Design Method of Data Collection

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Design Method of Data Collection
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Experiments Observational Studies

• Two other basic types of statistical study used
  for collecting data are experimental studies and
  observational studies.
• We use these when we are interested in studying
  the effect of a treatment on individuals or
  experimental units.

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Experiments Observational Studies

• We conduct an experiment when it is (ethically,
  physically etc) possible for the experimenter to
  determine which experimental units receive which
  treatment.

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Experiments Observational Studies

• Experiment Terminology
• Experimental Unit Treatment
  Response

patient drug
cholesterol car gasoline knocking tomatoes
fertilizer yield mouse
radiation mortality
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Experiments Observational Studies

• In an observational study, we compare the units
  that happen to have received each of the
  treatments.

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Experiments Observational Studies
Observational Study

• e.g. You cannot set up a control (non-smoking)
  group and treatment (smoking) group.

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Experiments Observational Studies

• Note
• Only a well-designed and well-executed
  experiment can reliably establish causation.
• An observational study is useful for identifying
  possible causes of effects, but it cannot
  reliably establish causation.

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Experimentation

• Guiding Principle
• Make comparisons fair try to make treatment
  groups as similar as possible except for
  treatments being used.

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1. Completely Randomized Design

• The treatments are allocated entirely by chance
  to the experimental units.

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1. Completely Randomized Design

• Example
• Which of two varieties of tomatoes (A B) yield
  a greater quantity of market quality fruit?
• Factors that may affect yield
• different soil fertility levels
• exposure to wind/sun
• soil pH levels
• soil water content etc.

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1. Completely Randomized Design

• Divide the field into plots and randomly
  allocate the tomato varieties (treatments) to
  each plot (unit).
• 8 plots 4 get variety A

UPHILL
(A)
(A)
(A)
(A)
Discuss for ½ Minute
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1. Completely Randomized Design

• Note
• Randomization is an attempt to make the
  treatment groups as similar as possible we can
  only expect to achieve this when there is a large
  number of plots.

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2. Blocking

• Group (block) experimental units by some known
  factor and then randomize within each block in an
  attempt to balance out the unknown factors.
• Use
• blocking for known factors (e.g. slope of field
  in previous example)
• and
• randomization for unknown factors to try to
  balance things out.

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2. Blocking

• Example continued
• It is recognized that there are two areas in the
  field well drained and poorly drained.
• Partition the field into two blocks and then
  randomly allocate the tomato varieties to plots
  within each block.

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2. Blocking
Well drained Poorly drained
1 (B)
2 (A)
4 (B)
3 (A)
6 (A)
5 (A)
8 (B)
7 (B)

• How should we allocate varieties to plots?
• Discuss in groups for 1/2 minute.

Randomly assign types to 4 well drained plots and
then to the 8 poorly drained plots.
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3. People as Experimental Units

• Example Cholesterol Drug Study Suppose we
  wish to determine whether a drug will help lower
  the cholesterol level of patients who take it.
• How should we design our study?
• Discuss for two minutes in groups.

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Polio Vaccine Example
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Polio Vaccine Example
Dr. Jonas Salk, vaccine pioneer 1914-95
Iron Lung
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The Salk Vaccine Field Trial

• 1954 Public Health Service organized an
  experiment to test the effectiveness of Salks
  vaccine.
• Need for experiment
• Polio, an epidemic disease with cases varying
  considerably from year to year. A drop in polio
  after vaccination could mean either
• Vaccine effective
• No epidemic that year

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The Salk Vaccine Field Trial

• Subjects 2 million, Grades 1, 2, and 3
• 500,000 were vaccinated
• (Treatment Group)
• 1 million deliberately not vaccinated
• (Control Group)
• 500,000 not vaccinated - parental permission
  denied

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The Salk Vaccine Field Trial

• NFIP Design
• Treatment Group Grade 2
• Control Group Grades 1 and 3 No Permission
• Flaws ? Discuss for 30 seconds.
• Polio contagious, spreading through contact.
  i.e. incidence could be greater in Grade 2 (bias
  against vaccine), or vice-versa.
• Control group included children without parental
  permission (usually children from lower income
  families) whereas Treatment group could not (bias
  against the vaccine).

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The Salk Vaccine Field Trial

• Double-Blinded Randomized Controlled
  Experimental Design
• Control group only chosen from those with
  parental permission for vaccination
• Random assignment to treatment or control group
• Use of placebo (control group given injection of
  salted water)
• Diagnosticians not told which group the subject
  came from (polio can be difficult to diagnose)
• i.e., a double-blind randomized controlled
  experiment

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The Salk Vaccine Field Trial

• The double-blind randomized controlled
  experiment (and NFIP) results

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3. People as Experimental Units

• control group
• Receive no treatment or an existing treatment
• blinding
• Subjects dont know which treatment they receive
• double blind
• Subjects and administers / diagnosticians are
  blinded
• placebo
• Inert dummy treatment

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3. People as Experimental Units

• placebo effect
• A common response in humans when they believe
  they have been treated.
• Approximately 35 of people respond positively to
  dummy treatments - the placebo effect

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Design Method of Data Collection
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Observational Studies

• There are two major types of observational
  studies

prospective
and retrospective studies
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Observational Studies

• 1. Prospective Studies
• (looking forward)

• Choose samples now, measure variables and follow
  up in the future.
• E.g., choose a group of smokers and non-smokers
  now and observe their health in the future.

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Observational Studies

• 2. Retrospective Studies
• (looking back)

• Looks back at the past.
• E.g., a case-control study
• Separate samples for cases and controls
  (non-cases).
• Look back into the past and compare histories.
• E.g. choose two groups lung cancer patients and
  non-lung cancer patients. Compare their smoking
  histories.

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Observational Studies

• Note
• 1. Observational studies should use some form of
  random sampling to obtain representative samples.
• Observational studies cannot reliably establish
  causation.

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Controlling for various factors

• A prospective study was carried out over 11 years
  on a group of smokers and non-smokers showed that
  there were 7 lung cancer deaths per 100,000 in
  the non-smoker sample, but 166 lung cancer deaths
  per 100,000 in the smoker sample.
• This still does not show smoking causes lung
  cancer because it could be that smokers smoke
  because of stress and that this stress causes
  lung cancer.

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Controlling for various factors

• To control for this factor we might divide our
  samples into different stress categories. We
  then compare smokers and non-smokers who are in
  the same stress category.
• This is called controlling for a confounding
  factor.

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Observational Studies

• Surgeon General of United States
• Presence of strong relationship (5 x higher)
• Strong research design
• Dose-response relationship
• Temporal relationship - cause should precede
  effect
• Reversible association
• Consistency different studies produce similar
  results
• Biological plausibility

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Example 1

• Home births give babies a good chance NZ
  Herald, 1990
• An Australian report was stated to have said that
  babies are twice as likely to die during or soon
  after a hospital delivery than those from a home
  birth.
• The report was based upon simple random samples
  of home births and hospital births.
• Q Does this mean hospitals are dangerous places
  to have babies in Australia? Why or why not?
  Discuss for 1 minute in groups.

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Example 2

• Lead Exposure Linked to Bad Teeth in Children
  USA Today
• The study involved 24,901 children ages 2 and
  older. It showed that the greater the childs
  exposure to lead, the more decayed or missing
  teeth.
• Q Does this show lead exposure causes tooth
  decay in children? Why or why not?
• Discuss for 1 minute.

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Example 2 contd

• Lead Exposure Linked to Bad Teeth in Children
  USA Today
• Researcher
• We controlled for income level, the proportion
  of diet due to carbohydrates, calcium in the diet
  and the number of days since the last dental
  visit.