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Some Tips for Planning an Experiment
Ask a question
Form an hypothesis
Choose the variables
Plan the treatments
Choose a control
Decide the number of replicates
Specify the constants
Glossary of related terms

Ask a question
When you want to do an experiment, a good way to start is by asking a question. For bioassays, the questions might be "Does salt inhibit germination of lettuce seeds?" or "Is salt toxic to Daphnia?"

Form your hypothesis
After you decide on a research question, then you are ready to state your hypothesis or prediction of what you think will happen. An example hypothesis: "The number of seeds that germinate will decrease with increasing concentration of NaCl."

Choose the variables
Your independent variable is the factor that you will change in your experiment. For example, in a lettuce seed bioassay the usual independent variable is the concentration of the solution to which the seeds are exposed.

The dependent variable is the factor that you predict will change as a result of variation in your independent variable. The number of seeds that germinate and the lengths of their roots are examples of two different dependent variables in lettuce seed bioassay experiments.

If you are confused about the independent and dependent variables, it may help to think back to your research question and then think about how you might want to present the results of your experiment. For example, for a bioassay using Daphnia, you might decide to set up a bar graph to display your results. On the x-axis (the horizontal axis), you would put your independent variable. These are the numbers that you know in advance, such as the concentrations of your test solutions. On the y-axis (the vertical axis), you would put your dependent variable. This is the factor you will be measuring in your experiment, such as the length of the lettuce roots or the number of Daphnia that die at each concentration.

Plan the treatments
A treatment is a factor that affects the outcome of a scientific experiment. In a bioassay, the experimental treatment usually is the concentration of the solution to which the seeds or organisms are exposed. (If you want to test the effect of temperature on seed germination or Daphnia growth, the treatment would be temperature rather than solution concentration.)

Choose a control
In a scientific experiment, the control is the group that serves as a standard of comparison. It is exposed to the same conditions as the treatment groups, except for the variable being tested. In bioassays, the control group is the set of seeds or Daphnia grown in distilled or culture water rather than in a test solution.

Decide the number of replicates
Replicates are individuals or groups that are exposed to exactly the same conditions in an experiment. In a lettuce seed bioassay, you might test 6 different solution concentrations, plus a distilled water control. For each of these concentrations, you might use 3 Petri dishes, each containing 5 lettuce seeds, or 3 beakers, each containing 10 Daphnia. In this experiment, you would have 3 replicates at each concentration (the 3 Petri dishes or beakers).

Specify the constants
The constants in an experiment are the factors that do not change. What your constants will be will depend on what question you are asking. In a bioassay experiment, the temperature usually is kept constant. However, if your question were, "How does temperature affect the life span of Daphnia?" then in this case the temperature would be a variable rather than a constant.


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