How to control experiments to create valid results

 When scientists create experiments, they must have a standard to compare their results to.  For example, we already know that most plants need sunlight and water to grow.  Therefore, if we were to design an experiment with plants, we would need to have at least one plant or group of plants that has “normal” conditions. This would be our CONTROL group.

 Don’t confuse our CONTROL GROUP with controlled variables.  Remember that variables are part of the experiment that CAN CHANGE.  There are some things we don’t want to change, since they might affect our results.  In the case of the plants, we would control variables (hold constant) such as WHEN we feed and water the plants, HOW MUCH we feed and water the plants, and THE TYPE of plants used.

 When we purposely change a variable, to test its affect on something, that is known as the INDEPENDENT variable.  The independent variable is changed because the scientist (YOU) makes the change.  For example, you want to test how different liquids affect plant growth. Your INDEPENDENT variable is the TYPE of liquid.  You would hold constant all the other variables- such as type of plant, amount of sunlight, and even amount of liquid.

 When we change something in the experiment, we are looking for an effect of that change.  We call this effect the DEPENDENT variable.  Often the dependent variable is what we measure or observe because of the change made.  The results you get DEPEND on what you change in the experiment (the INDEPENDENT variable).

 Scientists must produce RELIABLE results (This means experiments must be able to be REPEATED and REPLICATED).  The best way to do this is to hold all variables constant, except for the TEST variable…which means the experiment must be VALID first and foremost (Only 1 Test Variable allowed!).  On the next slide is an example of a TESTABLE experiment. From this experiment, choose the Test and Outcome variables, the Constants, the control group, and whether or not the results are reliable.

Identify the TEST and Outcome variables, the constants, and tell whether the experiment is valid (making the results reliable)—and why/why not. An equal amount of potting soil was placed in three identical pots. Twenty petunia seeds were used. Three seeds were planted in Pot A, seven seeds in Pot B, and ten seeds in Pot C. The pots were all placed on the same table. After they grew to a height of 10 cm, the plants from each pot were cut off at soil level, dried overnight, and the mass was found.

 Test (Independent) Variable: Amount of seeds  Outcome (Dependent) Variable: Mass of plants after being dried  Constants: same type of seeds, same size pots, amount of potting soil, place (same table)  Valid? Yes….at face value. Doesn’t mention if plants are given same amount of water. Assume all constants are implied.  If it is Valid, we could make it reliable by adding more trials.