BIOL 103
L
AB
#1 – T
HE
S
CIENTIFIC
M
ETHOD
: B
ACKGROUND
About this lab
In this lab you will form a hypothesis about the relationship between diet and weight gain for
crickets and use the scientific method to test your hypothesis.
The Scientific Method
The Scientific Method is a way of learning about the universe through observation and
experimentation. Observations that are recorded are called data (singular datum). Data may be
qualitative or quantitative. Qualitative data is descriptive. Examples of qualitative data include:
•
The shape of a snake’s scales
•
The pattern of remodeled bone (bones are remodeled as a result of healing after a break
or in response to changes in physical activity)
•
The nesting behaviors of mother birds
Quantitative data includes specific measurements of a particular parameter, such as time,
temperature, mass, or volume. Examples of quantitative data include:
•
How many triangular scales the snake has
•
The increase of bone remodeling cells (osteoclasts) base on the rate of exercise
•
The number of eggs a bird lays per nesting period
A general conclusion is drawn from specific observations. Let’s say you wake up and discover
that your toaster doesn’t toast. From this specific observation (my toaster doesn’t toast), you may
draw a general conclusion (the electrical outlet is broken).
The conclusion that the electrical outlet is broken is, of course, tentative. A tentative explanation
for observations is called a hypothesis. To find out if your hypothesis is correct, you will need to
conduct a controlled observation called an experiment. For example, you could plug your coffee
maker into the same outlet.
If the coffee maker works, you can conclude that the hypothesis “the electrical outlet is broken”
is incorrect. However, if the coffee maker doesn’t work, you cannot conclude that your
hypothesis is correct.
There are several possible explanations for the coffee maker not working: a fuse could be blown,
the electricity could have gone out, or maybe you just are having a really bad morning and all
your appliances have broken simultaneously. A good hypothesis can be proven wrong through
experimentation, but can never be proven correct.
BIOL 103
hypothesis
question
prediction
interpretation
experimentation
results
model
theory
A scientific hypothesis predicts specific, testable, results. For the hypothesis, “the outlet is
broken” the predicted result is “if the outlet is broken, my coffee maker won’t work when I plug
it in to the same outlet.”
Let’s assume that the coffee maker doesn’t work. This result indicates that hypothesis “the
electrical outlet is broken” could be correct, but so could several alternate hypotheses. You can
design other experiments to eliminate other possible explanations for this result. For example,
you might check the fuse, or see if the lights turn on.
It may take several rounds of generating hypotheses and testing to identify the problem.
Similarly, scientific advancement often involves cycles of generating hypotheses and
experimentation, as shown in the figure below.
If a hypothesis is supported, scientists will share their results, allowing others to benefit from
their discoveries. They will often design further experiments that look at the system in greater
depth.
Note that even a negative result (one which disproves the hypothesis) is useful because it
eliminates possible wrong explanations: if you find out that the outlet works, you are better off
than you were before as you have eliminated one possible explanation for why your toaster isn’t
working. It is important to remember that any hypothesis that can be tested experimentally is a
good hypothesis, even if it is proven wrong.
BIOL 103
Controlled Experiments
In a controlled experiment, one parameter is varied among test groups that are otherwise treated
the same. There are different types of controls.
Negative control groups do not receive treatment, and are compared to experimental groups that
do. For example, a researcher studying the efficacy of a new antibiotic would test an
experimental group of bacteria in the presence of the new antibiotic. The negative control group
of bacteria would only be treated with water.
Positive control groups are expected to produce a positive result. A positive control might be
used to compare the efficacy of the new antibiotic against an existing one that is known to be
effective at killing a particular strain of bacteria.
If the bacteria do not grow in the presence of the positive control antibiotic but do grow in the
presence of the experimental antibiotic, the researcher has evidence that the new antibiotic is
ineffective against that bacterial strain. If, on the other hand, the bacteria do not grow in even the
presence of plain water, the researcher has evidence that something went wrong with the
experiment. (For example, the culture he chose might be dead, or the bacteria may be unable to
grow under the conditions he selected.)
Other types of experimental setup may be used when a controlled experiment would be unethical
or impractical. For example, scientists use a natural controlled experiment to compare two
existing populations, such as smokers and non-smokers.
In controlled experiments, the parameter that is varied by the scientist is called the independent
variable. Another parameter, measured during the course of the experiment, is called the
dependent variable. In an experiment where the effect of caffeine on reaction time is tested,
consumption of caffeine is the independent variable, and reaction time is the dependent variable;
this is because the relative reaction time depends on whether or not caffeine was consumed.
An experiment can only have one independent variable. To understand why this is the case, think
about the bacteria experiment: if all the new antibiotic bacteria were grown at room temperature
(21 °C) and the positive control antibiotic were grown at 37 °C, the researcher would not know if
reaction time was dependent upon antibiotic type or temperature.
Laboratory Technique
Our model organism
All living organisms have specific nutritional requirements. In this lab you will be given
information about the natural diet of the field cricket (Gryllus pennsylvanicus). You will use this
information to form a hypothesis about the best food source for a newly discovered related
species of cricket, and conduct an experiment to test your hypothesis. Click on the Procedures
button at the top of the manual to begin your exploration.
