Designing experiments

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Control and Experiment Groups

Experiments have variables, or parts that change. You can design an experiment to investigate whether changing a variable between different groups has a specific outcome.

For example, imagine you want to find out whether adding fertilizer to soil affects the height of pea plants. You could investigate this question with the following experiment:

You grow one group of pea plants in soil with fertilizer and measure the height of the plants. This group shows you what happens when fertilizer is added to soil. Since fertilizer is the variable whose effect you are investigating, this group is an experimental group.

You grow another group of pea plants in soil without fertilizer and measure the height of the plants. Since this group shows you what happens when fertilizer is not added to the soil, it is a control group.

By comparing the results from the experimental group to the results from the control group, you can conclude whether adding fertilizer to the soil affects pea plant height.

Independent and Dependent Variables

Experiments have variables, or parts that change. You can design an experiment to find out how one variable affects another variable. For example, imagine that you want to find out if fertilizer affects the number of tomatoes a tomato plant grows. To answer this question, you decide to set up two equal groups of tomato plants. Then, you add fertilizer to the soil of the plants in one group but not in the other group. Later, you measure the effect of the fertilizer by counting the number of tomatoes on each plant.

In this experiment, the amount of fertilizer added to the soil and the number of tomatoes were both variables.

The amount of fertilizer added to the soil was an independent variable because it was the variable whose effect you were investigating. This type of variable is called independent because its value does not depend on what happens after the experiment begins. Instead, you decided to give fertilizer to some plants and not to others.

The number of tomatoes was a dependent variable because it was the variable you were measuring. This type of variable is called dependent because its value can depend on what happens in the experiment.

Experimental Question

Experiments can be designed to answer specific questions. How can you identify the questions that a certain experiment can answer? In order to do this, you need to figure out what was tested and what was measured during the experiment.

Imagine an experiment with two groups of daffodil plants. One group of plants was grown in sandy soil, and the other was grown in clay soil. Then, the height of each plant was measured.

First, identify the part of the experiment that was tested.

The part of an experiment that is tested usually involves the part of the experimental setup that is different or changed. In the experiment described above, each group of plants was grown in a different type of soil. So, the effect of growing plants in different soil types was tested.

Then, identify the part of the experiment that was measured.

The part of the experiment that is measured may include measurements and calculations. In the experiment described above, the heights of the plants in each group were measured.

Experiments can answer questions about how the part of the experiment that is tested affects the part that is measured. So, the experiment described above can answer questions about how soil type affects plant height.

Examples of questions that this experiment can answer include:

Does soil type affect the height of daffodil plants?
Do daffodil plants in sandy soil grow taller than daffodil plants in clay soil?
Are daffodil plants grown in sandy soil shorter than daffodil plants grown in clay soil?

Tested vs Measured

Experiments can be designed to answer specific questions. When designing an experiment, you must identify the supplies that are necessary to answer your question. In order to do this, you need to figure out what will be tested and what will be measured during the experiment.

Imagine that you are wondering if plants grow to different heights when planted in different types of soil. How might you decide what supplies are necessary to conduct this experiment?

First, you need to identify the part of the experiment that will be tested, which is the independent variable. This is usually the part of the experiment that is different or changed. In this case, you would like to know how plants grow in different types of soil. So, you must have different types of soil available.

Next, you need to identify the part of the experiment that will be measured or observed, which is the dependent variable. In this experiment, you would like to know if some plants grow taller than others. So, you must be able to compare the plants' heights.

To do this, you can observe which plants are taller by looking at them, or you can measure their exact heights with a meterstick.

So, if you have different types of soil and can observe or measure the heights of your plants, then you have the supplies you need to investigate your question with an experiment!

Experimental protocol

The instructions for how to carry out a science experiment can be found in an experimental protocol.

An experimental protocol often includes the list of materials you need for the experiment. The protocol also includes the procedure, which is a list of steps for carrying out the experiment. Some experimental protocols include data tables for recording the results of the experiment.

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