Constants
Constants are the factors within an experiment that are kept the same for all
groups or trials in an attempt to reduce the influence of additional variables.
Once you have chosen the independent variable, you must design an experiment
to take all of the other potential independent variables into account and
make them constant. Otherwise, you will not be able to support a clear relationship
between the two variables for which you have data.
How you decide to perform the experiment, meaning the step-by-step
procedure, is crucial and can greatly influence the integrity of your experiment.
Your treatment of each of the groups must be the same in every way.
When analyzing the data after the experiment, you will have to critique your
methods to see if something you may have done, or failed to do, influenced
the results. This is another reason why background research before starting
the experiment is so important.
You need to consider what it means to provide a constant environment
for all the groups. For example, in a plant experiment where different intensities
of light are used, it is likely that the soils will dry out at different rates.
Does keeping the water a constant mean that each plant gets watered the
exact amount, on the same days of the week? Or does keeping the water a
constant mean that each plant gets enough water so that its soil is moist 1 cm
below the surface? Although there is not always a “right” answer to these
types of questions, you need to do background research to determine which
methods would introduce the least amount of error.
In the reproduction/temperature experiment on planaria (pp. 17–18), the
constants might include the methods used to observe and handle the planaria,
the length of time each group receives light, and how often planaria are
fed and environments cleaned. The list of conditions to keep constant within
your experiment can be extensive. It is important to learn as much as possible
about the entity being studied AND about the independent variable.
You want to be as informed as possible about any additional factors that may
influence the results.
Be careful that by controlling for one extraneous variable you are not
introducing another one. An extraneous variable is a variable in addition to the
independent variable that may influence the results of an experiment. Extraneous
variables can introduce errors if they are not controlled or significantly
decreased. For example, if planaria specimens are placed in different rooms
to keep the varying temperatures from interfering with the experimental and
control groups, additional variables have now been introduced. The different
rooms might have varying amounts of light or might be used more or
less frequently by people. You will not be able to control everything, but you
will have to make decisions on what is least likely to influence the results. Be
ready to address any limitations of the experiment. Explaining the efforts that
went into reducing the effects of extraneous variables is important. Figure
2.4 shows a sample of what a biological experimental design might look like
when put into an experimental design table.