Courtesy of Dr. Jean Anastasia
Experimental Design
Biology is the study of life. Biologists observe life, ask questions about their
observations and propose explanations for their observations. Biologists then use what is
referred to as the “scientific method” to test if their explanations are valid. The scientific
method is a series of steps that are common to scientific investigations. Not all scientists
strictly follow these steps but the basic thought processes involved and certain
components of the method are usually present. The purpose of this assignment is to refamiliarize yourself with the process of scientific investigation and to build the skills that
will allow you to conduct and critique scientific investigations.
There are two portions of this assignment:
1. Designing your own scientific investigation
and
2. Critiquing other people’s scientific investigations
You will be required to identify a research question, state hypotheses and design
experiments to test your hypotheses. You will bring this assignment to lab. Students will
then randomly swap papers and critique each others assignments. The following
guidelines will help you in writing your own assignment and serve as a guideline for
what to look for when critiquing other student’s assignments.
The Question:
Scientists are naturally curious people who ask questions about the world based
on information from previous research or observations of natural phenomena. However,
not all questions can be answer by science. In order for a question to be answered
scientifically, it must be testable and the elements must be clearly defined and
measurable. Consider the question: Is euthanasia moral? Science could not answer this
question because morality cannot easily be defined or measured.
So, the first step in your assignment is to come up with a specific question that is
answerable scientifically.
Hypotheses:
Once a question is asked, scientists come up with possible explanations or
answers to the question, these are called hypotheses. For example, consider the question:
Does temperature affect the rate of digestion in ectothermic (“cold-blooded”) animals?
One hypothesis is that temperature has no affect on digestion rate. This type of
hypothesis that proposes not affect is typically called the null hypothesis. An alternative
hypothesis is that temperature does affect digestion rate. A more specific hypothesis
would be that increasing temperature causes an increase in digestion rate.
A good hypothesis must be testable and falsifiable. Most scientific investigation
is designed to provide data that will either falsify the hypothesis (i.e. prove it wrong) or
that will provide support for the hypothesis. Although hypotheses can be falsified, they
cannot be proved, only supported. Continuing our example, even if experiments show
that when temperature increases, there is an increase in digestion rate, causation had not
been proven. A correlation has been shown but there may be other variables that the
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scientist did not test that are also correlated. Scientific knowledge is there always open to
new data that may lead to modification and a better understanding of the science.
Testing hypotheses:
The scientific investigation used to test a hypothesis may be a carefully designed
controlled experiment or it may be the collection of observations or pooling of previous
research. In this exercise you will practice experimental design but it is important to
realize that this is not the only valid method of scientific inquiry.
Experimental design involves defining variables including controls and creating a
step by step procedure.
The Variables:
Dependent variable: this is the variable that will be measured, counted or
observed in the experiment. (in our example it is digestion rate)
Independent variable: this is the factor that will be manipulated by the scientist
(in our example it is temperature)
Controlled variables: All other factors, aside from the independent variable that
you are testing, that may have an effect on the dependent variable must be kept constant
or controlled. This allows the underlying assumption that the changes that you measure
in the dependent variable are only due to the changes you made in the independent
variable. In our example, the scientist may want to perform the experiments using
animals that are the same age and gender and are all fed the same food since each of these
factors may affect digestion rate. Therefore age, gender and food are all controlled
variables. It is very important to make sure that experiments are designed with
appropriate controls.
The Procedure:
The procedure is a sequence of steps that will be performed during the
experiment. Designing the procedure involves determining treatment levels, a control
treatment, the amount of replication and of course the specifics of equipment needed and
methodology. Once an experiment has been designed, scientists can make predictions of
the outcome based on their hypotheses.
Treatment Levels: This refers to the various values of the independent variable.
In our continuing example, the scientist would have to decide at which range of
temperatures to measure digestion rate. The treatment levels may be 10oC, 20 oC, 30 oC
and 40 oC or the scientists may simply have two treatment levels (10 oC and 40 oC) or
may have more. This decision is usually based on knowledge of the system, biological
significance of the treatment levels and sometimes time or money constraints.
Control: Experiments should include a control where the independent variable is
held at an established level or is omitted. In our example, measuring digestion rate at the
ambient temperature in the animal’s natural environment could be the control. As an
alternative example, if your question was how the addition of some chemical affects
metabolic rate, a control may be to expose the animal to water instead of the chemical.
Replication: Results from an experiment will not be considered widely applicable
if they were based on one experiment done on one or two individuals. Scientists
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therefore repeat experiments many times and test many subjects in attempts to average
out individual variation; this is replication. As a rule, more replication is better to get
more accurate results, however, time constraints and budgetary issues require a more
realistic amount of replication. When designing experiments try to determine what is a
manageable amount of replication that will still provide accurate results.
Predictions: Once an experiment is designed, the results can be predicted based
on the hypothesis. Scientists make these predications before conducting an experiment.
If clear predications cannot be made, then the procedure may need to be modified so that
is is truly going to provide data that will falsify or support the hypothesis. Predictions are
made as if, then statements: For example, If increasing temperature causes an increase
in digestion rate, then an increase in temperature will result in a decrease in the time it
takes for an object to move through the entire digestive tract.
After reading the preceding explanations, use the chart on the next page to check on your
own assignment and as a guideline to critique other students’ assignments.
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Template for your own Experimental Design Assignments (10 pts max)
The Question: (0-1 pt)
testable? Clearly defined and specific? Measurable?
The Hypothesis: (0-1pt)
Stated in correct format (i.e. as an answer to the question)?
Appropriate to the given question?
Falsifiable?
Experimental Design:
Are the following components clearly defined and appropriate?...(1 pt maximum for
each)
Dependent variable:
Independent Variable:
Controlled variables:
Treatment Levels:
Control:
Replication:
Predictions:
Step by step Procedure:
Specific?
Easy to follow/ clearly written?
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