Identifying dependent and independent variables
The dependent variable, independent variable and the level of the
independent variable have been identified in each example
experiment below.
Experiment 1
A pigeon is trained to peck a key if a green light is
illuminated but not if a red light is illuminated. Correct pecks
are rewarded by access to grain and the numbers of pecks
are recorded.
The dependent variable is: The numbers of correct pecks are
recorded to see if manipulating the independent variable has had an
effect and are therefore the dependent variable.
The independent variable is: The colour of the light is the
variable that is manipulated and is therefore the independent
variable.
The levels of the independent variable are: Red light and green
light are levels of the independent variable as they both represent
single conditions or treatments in this experiment.
Experiment 2
A car manufacturer wants to know how bright brake lights
should be to minimise the time required for the driver of the
following car to realise that the car in front is stopping and
to brake themselves. An experiment was conducted to
answer this question. Car 1 was the lead car and Car 2 the
following car. Identify the variables.
The dependent variable is: The time taken for Car 2 to apply
brakes after car 1 has applied brakes is observed and measured to
see if manipulating the independent variable has had an effect.
The independent variable is: The brightness of brake lights on
car 1 is the variable that is manipulated and is therefore the
independent variable.
The levels of the independent variable are: The different brake
light brightness’s tested on Car 1 are levels of the independent
From: The Research Observatory @ The University of the West of England,
Bristol. Last edited 10th February 2009.
variable as they represent single conditions or treatments in this
experiment.
Experiment 3
A higher education (post 16 years) college (college A) has
decided to offer a £20.00 reward to its students for each AS
level they achieve at grade A or A+. Another college (college
B) in the same area has decided not to give any monetary
awards to its students. Both colleges are interested to see if
this incentive will increase the number of A and A+ grade
passes. Identify the variables.
The dependent variable is: The number of A and A+ grades
achieved by the students at each college are observed and
measured to see if manipulating the independent variable has had
an effect.
The independent variable is: The £20.00 reward is the variable
that is manipulated and is therefore the independent variable.
The levels of the independent variable are: The £20.00 reward
and no monetary reward are levels of the independent variable as
they represent single conditions or treatments in this experiment.
Experiment 4
A number of plants are grown in either blue or red coloured
light to see if this has an effect on the length of the plants
leaves.
The dependent variable is: The leaf length is observed and
measured to see if manipulating the independent variable has had
an effect.
The independent variable is: The colour of the
manipulated and is therefore the independent variable.
light
is
The levels of the independent variable are: Blue light and red
light are both levels of the independent variable as they represent
single conditions or treatments in this experiment.
From: The Research Observatory @ The University of the West of England,
Bristol. Last edited 10th February 2009.