Enhancing Thinking Skills in Science
A scientist, Dr. Chan, wants to design an experiment to
investigate the following topic:
The effect of taking a nap (a short sleep during the day)
1. Please design an experiment to study this research topic
using a control group and an experimental group.
(Use a flow chart / mind map to show your experimental
2. Please comment on the difficulties Dr Chan
may face if he carries out his research
according to your design.
3. What are the advantages of this kind of
experimental design (Randomised
Experimental Design) in scientific
4. Please comment on the difficulties Dr Chan may face if
he carries out his research according to your design.
Expensive and time-consuming (for years) to
carry out the experiment;
Must involve a large number of subjects for the
experiment in order to have statistically
Must keep other variables constant (fair test) but
in reality it is difficult to control.
If a scientist wants to study the effect of
a certain poison on humans, he/she is
not allowed to choose an experimental
and control group at random from a
5. What are the advantages of this kind of experimental
design (Randomised Experimental Design) in Scientific
Randomised Experimental Design can
provide strong evidence to support an
hypothesis because other possible
causal factors are controlled in the
Dr. Chan wanted to investigate the link between eating fruits and immunity
from cold. He selected 5000 students randomly from 500secondary schools
in Hong Kong. Then he sent questionnaires to these students to collect
information about their eating habits. He divided them into 2 groups: those
who eat fruits regularly (eating fruits every day) and those who seldom eat
fruits (eating fruits less than three times a week). The former became the
experimental group and the latter became the control group. He found that
the number of students in the experimental group outnumbered the number
of students in the control group. He then pared down the size of the
experimental group so that the numbers of students in the experimental and
control group were the same.
He then sent another questionnaire to
them to gather information from them on
their frequency of getting cold in the
following 6 months.
1. Use a flow chart to illustrate the above experimental design.
2. The experimental design in (1) is called Prospective
Experimental Design. What are its special features?
3. What are the advantages of Prospective Experimental
Design over Randomised Experimental Design?
4. What are the limitations of this kind of experimental design?
5.The experimental design in (1) is called Prospective
Experimental Design. What are the special features of the
Prospective Experimental Design?
The subjects who have been exposed to the
suspected cause (eating fruits regularly in (1)) are
selected for the experimental group while the
subjects of the control group who have not been
exposed to the suspected cause (eating fruits less
often) are selected for the control group.
6. What are the advantages of Prospective
Experimental Design over Randomised Experimental
Easier and less expensive to carry out the
May avoid ethical objections
Can pool data from very large group of
subjects thus increasing the accuracy of the
7. What are the limitations of this kind of
The selection process of experimental and
control group usually focuses on one single
causal factor, hence it is possible that other
factors will come into play at the very early
stage of the selection process (i.e. the
selected subjects having the suspected
cause already affected by other factor(s)).
Another Design involves the collection of data prior to the setting of
the objectives and design of the research. The data can be abstracted
from past studies or researches. This kind of design is called
Retrospective Design: to find the Cause and Effect from past data and
Suspected Cause (independent variable)
Suspected Effect (dependent variable)
Experimental group: a group of subjects chosen from the population
with the suspected effect (or with improvement after treatment)
Control group: a group of subjects chosen from the population
without the suspected effect (or without improvement after treatment)
1.What would the result of Dr. X’s study be if
acupuncture treatment can prolong the life
span of patients with last stage liver cancer?
2.What are the pros and cons of Retrospective
3. What would the result of Dr. X’s be if
acupuncture treatment can prolong the life span of
patients with last stage liver cancer?
We may look for the percentage difference (life
span) between the experimental and control
group. If there is a higher and significant % of
patients who can still live after 1 year in the
experimental group than that of the control
group, the study may support the causal link
between acupuncture and prolonging of life
span of patients with last stage liver cancer.
4. What are the pros and cons of
Retrospective Experimental Design?
(i) The advantages of retrospective experimental design are that it
is quick and inexpensive. We only need to analyse the data of
past research/study carefully.
(ii) The disadvantages of retrospective experimental design are as
Can only provide weak evidence for a causal link because
there may be other potential causal factors which are difficult to
control in this kind of experimental design (these potential
causal factors other than the one to be tested for may
automatically be built in the experimental and control group).
Furthermore, retrospective experimental design cannot
estimate the level of difference of the effect being studied.
Reference: Stephen S. Carey (1998)
Read the following news and answer the
Inhalation of air with high oxygen
concentration enhances memory
A Japanese electric appliance company X conducted a
research and claimed that inhalation of air with high oxygen
concentration can enhance memory. 80 students were
divided into 2 groups. They were required to take a test on
English vocabulary. Then the experimental group of
students inhaled air of high concentration of oxygen while
the control group breathed ordinary air. Both groups of
students were allowed to have some revision before taking
the same vocabulary test again. It was found that the
experimental group could memorise the vocabulary items
15% more than that of the control group.
1. Name the experimental design used by
2. Comment on the experimental result and
judge the reliability of the research