Objective: Identify and use the four principles of experimental design.

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Objective Identify and use the four principles
of experimental design.

• HW Read pp. 290-293 and complete exercises 5.31,
  5.32, 5.33
• Then read pp. 294-298 and complete exercise 5.38
  (which is an extension of 5.32).
• Do Now Answer the following questions in your
  notebook. If you
• dont know, simply leave the question blank
• What did you eat last night for dinner two nights
  ago?
• What color shirt did you wear yesterday?
• What time did you wake up yesterday morning?
• How confident are you that your answers to the
  above questions are accurate?

2
Observational Studies

• Observes people in the wild.
• Researchers dont assign choices or manipulate
  subjects. They simply observe them.
• What do you think the difference between a
  retrospective and prospective study are?

3

• Retrospective data identifies subjects and then
  looks at data from their past while prospective
  data identifies subjects and then collects data
  as the events unfold.
• Observational studies may identify important
  variables related to the outcome we are
  interested in, but there is no guarantee that we
  have found the right or most important related
  variables.

4

• Is it ever possible to prove a cause
• and effect relationship?
• Yes! But only if we run an EXPERIMENT.

5
Does taking vitamin C reduce the chance of
getting a cold?
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• The individuals on whom or which we experiment
  are called experimental units. Human
  experimental units are called subjects or
  participants.

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Does taking vitamin C reduce the chance of
getting a cold?

• Our experimental units are probably humans
• in this scenario.

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• Experiments study the relationship between two or
  more variables.
• An experimenter must identify one explanatory
  variable, called a factor, to manipulate and at
  least one response variable to measure.
• An experimenter manipulates the factors to
  control the details of possible treatments.

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Does taking vitamin C reduce the chance of
getting a cold?

• Explanatory Variable / Factor (to
• manipulate)
• Response Variable (to measure)

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• The specific levels that the experimenter chooses
  for a factor are called the levels of a factor.
• The combination of specific levels from all the
  factors that an experimental unit receives is
  known as its treatment.

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Does taking vitamin C reduce the chance of
getting a cold?

• Lets choose levels for our factor

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Response Variable

• How will you measure your response variable?

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• An experiment requires a random assignment of
  subjects to treatments.

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DIAGRAMS

• Group 1 Treatment 1
• Random allocation
  Compare
• Group 2 Treatment 2

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The Four Principles of Experimental Design
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CONTROL

• Control sources of variation other than the
  factors you are testing by making conditions as
  similar as possible for all treatment groups.

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RANDOMIZE

• Randomization allows us to equalize the effects
  of unknown or uncontrollable sources of variation.

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REPLICATE

• Repeat the experiment with different subjects.
  Only with replication, can we estimate the
  variability in responses.
• If we experiment on a single subject, the outcome
  is an anecdote.

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Because I know you love them
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REPLICATE

• We also can talk about replication of an entire
  experiment at different levels (in other parts of
  the country, with people of different ages, at
  different times of year).

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BLOCK

• This is the only one that is not required for
  every experiment. This is analogous to
  stratifying for sampling design in surveys.
• We can group similar individuals together and
  then randomize within each block.

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Designing Experiments

• An ad for OptiGro plant fertilizer claims that
  with this product, you will grow juicier,
  tastier tomatoes. Youd like to taste this
  claim, and wonder whether you might be able to
  get away with half the specified dose. How can
  you set up an experiment to test this claim?

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• State what you want to know
• I want to know if OptiGro makes tomato plants
  tastier or juicier than plants without
  fertilizer.
• 2) Specify the response variable
• The level of juicy and tastiness in the
  tomatoes.
• 3) Specify the factor levels and treatments
• I will grow tomatoes at three different
  levels- some with no fertilizer, some with half
  the specified amount, and some with the full
  dose. These are the three treatments.
• 4) Specify the experimental units
• 24 lovely tomatoes (possibly organic and of the
  same variety) from the same store.

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• Observe the principles of experimental design
• -Control any sources of variability you know of
  and can control
• -Randomly assign experimental units to treatments
  to equalize the effects of unknown or
  uncontrollable variation
• -Specify how the random numbers needed for
  randomization will be obtained.
• -Replicate results by placing more than one plant
  in each treatment group.

Grow tomato plants in the same soil / plot of
land, take into account environmental things like
trees, etc. Take care of them consistently (same
amount of watering, pesticides, etc) I will use
my calculator to randomly select tomatoes to put
into 3 different groups. 24 Tomatoes put 8 in
each treatment group
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Other Details

• Specify other experiment details. You must give
  enough details so that another experimenter could
  exactly replicate your experiment. Its
  generally better to include details that seem
  irrelevant than to leave out matters that could
  turn out to be important.
• Specify how to measure the response.