An experiment is a controlled procedure designed to test a hypothesis.  There are several parts of any good scientific experiment.

A good experiment is one that is:   







HYPOTHESIS:  How  ____________ affects _______________.

Cause (IV)       and      Effect (DV).

More than a prediction about the outcome of an experiment, a hypothesis provides an explanation for why phenomena occur. A valid hypothesis includes the property that is changed (the cause or the independent variable) and the property that is measured (the effect or dependent variable) which may change as a result.  In a good experiment, there is only one cause for the effect.

Example: Increasing the amount of light (cause) increases a plant’s height (effect).  (Light affects height)


Both of these properties can be measured.  When they are graphed, the independent variable is placed on the bottom (x) axis, and the dependent variable is placed on the side (y) axis.


INDEPENDENT VARIABLE (IV):  This is the factor, or variable, that you change. This is the physical property that you have direct control over to change. It should be the ONLY difference between the two groups for it to be a good experiment.  It is the CAUSE property mentioned in the hypothesis.

Example: the amount of light.


DEPENDENT VARIABLE (DV):  This is the factor, or property, that you measure for, or the result.  It could be different between the groups, or it could be the same.  You don't know the value of this variable until the end of the experiment.  This is the EFFECT property mentioned in the hypothesis.

Example: how high the plant grows, the height


CONSTANTS/CONTROLLED EXPERIMENT:  All other properties and factors should be the SAME in all groups, or they should be CONTROLLED.

Example: the amount of food, the amount of air, the type of plant, are all kept the same.


A CONTROL GROUP:  The group that is used as the basis for comparison.  It could be: the BEFORE part of a before and after experiment (mixing two chemicals to see a color change, the control group is the set up before they were mixed).  It could be: the "normal,” or it could be the group in which the value of the independent variable is zero.

Example: a plant set in a room with no light bulb.


EXPERIMENTAL GROUP (S):  The experimental group differs from the control group in just ONE factor or variable.  This is the group that is usually mentioned in the prediction.  It can also be the "after" part of a before and after experiment.  It is the actual physical set of objects that you have changed or are doing something to.  A good experiment usually has at least two or three experimental groups, or data points.

Example: a plant set in a room with twenty light bulbs.

CONCLUSION: You organize your results and data and summarize patterns in the data. The conclusion statement should indicate whether the evidence supports your initial expectations and describes what you found out about the hypothesis and prediction. The conclusion should also discuss the validity and reliability of your experimental results.


Amount of Light (Cause, IV) affects the Height (Effect, DV) of a plant.

A good experiment, besides having careful observations, and using instruments, will always have a control group to compare to. It is also very important to have groups in which only ONE property or variable is changed at a time, so that you can be sure that the property is the cause of whatever effect you are measuring.


Amount of Light                                             Average Height of plant after 24 hours

                        1 foot candle                                                    (19 cm, 20cm, 21 cm)      20 cm

                        2 foot candle                                                    (30 cm, 25 cm, 29 cm)     28 cm

                        10 foot candles                                                (110 cm, 113 cm, 119 cm)            114 cm




            To determine the relationship, a student could find a "best fit" line or curve.


            Y= 2X + 10 , so with NO light, the plant would be at 10 cm (Control Group= Y Intercept)


If there is more than one independent variable, bad experiment = Not a Function!