Observational Studies
 Simply
observing what
happens
–A sample survey is an
observational study.
–There are other observational
studies that are not surveys.
1
Tanning and Skin Cancer
 The
observational study involved
1,500 people.
 Checked medical records to see if a
person had experienced skin cancer or
had never had skin cancer.
 Asked all participants whether they
had used tanning beds.
2
Sodium and Blood Pressure
 Enroll
100 individuals in the
study.
 Give them diet diaries where they
record everything they eat each
day for a month. From this the
amount of sodium in the diet is
found.
 Measure their blood pressure.
3
Differences
– look at past records
and historical data.
 Retrospective
– Tanning and Skin Cancer
– identify subjects and
collect data as events unfold.
 Prospective
– Sodium and Blood Pressure
4
Experiments
 Explanatory
variable – Factor.
 Treatments.
– Participants –
Experimental Units.
 Response variable.
 Subjects
5
Experiments
 The
experimenter must actively
and deliberately manipulate the
factor(s) to establish the method
of treatment. Experimental units
are assigned at random to the
treatments.
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Sodium and Blood Pressure
 20
subjects.
 Factor – amount of dietary sodium.
 Treatments: low sodium diet and high
sodium diet.
 10 subjects randomly assigned to
each treatment.
 Response – systolic blood pressure.
7
Experimental Principles
 Control
 Randomize
 Replicate
– within an experiment.
– repeating the entire experiment.
 Block
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Control
 Control
outside variables that may
affect the response.
– Have subjects of the same age,
gender, general health, ethnic
group.
– By controlling outside variables
you prevent those variables from
causing variation in the response.
9
Randomization
 Randomization
tends to spread the
effects of uncontrolled outside
variables evenly across the
treatment groups.
 Randomization reduces the chance
that an uncontrolled outside
variable will bias the results.
10
Replication
 Within
an experiment.
–must have several experimental
units in each treatment group.
–can assess the natural variation
in the response for units treated
the same.
11
“Replication”
 Repeating
the entire experiment.
– This is especially important if the
subjects in an experiment are not a
random sample from a population.
– Are the results of the entire
experiment repeatable?
12
Diagram
Group 1
several
subjects
Subjects
Treatment 1
random
allocation
Group 2
several
subjects
Compare
Response
Treatment 2
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Block
 There
may be attributes of the
experimental units that can’t be
controlled but may affect the
response.
 Group similar experimental units
into blocks and then randomize
the assignment of treatments
within each block.
14
Blocking
 Math
ability
–very high, high, average, low,
and very low.
–assign, at random, students from
each math ability group to each
treatment.
15
More Ideas
 Control
“treatment”.
 Blinding.
–Single blind and double blind.
 Placebos.
16
Multiple Factors
 Factors
– can use calculator (yes, no)
– can use a formula sheet (yes, no).
 Treatments
– calculator and formulas, calculator but
no formulas, formulas but no calculator,
no calculator and no formulas.
17
Confounding
 Sodium
and blood pressure.
– All subjects on the low sodium diet had
their blood pressure measured by a
registered nurse using a standard
manual cuff and stethoscope.
– All subjects on the high sodium diet had
their blood pressure measured using an
automated cuff and digital readout.
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