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# 1.2: Science Experiments

Difficulty Level: At Grade Created by: CK-12
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So what exactly is an experiment?

At first you may picture a science laboratory with microscopes and chemicals and people in white lab coats. But do all experiments have to be done in a lab? And do all scientists have to wear lab coats?

### Experiments

Figure below shows a laboratory experiment involving plants. An experiment is a special type of scientific investigation that is performed under controlled conditions, usually in a laboratory. Some experiments can be very simple, but even the simplest can contribute important evidence that helps scientists better understand the natural world. An example experiment can be seen here http://www.youtube.com/watch?v=dVRBDRAsP6U or here http://www.youtube.com/watch?v=F10EyGwd57M.

A laboratory experiment studying plant growth. What might this experiment involve?

In this experiment, a scientist is conducting research (and taking notes) while looking through a microscope.

#### Medicine From the Ocean Floor

Some experiments are much more sophisticated than those shown above. For example, scientists at the University of California, Santa Cruz are using robots to sort through thousands of marine chemicals in search of cures for diseases like cholera, breast cancer, and malaria. These experiments are described in the following links:

#### Variables

An experiment generally tests how one variable is affected by another. The affected variable is called the dependent variable. In the plant experiment shown above, the dependent variable is plant growth. The variable that affects the dependent variable is called the independent variable. In the plant experiment, the independent variable is fertilizer—some plants will get fertilizer, others will not. In any experiment, other factors that might affect the dependent variable must be controlled. In the plant experiment, what factors do you think should be controlled? (Hint: What other factors might affect plant growth?)

#### Sample Size and Repetition

The sample in an experiment or other investigation consists of the individuals or events that are studied, and the size of the sample (or sample size) directly affects the interpretation of the results. Typically, the sample is much smaller than all such individuals or events that exist in the world. Whether the results based on the sample are true in general cannot be known for certain. However, the larger the sample is, the more likely it is that the results are generally true.

Similarly, the more times that an experiment is repeated (which is known as repetition) and the same results obtained, the more likely the results are valid. This is why scientific experiments should always be repeated.

### Summary

• An experiment is a special type of scientific investigation that is performed under controlled conditions, usually in a laboratory.
• An experiment generally tests how one variable is affected by another.
• The sample size in an experiment directly affects the interpretation of the results.
• Repetition is the repeating of an experiment, validating the results.

### Practice

Use this resource to answer the questions that follow.

• http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: How Do Biologists Study?
1. What is a hypothesis?
2. What is a controlled scientific experiment?
3. What causes Lyme disease?
4. What is the relationship between the number of deer and Lyme disease?
5. Was the original hypothesis correct?
6. If the hypothesis is not correct, what must be done next?

### Review

1. What is an experiment?

2. Identify the independent and dependent variables in the following experiment:

A scientist grew bacteria on gel in her lab. She wanted to find out if the bacteria would grow faster on gel A or gel B. She placed a few bacteria on gel A and a few on gel B. After 24 hours, she observed how many bacteria were present on each type of gel.

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

### Vocabulary Language: English Spanish

dependent variable

Variable in a scientific experiment that is being measured as the independent variable is changed.

experiment

Special type of scientific investigation that is performed under controlled conditions to test the validity of a hypothesis.

repetition

The process of repeating an experiment.

sample size

The size of the sample investigated.

variable

A changeable feature of an experiment.

independent variable

Variable in a scientific experiment that is manipulated by the researcher to investigate its effect on the dependent variable.

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