SCIENTIFIC METHOD PRACTICE - KEY
Note: this key is for the long version, you may not have had all these questions.
For each of the following, indicate whether the statement would be useful as a scientific hypothesis. Remember
that a hypothesis must be testable and falsifiable (able to be proved incorrect). If you do not think that a
statement is testable or falsifiable, indicate the reason why.
1.
Crime rates increase during the full moon.
Yes, this hypothesis is both testable and falsifiable. You could easily design a study
to examine crime rates during the full moon. (By the way, it is true!)
2.
Positive kitty emotions prolong life.
No, this hypothesis would be impossible to measure. How will “positive kitty
emotions” be examined or measured? You cannot with any reliability. When
scientists do studies like this on people, they must be sure that they are measuring
something objectively. For example, they could ask participants in the study to rate
their emotions on a scale from positive to negative over time. It’s still very vague
and results from studies like this should be taken with a grain of salt.
3.
The extinct dodo is closely related to other large flightless birds, such
as ostriches and rheas.
Examinations of the anatomical structures of the dodo (it’s bone and muscle
structure, for example) could be compared to the structures of other birds. The
DNA of the dodo could also be compared to other birds. This hypothesis works.
4.
People with larger heads have higher IQs.
This hypothesis is fine. You could measure head size and correlate that with results
from an IQ test. It would be important to have all the participants in the study take
the same IQ test, however.
5.
The rate of carbon dioxide produced for yeast cells growing with and
without the spice cinnamon.
INDEPENDENT VARIABLE: cinnamon
DEPENDENT VARIABLE: rate of carbon dioxide production
6.
Number and size of cubs born to polar bears living in habitats reduced
by melting sea ice.
INDEPENDENT VARIABLE: size of habitat
DEPENDENT VARIABLE: number and size of cubs
7.
The beak length for ground finches recorded over 3 years.
INDEPENDENT VARIABLE: time (3 years)
DEPENDENT VARIABLE: beak length
Suggest a control group that could be used for each of the following experiments.
For each of the experiments, explain why you chose the control group you did.
8.
Rats are fed artificial sweeteners to determine if the sweeteners cause
cancer.
Rats that are not fed artificial sweeteners, but are kept under the same conditions
as the rats that are fed sweeteners. You need a control that will determine the
background cancer rate in your rats under your conditions (feed, environment, etc).
9.
Alzheimer patients aged 40-55 are tested for the presence of three
copies of chromosome 21.
People aged 40-55 who do not have Alzheimers. This study might be somewhat
complicated by the fact that most people who develop the disease do not show
symptoms until later in life. The study would have to follow the participants until old
age to ensure that they didn’t go on to develop symptoms.
10. Black worms are exposed to a solution of saline. Worm heart rates are
recorded at 2-minute intervals.
Black worms that are exposed to a solution of fresh water. In this case, it is
important to expose the worms to something and it should be similar to saline (salt
water), but without the thing you are testing (the salt). Just saying “worms not
exposed to saline” wouldn’t be a good enough control because it may be that
sticking the worms into any solution would affect their heart rate. An ideal control
would be to create a solution that has the same concentration of salt as their
normal environment. You would not expect this solution to affect the heart rate.
11. Frogs are captured from ponds where the parasitic worm Robeiroia is
abundant. The number of limb deformities is recorded.
Frogs captured from ponds that are free from the parasitic worm. This experiment
would be more convincing if you tested frogs from ponds that contained various
levels of Robeiroira to see if the number of limb deformities correlated with the level
of Robeiroira.
EXPERIMENT 1: John wanted to know if there would be a difference in the rate
that tomato plants would grow at different temperatures. He thought that if tomato
plants were grown at warmer temperatures, then they would grow faster. He took
one tomato plant and put it at room temperature (72°F) and put a second tomato
plant in a room where the heater was on (85°F). He measured the height of the
plants every 3 days for 4 weeks.
QUESTION: Will tomato plants grow faster at warmer temperatures?
HYPOTHESIS: If tomato plants are grown at warmer temperatures, then they
will grow faster.
INDEPENDENT VARIABLE: Temperature
DEPENDENT VARIABLE: Height of tomato plants
CONSTANTS: amount of sunlight, age / size of plant, type of tomato, soil,
pot, amount of water, etc.
CONTROL GROUP: Plant grown at room temperature
EXPERIMENTAL GROUP: Plant grown at 85°F
POSSIBLE SOURCES OF ERROR: Only one plant was used at each
temperature; plant at higher temperature may dry out faster (which would be
2 independent variables: temperature and amount of water in soil); no
constants are specified (were the plants the same age when started? are they
getting the same amount of sun?)
EXPERIMENT 2: Sara was curious about whether plants needed day AND night to
grow. She thought that if she grew plants under light 24 hours a day, then they
would grow faster. She took three plants put them outside, where they would get
night and day. She took three plants and put them in a room where she left the
lights on all the time. She counted how many leaves the plants had every 3 days.
QUESTION: Do plants need day and night to grow?
HYPOTHESIS: If plants are grown under 24 hours of light, then they will grow
faster.
INDEPENDENT VARIABLE: amount of light
DEPENDENT VARIABLE: number of leaves
CONSTANTS: type of plant, soil, water, starting size, temperature, etc.
CONTROL GROUP: day and night plants
EXPERIMENTAL GROUP: 24 hours of light plants
POSSIBLE SOURCES OF ERROR: the environment outside is quite different
from the environment inside, so multiple things are probably not constant
(weather, amount of water, insects, etc); sunlight and artificial light (indoors)
are not necessarily the same for the plant
EXPERIMENT 3: Mike had heard that eating chocolate would
make you smarter. He decided to test the speed that rats
could run through a maze before and after eating chocolate.
He thought that rats would run through the maze more
quickly if they had eaten chocolate. He timed a rat as it ran
through a maze, then he fed it chocolate and timed it again
as it ran through the same maze.
QUESTION: Will chocolate make you smarter?
HYPOTHESIS: If a rat is fed chocolate, then it will be smarter.
INDEPENDENT VARIABLE: eaten chocolate or not
DEPENDENT VARIABLE: speed through maze
CONSTANTS: length of maze (if same maze isn’t used), food other than
chocolate, difficulty of maze, in this experiment the rat is constant although
that wouldn’t be the case if there were more than one trial done.
CONTROL GROUP: no-chocolate eating rats
EXPERIMENTAL GROUP: chocolate eating rats
POSSIBLE SOURCES OF ERROR: This experiment is very poorly set up. Speed
through a maze does not necessarily test how intelligent a rat is; it tests how
fast the rat is. The chocolate could make the rat faster, but not smarter
(sugar or caffeine). The rat could be tired after the first test, or it could be
more familiar with the maze and go faster – several rats should be used for
more consistent results and this is not clearly specified in the experiment.
EXPERIMENT 4: The class wanted to know who in the class could jump the
highest. We hypothesized that it would be the person with the longest legs. We
measured the length of each student’s legs and then measured the height that each
student could jump.
QUESTION: Does leg length affect the height a person can jump?
HYPOTHESIS: If a person has longer legs, then they will be able to jump
further.
INDEPENDENT VARIABLE: leg length
DEPENDENT VARIABLE: jump height
CONSTANTS: method of measuring (are you measuring how high the student
can reach? how far their feet get off the ground?), type of shoes (or no
shoes), what the person ate that day, etc.
CONTROL GROUP: there really isn’t a control group in this experiment
because all the results will be compared to each other. You could set one person as
the standard and compare all the other results to that person (the middle-most leg
length or the shortest legs).
EXPERIMENTAL GROUP: all the other jumpers
POSSIBLE SOURCES OF ERROR: There are a lot of other factors at play here
other than leg length: male vs female, how athletic a person is, what type of shoes
they are wearing, how much effort they put into their jump, etc. This would be a
very difficult experiment to get good results for. It might also be difficult to
measure leg length consistently (measure from where to where?).