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?
The 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. As many different types of experiments are possible, an experiment must be designed to produce data that can help confirm or reject the hypothesis.
An experiment generally tests how one variable is affected by another. The affected variable is called the dependent variable. In the plant experiment shown in the Figure below, 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 could be fertilizer—some plants will get fertilizer, others will not. The scientists change the amount of the independent variable (the fertilizer) to observe the effects on the dependent variable (plant growth). An experiment needs to be run simultaneously in which no fertilizer is given to the plant. This would be known as a control experiment. The plants that do not receive the fertilizer are the control group. An experiment must have a control group. In the control group, nothing is changed. It is not subjected to the independent variable. 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?)
The independent variable is the variable under the control of the experimenter.
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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.
Scientists make many measurements during experiments. Humans and equipment can make mistakes. Errors are often unavoidable. This is called experimental error. Errors may be due to the equipment being used. In this case, the numbers may be always skewed in one direction. For example, a scale may always measure one-half of an ounce high. People can also make errors. A measurement may not be made precisely. For example, a stopwatch may be stopped too soon or too late. But probably not always in the same direction. To correct for this, many measurements are taken. The measurements are then averaged. Experiments always have a margin of error associated with them.
Microscopes are arguably one of the most important tools of the biologist. They allow the visualization of smaller and smaller biological organisms and molecules. With greatly magnified powers, these instruments are becoming increasingly important in modern day research.
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