Control groups can also be separated into two other types: positive or negative. Positive control groups are groups where the conditions of the experiment are set to guarantee a positive result.
A positive control group can show the experiment is functioning properly as planned. Negative control groups are groups where the conditions of the experiment are set to cause a negative outcome. Control groups are not necessary for all scientific experiments.
Controls are extremely useful where the experimental conditions are complex and difficult to isolate. Negative control groups are particularly common in science fair experiments , to teach students how to identify the independent variable.
A simple example of a control group can be seen in an experiment in which the researcher tests whether or not a new fertilizer has an effect on plant growth.
The negative control group would be the set of plants grown without the fertilizer, but under the exact same conditions as the experimental group. The only difference between the experimental group would be whether or not the fertilizer was used.
There could be several experimental groups, differing in the concentration of fertilizer used, its method of application, etc. The null hypothesis would be that the fertilizer has no effect on plant growth. Then, if a difference is seen in the growth rate of the plants or the height of plants over time, a strong correlation between the fertilizer and growth would be established.
Note the fertilizer could have a negative impact on growth rather than a positive impact. Or, for some reason, the plants might not grow at all. The negative control group helps establish that the experimental variable is the cause of atypical growth, rather than some other possibly unforeseen variable. A positive control demonstrates an experiment is capable of producing a positive result.
For example, let's say you are examining bacterial susceptibility to a drug. You might use a positive control to make sure the growth medium is capable of supporting any bacteria. You could culture bacteria known to carry the drug resistance marker, so they should be capable of surviving on a drug-treated medium. If these bacteria grow, you have a positive control that shows other drug-resistance bacteria should be capable of surviving the test. What is an Independent Variable?
What is a Dependent Variable? What is a Control Variable? Teacher Tool Box. Water faucet opening closed, half open, fully open Amount of water flowing, measured in liters per minute The faucet Water pressure, or how much the water is "pushing" "Different water pressure might also cause different amounts of water to flow and different faucets may behave differently, so to ensure a fair test, I want to keep the water pressure and the faucet the same for each faucet opening that I test.
Temperature of the water measured in degrees Celsius Amount of sugar that dissolves completely, measured in grams Stirring Type of sugar "More stirring might also increase the amount of sugar that dissolves, and different sugars might dissolve in different amounts, so to ensure a fair test I want to keep these variables the same for each cup of water. Amount of fertilizer, measured in grams Growth of the plant, measured by its height Growth of the plant, measured by the number of leaves See Measuring Plant Growth for more ways to measure plant growth.
Same type of fertilizer Same pot size for each plant Same plant type in each pot Same type and amount of soil in each pot Same amount of water and light Make measurements of growth for each plant at the same time "The many variables above can each change how fast a plant grows, so to ensure a fair test of the fertilizer, each of them must be kept the same for every pot.
Voltage of the electricity, measured in volts Speed of rotation, measured in revolutions per minute RPMs Same motor for every test The motor should be doing the same work for each test turning the same wheel, propeller, or whatever "The work that a motor performs has a big impact on its speed, so to ensure a fair test, I must keep that variable the same. Time measured, in minutes Height of candle, measured in centimeters, at regular intervals of time for example, every 5 minutes Use same type of candle for every test Wind—make sure there is none.
Groups receiving the survey: Teenagers or parents Amount of time that each person listens to music per day, measured in hours Ask the question in exactly the same way to each individual. Teacher location: The teacher is either in the room or not in the room. Fenders: The bicycle either has fenders or it does not "Many engineering projects have alternative designs with independent variables like this one with and without fenders.
Riding at the same speed Same size and depth of puddle. Sample Here is a sample containing the variables and hypothesis. Explore Our Science Videos. Test Different Pinwheel Designs. Build A Gumdrop Geodesic Dome! The faucet Water pressure, or how much the water is "pushing" "Different water pressure might also cause different amounts of water to flow and different faucets may behave differently, so to ensure a fair test, I want to keep the water pressure and the faucet the same for each faucet opening that I test.
Stirring Type of sugar "More stirring might also increase the amount of sugar that dissolves, and different sugars might dissolve in different amounts, so to ensure a fair test I want to keep these variables the same for each cup of water.
Growth of the plant, measured by its height Growth of the plant, measured by the number of leaves See Measuring Plant Growth for more ways to measure plant growth. Same motor for every test The motor should be doing the same work for each test turning the same wheel, propeller, or whatever "The work that a motor performs has a big impact on its speed, so to ensure a fair test, I must keep that variable the same.
Height of candle, measured in centimeters, at regular intervals of time for example, every 5 minutes. Use same type of candle for every test Wind—make sure there is none. Same classroom Same students Same time of day.
The rider either gets wet or does not. Remember that the control should never change. If your independent variable always remains the same, odds are it is your control.
Not all science experiments require a control, but many do. You can create your own control sample by following a few simple steps. One great example of creating a control in a relatively simple experiment is working with plants. The basis is to determine how plants grow in different types of soil mixtures. The control pot uses regular potting soil and the same daily routine of water and sun. The other pots have different soil mixtures and may be exposed to varying lights and temperatures. Depending on your science experiment, determine a variable or sample set that must remain the same at all times.
The control may directly apply to every portion of your experiment, or it can be relative, such as the plant experiment. The control would be using the same amount of water in the exact same size container.
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