Lesson Two: Design an Experiment

Step One: Ask a Question

The first step in an experiment is to ask a question about whatever you want to find out from hydroponics. This could be as simple as "Can I grow radish plants without soil?" For a more ambitious experiment, you might ask a question about plant nutrition such as "How much magnesium do radish plants need to grow?" or "How do radish plants react to concentrated nitric acid?"

For a question to be answered scientifically, it must be clear and testable, and the phenomenon that you question must be measurable and controllable. For instance, you could not ask "Is hydroponics superior to soil for growing plants?" because whether something is "superior" is a matter of opinion.

Step Two: Form a Hypothesis

The next step in an experiment is to come up with a hypothesis, a tentative explanation for a scientific phenomenon. For instance, you might hypothesize:
Radish plants can grow in a nutrient solution without soil.
Magnesium is essential for normal radish plant growth.
Concentrated nitric acid is harmful to radish plants.

Make sure your hypothesis can be proven wrong. If it cannot be proven wrong, it is useless to conduct an experiment to test it.

Along with a hypothesis comes a prediction. A prediction is what you think will happen in the experiment. It takes the form of an if/then statement: IF the hypothesis is true, THEN these are the results I expect.

Examples:
IF radishes can grow in a nutrient solution without soil, THEN these plants will grow.
IF magnesium is essential to radish growth, THEN radishes without magnesium in their solution will develop chlorosis.
IF concentrated nitric acid is harmful to radish plants, THEN plants fed concentrated nitric acid will drop dead within one hour.

Step Three: Determine Variables

There are three kinds of variables that you must account for in an experiment. The independent variable is what you change in the experiment. For instance, if you are trying to find out how much magnesium radish plants need to grow, your independent variable might be concentration of magnesium in the nutrient solution. It is important that you have only one independent variable in your experiment. For example, you cannot vary both the magnesium concentration and the temperature conditions of your radish plants. You would not be able to draw reliable conclusions from the experiment if you altered more than one experimental condition.

The dependent variable is what you measure in the experiment. Unlike the independent variable, an experiment can have several dependent variables because variations in the independent variable can have many different effects. For example, you might measure length of leaves and weight of roots to assess the growth of radish plants. Dependent variables can include qualitative as well as quantitative data: you might also examine the color of the radish leaves and eat the roots to see how they taste. Such data cannot be measured but is still useful when you describe and compare it.

Any other conditions in the experiment are called controlled variables. You must keep these conditions constant for all plants in the experiment. Controlled variables might include light exposure, humidity, pH of solution, ambient noise, etc. If you change these variables, they become independent variables, and remember that you cannot have more than one independent variable in a scientific experiment.

Step Four: Design a Procedure

The procedure is the exact steps you take to carry out your experiment. This may change during the experiment if you discover a better way to do something than your original procedure. Be sure to note all changes in your procedure.

One important thing to include in your procedure is an appropriate level of treatment. The level of treatment is the extent to which you change your independent variable. For example, if you are testing the effects of magnesium concentration on radish growth, your levels of treatment might include no magnesium (0%), normal magnesium (100%), and double magnesium (200%). Note that these values are relative to the "normal" value, which is given in the recipe for the nutrient solution. Make sure you note the numeric value of "normal" (i.e. concentration in moles per liter). More extreme levels of treatment usually get more visible results, but less extreme levels of treatment usually simulate real-world conditions better.

Replication is the number of times you repeat a specific procedure. This is important to ensure that your experimental data is reliable and less subject to chance variation. For example, in the magnesium experiment, you might have two pots of radish plants in each nutrient solution and three plants in each pot. This way, some plants may grow tall and others may not grow much at all, but you can compare the general growth pattern of all the plants with the general growth pattern of the plants in the other nutrient solutions.

The control group is the group of plants in which the independent variable is held at a "normal" level. The purpose of a control group is to show what would normally happen and compare it with what happens when you change the independent variable. This shows if the independent variable is really responsible for your observations. For example, in the magnesium experiment, the control group would be the radishes with 100% of normal magnesium concentration in their nutrient solution.

Be careful not to confuse the control group with the controlled variables. Remember, the control group is the group in which the independent variable isn't changed, and the controlled variables are the variables that never change in any group.

Source for entire lesson: Campbell, Neil A. "Lab Topic 1: Scientific Investigation." Lab Manual for Campbell, Fifth Edition. Ed. Dan Wivagg. Menlo Park, California: Benjamin/Cummings, 2000, pp. 1-27.

Focus Questions

Design an entire experiment to test something about plant nutrition using hydroponics.

Ask a question about plant nutrition
Form a hypothesis about the answer to that question
Make a prediction about what will happen if your hypothesis is true
Design a procedure to test your hypothesis
Be sure to include an independent variable, dependent variables, and controlled variables in your experiment and identify them
Be sure to state the level of treatment, replication, and control group in your experiment.

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