Experiment
Design
Scientists
follow a specific method for setting up experiments to test their
hypotheses. In an experiment, we want to know if a change happens, and
we want to be sure we can identify what caused a change to happen.
Therefore, an experiment must include these basic parts:
Independent
Variable:
This is the part of the
experiment that you manipulate to see what happens. For example, let’s
say we have a hypothesis that a tomato plant would grow just as quickly
using organic compost as it would using Miracle-Gro ® water soluble
tomato plant food. In an experiment designed to test this hypothesis
the independent variables are the Miracle-Gro ® and the
organic compost. These are the soil treatments we are adding to see
what happens with the tomato plants.
Dependent
Variable:
This is the part of the
experiment that you observe to see what happened. For example, in our
tomato plant experiment the dependent variables are the tomato plants
themselves, specifically, the growth rate of the tomato plants. This is
what we will be measuring to see if there is any difference.
Control
Variables:
These are the parts of the
experiment that you make sure stay the same. This can be the most
important part of the experiment. If you haven’t carefully controlled
all the other variables, you cannot be sure if it is the independent
variable you are testing that has actually produced the difference. For
example, in the tomato plant experiment, we want to be sure that we are
using the same base soil mixture, the same size and type of container,
and the same variety of tomato seeds. We want to be sure all the plants
are getting the same amount of water and light. We want to be as sure
as possible that the ONLY difference between the tomato plants is the
type of soil treatment we’ve added.
In addition to the 3 types
of variables, a good experiment should also have the following:
Multiple
trials:
If we were to design our
tomato plant experiment to have only two plants, one with Miracle-Gro
® and one with organic compost, then it would not be a very good
experiment. There are unaccounted for differences in the genes of any
two specific plants (just like differences between my genes and your
genes that make us appear different even though we are both the same
species). One plant might just grow faster because it has the genes to,
or for some other reason besides the soil treatment we’ve added. In
order to reduce the impact of this random variation on our results, we
want to apply our independent variable to multiple test subjects and
measure the dependent variable multiple times. These repetitions are
called trials. So we would plant 3 or more tomatoes in organic compost
and 3 or more in Miracle-Gro ®. The more we plant, the more sure we
can be of our results. But to be sure, we still need a...
Control
group:
The control group is the
group of trials that you do not apply the independent variable to. It
provides the baseline against which you compare the experimental group
(the group in which you are testing the independent variable). In our
tomato plant experiment, the experimental group are all 6 plants, 3
with organic compost and 3 with Miracle-Gro ®. We would add another
3 tomato plants in our control group. These plants would get neither
soil treatment. But we would meticulously ensure that all the control
variables remained the same for this control group--the same base soil,
the same tomato seeds, the same amount of light and water, etc. We
would measure the dependent variable (the growth rate) for this control
group whenever we measure the dependent variable of the experimental
group.
Random
selection:
Let’s say that we start
this experiment with nine 14-day-old seedlings. We need to assign each
of these seedlings to a group: organic compost group, Miracle-Gro ®
group, or control group. If we assign all the biggest, healthiest
seedlings to the organic compost group and all the scrawniest seedlings
to the Miracle-Gro ® group because we want to support our
hypothesis, we are biasing our experiment, and the results will not be
reliable. We can even bias the experiment in this way without
consciously trying to. So before we begin our experiment we need to
devise a method to make sure we are selecting seedlings for each group
randomly. Maybe we could ask a friend who doesn’t know our hypothesis
to shuffle them up and divide them into three random groups for us.
Thinking out how to randomize the groups is an important part of the
experiment design.
Whenever you are asked to
design an experiment for a science class, be sure you carefully think
through and include all of the parts above.