SCIENTIFIC METHOD INVESTIGATIONS: VARIABLES and GRAPHS
Independent and dependent variables are mathematical tools used in an experiment
to keep track of what's going on. They allow you to maintain control over your
experiment in a quantitative way. That is, using them, you will be able to measure
your results and draw accurate conclusions.
Independent and dependent variables are related to one another. The Independent
part is what you, the experimenter, changes or enacts in order to do your
experiment. The dependent variable is what changes when the independent variable
changes - the dependent variable depends on the outcome of the independent
variable. There is ALWAYS one independent variable and usually one dependent
variable.
*If you have more than one independent variable in your experiment (more then
one variable that is affecting your dependent variables) that probably indicates that
you have not properly identified and dealt with your fixed/controlled variables.
Breaking this down further, we summarize variables in this way:
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Independent Variables are changes that occur in an experiment that are
directly caused by the experimenter (you) – data will go on X axis of graph.
Dependent Variables are changes that occur due to independent variables –
data will go on Y axis of graph.
A Fixed/Controlled Variable is anything else that could influence the
dependent variables but fixed/controlled variables must be carefully
monitored and kept equal in your experiments - otherwise they could mess up
your experiment by making your results false or unreliable – not on the graph.
Many people have trouble remembering which is the independent variable and which
is the dependent variable. An easy way to remember is to insert the names of the
two variables you are using in this sentence in they way that makes the most sense.
Then you can figure out which is the independent variable and which is the
dependent variable:
(Independent variable) causes a change in (Dependent Variable) and it isn't possible
that (Dependent Variable) could cause a change in (Independent Variable).
For example 1: (Time Spent Studying) causes a change in (Test Score) and it isn't
possible that (Test Score) could cause a change in (Time Spent Studying).
We see that "Time Spent Studying" must be the independent variable and "Test
Score" must be the dependent variable because the sentence doesn't make sense
the other way around.
For example 2: If you were measuring the growth rate of plants under full sunlight
for 8 hours a day versus plants that only have 4 hours of full sunlight per day, the
amount of time per day of full sunlight would be the independent variable - the
variable that you control. The growth rate of the plants would be a dependent
variable.
A dependent variable? Yes, there can be more then one dependent variable. In
our example, for instance, the growth rate of the plants might be one dependent
variable and the overall height of the plants might be another dependent
variable. Both of these variables depend upon the independent variable.
CREATING GRAPHS: How to Choose Which Type of Graph to Use?
When to Use . . .
. . . a Line graph.
Line graphs are used to track changes over short and long periods of time. When smaller
changes exist, line graphs are better to use than bar graphs. Line graphs can also be sued to
compare changes over the same period of time for more than one group.
. . . a Pie Chart.
Pie charts are best to use when you are trying to compare parts of a whole. They do not show
changes over time.
. . . a Bar Graph.
Bar graphs are used to compare things between different groups or to track changes over time.
However, when trying to measure change over time, bar graphs are best when the changes
are larger.
PERFECTING PIE CHARTS
PIE CHARTS CAN BE USED TO SHOW PERCENTAGES OF A WHOLE, AND REPRESENTS
PERCENTAGES AT A SET POINT IN TIME. UNLIKE BAR GRAPHS AND LINE GRAPHS, PIE
CHARTS DO NOT SHOW CHANGES OVER TIME. THE FOLLOWING PAGES DESCRIBE THE
DIFFERENT PARTS OF A PIE CHART.
The Title The title offers a short explanation of what is in your graph. This helps the reader
identify what they are about to look at. It can be creative or simple as long as it tells what is
in the chart. The title of this chart tells the reader that the graph contains information about
how money is spent for public education for the average student.
The Key The key tells what each slice represents. Just like on a map, the legend helps the
reader understand what they are looking at. This legend tells us that the green slice
represents money spent on instruction, the blue slice represents money spent on support
services, and the orange slice represents money spent on non-instruction activities.
The Data The most important part of your chart is the information, or data, it contains. Pie
charts represent data as part of 100 (a percentage). Each slice represents a different piece of
data.
Information and trial questions from:
http://cnx.org/content/m10802/latest/
http://www.cool-science-projects.com/independent-and-dependent-variables.html
http://www.lhup.edu/sboland/independent_and_dependent_variab.htm
http://nces.ed.gov/nceskids/help/user_guide/graph/variables.asp
INDEPENDENT AND DEPENDENT VARIABLES: EXAMPLES TO TRY
Identify the independent and dependent variables in the following studies.
(The answers are at the end of the page – scroll down to find them.)
1. A group of college students were given a short course in speed-reading.
The instructor was curious if a monetary incentive would influence
performance on a reading test taken at the end of the course. Half the
students were offered $5 for obtaining a certain level of performance on the
test, the other half were not offered money.
Independent variable:
Dependent variable:
2. A social psychologist thinks that people are more likely to conform to a
large crowd than to a single person. To test this hypothesis, the social
psychologist had either one person or five persons stand on a busy walking
path on campus and look up. The psychologist stood nearby and counted the
number of people passing by who also looked up.
Independent variable:
Dependent variable:
3. To test a new voice feature in a cockpit design a flight simulator was
used. The simulator was programmed to give visual readings of flight
information, or to give visual and auditory (voice) readings of flight
information. All test pilots were put through a simulated emergency landing
procedure, but were randomly assigned to the visual, or visual and auditory
conditions. Flight experts rated each pilot’s performance in the simulator on
a scale of 1 (very poor) to 10 (excellent).
Independent variable:
Fixed variables:
Dependent variable:
4. Can blueberries slow down aging? A study indicates that antioxidants
found in blueberries may slow down the process of aging. In this study, 19month old rats (equivalent to 60-year old humans) were fed either their
standard diet or a diet supplemented by either blueberry, strawberry, or
spinach powder. After eight weeks, the rats were given memory and motor
tests. Although all supplemented rats showed improvement, those
supplemented with blueberry powder showed the most notable improvement.
Independent variable:
Dependent variables:
5. Does beta-carotene protect against cancer? Beta-carotene supplements
have been thought to protect against cancer. However, a study published in
the Journal of the National Cancer Institute suggests this is false. The study
was conducted with 39,000 women aged 45 and up. These women were
randomly assigned to receive a beta-carotene supplement or a placebo, and
their health was studied over their lifetime. Cancer rates for women taking
the beta-carotene supplement did not differ systematically from the cancer
rates of those women taking the placebo.
Independent variable:
Dependent variable:
6. How bright is right? An automobile manufacturer wants to know how
bright brake lights should be in order to minimize the time required for the
driver of a following car to realize that the car in front is stopping and to hit
the brakes.
Independent variable:
Dependent variable:
7. In a study done by gr 11 boys at a well known boys school, the boys
wanted to find out the conditions in which milk stayed fresh the longest. The
boys took equal quantities of fresh 2% milk and stored the milk at 4oC, 23oC
and 18oC and tested for freshness each day.
Name the independent variable:
Name the dependent variable:
8. A friend of yours puts forward the hypothesis that over half of boys at
Bishops believe that species are immutable (unchanging) and that they came
into existence through special creation rather than being evolving from
common ancestors. He suggests that you survey a group of your 18 year old
male friends in order to test the hypothesis.
Name the independent variable:
Name the dependent variable:(HINT:
look at the hypothesis itself as both the independent and dependent variables
should be listed IN the hypothesis).
Answers:
1. A group of college students were given a short course in speed-reading.
The instructor was curious if a monetary incentive would influence
performance on a reading test taken at the end of the course. Half the
students were offered $5 for obtaining a certain level of performance on the
test, the other half were not offered money.
Independent variable: Monetary incentive ($5 or no money)
Dependent variable: Performance on reading test
2. A social psychologist thinks that people are more likely to conform to a
large crowd than to a single person. To test this hypothesis, the social
psychologist had either one person or five persons stand on a busy walking
path on campus and look up. The psychologist stood nearby and counted
the number of people passing by who also looked up.
Independent variable: Size of group (5 people or 1 person)
Dependent variable: Conformity (measured by number of people looking
up)
3. To test a new voice feature in a cockpit design a flight simulator was
used. The simulator was programmed to give visual readings of flight
information, or to give visual and auditory (voice) readings of flight
information. All test pilots were put through a simulated emergency landing
procedure, but were randomly assigned to the visual, or visual and auditory
conditions. Flight experts rated each pilot’s performance in the simulator
on a scale of 1 (very poor) to 10 (excellent).
Independent variable: Presence or absence of auditory (voice)
readings. (All pilots received visual readings, so that is a constant
variable.)
Dependent variable: Ratings of pilots’ performance
4. Can blueberries slow down aging? A study indicates that antioxidants
found in blueberries may slow down the process of aging. In this study, 19month old rats (equivalent to 60-year old humans) were fed either their
standard diet or a diet supplemented by either blueberry, strawberry, or
spinach powder. After eight weeks, the rats were given memory and motor
tests. Although all supplemented rats showed improvement, those
supplemented with blueberry powder showed the most notable
improvement.
independent variable? (diet: blueberries or no blueberries)
dependent variables? (memory test and motor skills test)
5. Does beta-carotene protect against cancer? Beta-carotene supplements
have been thought to protect against cancer. However, a study published in
the Journal of the National Cancer Institute suggests this is false. The study
was conducted with 39,000 women aged 45 and up. These women were
randomly assigned to receive a beta-carotene supplement or a placebo, and
their health was studied over their lifetime. Cancer rates for women taking
the beta-carotene supplement did not differ systematically from the cancer
rates of those women taking the placebo.
Independent variable? (supplements: beta-carotene or placebo)
Dependent variable? (occurrence of cancer)
6. How bright is right? An automobile manufacturer wants to know how
bright brake lights should be in order to minimize the time required for the
driver of a following car to realize that the car in front is stopping and to hit
the brakes.
Independent variable? (brightness of brake light)
Dependent variable? (time to hit brake)
7. In a study done by gr 11 boys at a well known boys school, the boys
wanted to find out the conditions in which milk stayed fresh the longest.
The boys took equal quantities of fresh 2% milk and stored the milk at 4 oC,
23oC and 18oC and tested for freshness each day.
Name the independent variable:
Name the dependent variable:
temperatures at which milk is kept
freshness of the milk
8. A friend of yours puts forward the hypothesis that over half of boys at
Bishops believe that species are immutable (unchanging) and that they
came into existence through special creation rather than being evolving
from common ancestors. He suggests that you survey a group of your 18
year old male friends in order to test the hypothesis.
Name the independent variable: Bishops boys
Name the dependent variable:
immutable
number that believe that species are
Fixed variables: same age, same gender, all have same amount of
knowledge about evolution, all are friends who will answer truthfully
Sample size: as big as possible to ensure that data is reliable