Download As it appears in Units 1–4

Biology
Teach Yourself Series
Topic 1: Experimental Design and Data Interpretation
A: Level 14, 474 Flinders Street Melbourne VIC 3000
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Contents
Hypotheses ..................................................................................................................................................... 3
As it appears in Units 1–4 .............................................................................................................................. 3
Terminology ……………………………………………………………………………………………..…4
As it appears in Units 1–4 .................................................................................................................................. 4
Review Questions .............................................................................................................................................. 4
Experimental Reliability...................................................................................................................................
As it appears in Units 1–4 ................................................................................................................................
Review Questions ................................................................................................................................................
Data Interpretation ............................................................................................................................................
As it appears in Units 1–4 ................................................................................................................................
Review Questions ................................................................................................................................................
Solutions to Review Questions .................................................................................................................. 6
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Experimental Design and Data Interpretation
Experimental design and data interpretation are a vital part of science and they are a key part of the current
Biology Study Design, specifically fallingunder the Units 1–4 key skills. Scientists need to be able to design
and evaluate fair and accurate experimental procedures that are used to test hypotheses. The proper
interpretation of data and the ability to draw valid conclusions that are consistent with the data are also vital
skills required by all scientists.
Hypotheses
As it appears in Units1–4
A hypothesis is a potential explanation for an observation and it should be based on relevant knowledge. A
hypothesis needs to be tested under controlled conditions and the results should either support or refute the
original hypothesis. If a hypothesis is supported by experimental results this does not mean that the
hypothesis is correct. It means that the results support the hypothesis. Conversely, if the results refute the
hypothesis it does not mean that the hypothesis is incorrect but that the results do not support the hypothesis.
Future experimentation may disprove any currently held hypotheses. For example, the experiments of
Francesco Redi, and later, Louis Pasteur disproved the theory of spontaneous generation.
To accurately test a hypothesis scientists must ensure that the experiment is controlled. This means that there
is only one independent variable being tested in an experiment. Experiments are often conducted by setting
up the experiment in two parts. These two parts are identical in every way except for the independent
variable being tested. In the first part, all variables are kept constant throughout the experiment. This is
referred to as the experimental control. In the second part, one variable is altered throughout the course of
the experiment. If the results of these two parts are different, it can be reasonably assumed that exposure to
the independent variable was the cause of the difference and that it was not the result of an extraneous event.
These results can then be used to support or refute the original hypothesis.
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Terminology
As it appears in Units1–4
When completing an assessed task or examination, it is important that you use terminology appropriately.
Independent Variable: the variable that is being manipulated or tested by the experimenter.
Dependent Variable: the response that is being measured.
This means that the independent variable is the presumed cause and the dependent variable is the effect
which is observed.
Controlled Variables: all other factors that could affect the experiment are held constant in order to
establish the relationship between the independent and dependent variables.
Experimental control (control group): this consists of a group of subjects that are not subjected to the
independent variable. As a result the control group can be used to establish whether the exposure to the
independent variable causes the result or not.
Treatment group: this is the group/s of subjects that are subjected to the independent variable or different
levels of the independent variable, for example if testing the effect of fertiliser on plant growth there would
be different groups of plants with each group being exposed to different amounts of fertiliser.
Quantitative: data that is able to be expressed as a quantity e.g. grams, litres or centimetres.
Qualitative: data that involves a description rather than a precise measurement e.g. an indicator changes
colour from yellow to pink. Qualitative data is more subjective or open to different interpretation than
quantitative data.
Sample Size: this refers to the number of subjects in each group. For examination purposes, each group
should contain a minimum of five subjects.
Review Questions
1. For the experiment below identify the following:
An agriculturist wishes to determine if the use of fertiliser has an effect upon the growth of pot plants.
Aim: To establish if increasing the amount of fertiliser increases plant growth.
a. The independent variable _________________________________________________________
b. The dependent variable ___________________________________________________________
c. All controlled variables ___________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
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d. Identify an appropriate experimental control for this experiment.
____________________________________________________________________________
____________________________________________________________________________
____________________________________________________________________________
2. Explain the importance of having a single independent variable in an experiment.
Consider the following experiment, conducted by a VCE Biology student.
Aim: To determine the role sunlight plays in the production of starch
Hypothesis: Sunlight is required to produce starch
Materials:A plant with numerous leaves wrapped in aluminium foil for a number of days, ethanol, one
500mL beaker and one200mL beaker, iodine solution, Bunsen burner with heat mat, tripod and
gauze,tweezers, watch-glass, test tube holder.
Method:
1. Take one leaf that has been wrapped in aluminium foil and onethat has been exposed to sunlight.
Make sure that the leaves are of different sizes so that you are able to tell them apart.
2. Fill the 200mL beaker with 50mL of ethanol and both leaves and boil the ethanol over the Bunsen
burner until most of the greencolour has come out.
3. Remove the leaves from the beaker with the tweezers andplace them on the watch glass.
4. Cover the leaves with iodine and observe their appearance.
The student wrote the following conclusion:
When iodine was added to leaf one, which had been wrapped in aluminium foil, there was no colour
change and the leaf remained pale suggesting that starch was not present. When iodine was added to
leaf two, which had been exposed to sunlight, parts of the leaf became a blue/black colour suggesting
that starch was present. The hypothesis that “Sunlight is required to produce starch” is supported by
the experimental results.
3. In the experiment above, which of the two leaves represented the control? Why?
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Solutions to Review Questions
1. a. The application of fertiliser.
b. The height of the plants.
c. All pots used should be the same size and made out of the same materials. The same type of plants
should be grown in each pot, these plants should be grown from seeds all from the same parent plant, the
same amount of water should be used, the same amount and type of soil should be used, all plants should
be exposed to the same amount of light for the same period of time. All plants should be grown for the
same period of time.
d. The experimental control should consist of a group of plants that were not exposed to the fertiliser.
2. If there is only one independent variable then any difference between the results produced in the control
and treatment groups can be attributed to exposure to the independent variable. If there is more than one
independent variable, then it cannot be stated which was responsible for the result, or potentially both
variables
3. Answer: The leaf that was exposed to sunlight is the control in this experiment
Explanation: This leaf is the control as it is the leaf where all of the variables are kept constant; in this
case, this is the leaf that has had nothing done to it.
4.
a. Answer: There are two variables in this experiment, access to sunlight and size of the leaf.
Explanation: The experiment requires that two leaves are used, one is exposed to sunlight and the
other is wrapped in aluminium foil, thus preventing sunlight to reaching the leaf. The experiment
also states that two leaves of different sizes be selected; this is a variable as the different sizes
could alter the results.
b. Answer: This experiment should have only one variable, access to sunlight.
Explanation: To accurately test a hypothesis there should only be one variable to ensure that any
variation in results is due to that one variable.
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