Experiment Terms
Theory vs. Hypothesis
 A hypothesis is a proposed testable
explanation for an observable phenomenon.
 When a set of hypothesis is
confirmed to be true many
times, it is then reclassified
as a theory!
Some Example Theories
 Basic theories of biology are:
Cell theory: all organisms are made of cells.
Biogenesis theory: life only comes from life.
Evolution theory: living things have a common
ancestor and are adapted.
Gene theory: Organisms contain coded
information that determines their form, function,
and behavior.
 Other Theories:
Germ Theory - proposes that microorganisms
are the causes of many diseases
Heliocentric Theory - the sun is the center of the
solar system, planets revolve around the sun
Law of Gravity - masses attract each other
Practice Question
 Which is NOT a valid hypothesis?
A. Bluebirds sing to attract mates.
B. Bluebird songs are beautiful.
C. Only male bluebirds sing.
D. Sparrows will leave territories where they
hear bluebird songs
Control group vs. experimental
group
 Control group – provides a normal
standard to which the experimental group
can be compared. It is CONSTANT!
 Experimental group – experimental group
consists of what is being MANIPULATED!
Practice Question
 The purpose of including a control in a
scientific investigation is to provide what?
a. A basis for comparison.
b. A correction for experimental errors.
c. A preliminary trial of the methods.
d. An opportunity for repetition of the experiment.

Independent vs. Dependent
Variable
 Used in an experiment to keep track of
what's going on.
 Used to measure your results and draw
accurate conclusions.
 Are related to one another.
Independent vs. Dependent
Variable
 Independent variable – what you change
in order to do your experiment.
 There should be only one independent
variable for any valid experiment.
Independent vs. Dependent
Variable
 Dependent variable - what changes when
the independent variable changes
 the dependent variable depends on the
outcome of the independent variable.
 There can be more than one dependent
variable.
Independent vs. Dependent
Variable
 Example experiment:
You want to measure
and compare 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.
Independent vs. Dependent
Variable
 Independent variable – the amount of time
per day of full sunlight (you control this!)
 Dependent variable - the growth rate of
the plants.
Controlled Variable
 A Controlled Variable is anything else that
could influence the dependent variables.
 must be carefully monitored and kept equal in
your experiments.
 Example: Could the amount of water
affect the growth of our plants?
 Yes, so you need to make sure that each
plant gets the same amount of water in the
experiment.
 In this way you eliminate water as a possible
source of difference between your plants.
Practice Questions
 A researcher investigated two groups of
fruit flies.
Population A was kept in a 0.5 L container.
Population B was kept in a l.0 L container.
 Below is a graph showing the growth of
these two populations.
A researcher investigated two groups of fruit flies.
Population A was kept in a 0.5 L container.
Population B was kept in a l.0 L container.
 What is the manipulated (independent) variable?
A. The number of flies.
B. The time in days.
C. The numbers of groups studied.
D. The size of the containers.
A researcher investigated two groups of fruit flies.
Population A was kept in a 0.5 L container.
Population B was kept in a l.0 L container.
 What is the dependent variable?
A. The number of flies.
B. The time in days.
C. The numbers of groups studied.
D. The size of the containers.
 Which of the following is a logical inference
based on the contents of the graph?
A. The flies in Group B were healthier than those
in Group A.
B. A fly population with more available space will
grow larger than a population with less space.
C. If Group B were observed for 40 more days, the
size of the population would double.
D. In 40 days, the sizes of both populations would
decrease at the same rate.
Practice Questions
 Which variable is measured rather
than manipulated?
A. Independent variable
B. Dependent variable
Practice Questions

Positive people have more friends
than negative people. Which
statement is correct?
A. The dependent variable is the positivity of the
person
B. The dependent variable is the number of
friends
Practice Questions
 Smoking causes lung cancer.
Which statement is correct?
A. The independent variable is lung cancer
B. The independent variable is smoking