Transcript Observational Study vs . Experiment

Observational Study vs. Experiment
• An observational study
observes individuals
and measures variables
of interest but does not
influence the
responses.
• An experiment
deliberately imposes
some treatment on
individuals to measure
their responses.
• Experiments give good
evidence for causation
Language of experiments
• Treatment a specific condition applied to the
individuals in an experiment.
– If an experiment has several explanatory variables,
a treatment is a combination of specific values of
these variables.
The experimental units are the smallest collection of
individuals to which treatments are applied.
When the units are human beings, they often are
called subjects.
Language of Experiments
• A response variable measures and outcome of a study.
• An explanatory variable may help explain or influence
changes in a response variable. Another name is a
factor.
• A lurking variable is a variable that is not among the
explanatory or response variables but may influence
the response variable . Lurking variables create
problems in two ways.
• Confounding occurs when two variables are associated
in such a way that their effects on a response variable
cannot be distinguished from each other.
Factors and Levels
• The experimental
variables in an
experiment are called
factors
• Many experiments
study the joint effect of
several factors. Each
treatment is formed by
combining a specific
value called a level of
each of the factors.
The Randomized Comparative
Experiment
Random
Assignment
• Balance out the effects
of lurking variables
• Forms groups of
experimental units that
should be similar in all
respects before the
treatments are applied.
Control
Replication
• Ensure that influences
other than the
experimental
treatments operate
equally on all groups.
• Use enough
experimental units in
each group so that any
differences in the effects
of the treatments can
be distinguished from
chance differences
between the groups.
What can go wrong
The placebo effect.
• The response to a
dummy treatment.
• Used as control for
comparison
Double Blind
Statistically Significant.
• Neither the subject nor
those who interact with
them and measure the
response variable know
which treatment a
subject receives.
• Single blind. The
subjects are unaware
of which treatment
they are receiving but
the people interacting
with them and
measuring the response
variable do know
• An observed effect so
large it would rarely
occur by chance
• Gives good evidence
that the treatments
actually caused these
differences. A
statistically significant
association in data from
a well designed
experiment does imply
causation.
Blocking
A group of experimental units that are known before
the experiment to be similar in some way that is
expected to affect the response to the treatment.
In a randomized block design the random assignment of experimental
units to treatments is carried out separately within each block
Another form of control. Also allows separate
conclusions about each block.
Matched Pairs Design
Special case of randomized block
design that uses blocks of size two
Subjects serve as their own
control
• The random assignment of
subjects to treatments is
done within each matched
pairs. Helps reduce the
effect of variation among
the experimental units.
One subject gets the first
treatment and the second
subject gets the other
treatment.
• Each pair consists of just
one experimental unit that
gets both treatments one
after the other. The order
of the treatment can
influence the response so
we randomize the order for
each experimental unit.