactly every time you do the experiment.

Practice Problem: You want to determine the effects of a certain fertilizer on the growth of corn grown in a greenhouse. Materials that are available to you include: a greenhouse, 100 corn plants, water, fertilizer, and soil. You want to know if the corn will grow best with a weak concentration of fertilizer, a medium concentration of fertilizer, or a high concentration of fertilizer. How will you design an experiment to test different concentrations of this fertilizer? State your hypothesis: Possible hypothesis: If you increase the amount of fertilizer, then the corn will grow better.

How will you set up an experiment? The 100 plants will be divided into 4 groups as follows: Group 1: 25 plants will receive plain water. Group 2: 25 plants will receive a weak concentration of fertilizer. Group 3: 25 plants will receive a medium concentration of fertilizer. Group 4: 25 plants will receive a high concentration of fertilizer. The plants will be watered daily. Over a period of a month, the plants will be measured to see which ones grew the tallest.

Control Group Experimental Group What is the control group in this experiment? The control group consists of the 25 plants that are receiving plain water. What is the experimental group in this experiment? The experimental group consists of the 75 plants that are receiving various concentrations of fertilizer.

In a good experiment, all variables must be kept constant except the one variable that is being changed. What variables must be kept constant in this experiment? All plants must receive the same amount of water/fertilizer each day. All plants are grown in pots of equal size. All plants are grown at the same temperature. All plants receive the same amount of sunlight. All plants are the same species. What variable is being changed in this experiment? The variable being changed is the amount of fertilizer received by each group of plants.

After one month of measuring the corn, the following data is obtained: Group 1 (Control Group): Grew to an average height of 15 cm. Group 2 (Weak conc.): Grew to an average height of 35 cm. Group 3 (Medium conc.): Grew to an average height of 28 cm. Group 4 (High conc.): Grew to an average height of 10 cm. Is your hypothesis supported or disproved by these results? We hypothesized that the corn would grow best with a high concentration of fertilizer. The results do not support our hypothesis.

Question Why is it important to have a large sample size in any experiment? If the sample size is too small, an inaccurate conclusion may be reached. Ex. An accident occurs and the one control plant dies. Now we don t have anything to compare our experimental plants to. Vs.

Question Why is it important to repeat the experiment many times? Experiments should be repeated to see if the same results are obtained each time. This makes the results more believable. + +

Question What is the importance of the control? The control shows what will happen under normal circumstances. Without the control, there is nothing to compare your results to and no way to know how your independent variable effected your experiment.

In your notebook, I want you to come up with and create a picture that demonstrates the relationship between a pair of independent and dependent variables. Similar to the one below.