Experimental Design

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					Experimental Design

     Controlled Experiments

   Scientific research begins with making
    observations. Observations give rise to
       Some questions can be answered simply
        through observation.
           Ex: what do crayfish eat?
       Other questions can be answered only through
           Do crayfish prefer to eat animal matter or plant matter?

   Once a scientist has a question, they
    must form a hypothesis before
    conducting a controlled experiment.
         Question:“Does eating high-sodium foods
          cause high blood pressure”?
   Hypothesis: a testable statement of
    what is thought to be true based on
    initial observations, prior knowledge
    and scientific evidence.
“If…then…” statements
   The format for writing a hypothesis is the
    “If…then…” format.
   If…[what is thought to be true],
    then…[testable statement].
   Complete the If…then…hypothesis for the
    example scientific question.
     If a high-sodium diet causes high
       blood pressure, then
Example: Hypothesis

   If a high-sodium diet causes high
    blood pressure, then people who
    consume a high-sodium diet will
    have higher blood pressure after
    one year than people who eat a low-
    sodium diet.
Biologists use controlled experiments
to test their hypothesis

   Controlled Experiment: An
    experiment in which two or more
    groups are set up for comparison;
    only one variable is different
    between the groups.
       Example:
           What two groups would be compared in an
            experiment to answer this question?
    Control & Experimental Groups
   Control Group:              Experimental Group:
     Either,                     Either,
       Does not get                Gets the
         the treatment,                treatment, or
         or                         Is followed under
       Is followed                    altered conditions.
         under “normal”           Ex: Eats a diet that
         conditions.               exceeds the
     Ex: Eats a diet that         recommended value
      meets the daily              for sodium.
      recommended                   Can have more than 1 group.
      value for sodium.              Different groups can eat
                                     varying amounts of sodium, all
                                     exceeding the
Variables of a Controlled Experiment

   Independent Variable: the variable that
    differs between the control and
    experimental groups. It is what is purposely
   Dependent Variable: the variable that will
    be measured/observed to see if the I.V.
    made a difference.
   Controlled Variables: the variables that will
    be the same (constant) between the two
Example Question: What are the I.V.,
D.V., and C.V. of this experiment?

   Independent Variable:
       sodium level in diet
   Dependent Variable:
       blood pressure, measured 2x per
        month for 1 year
   Controlled variables:
       age, health, family health history,
        gender, # of calories eaten, exercise,
        medication taken…
Other evidence
supports many
related hypotheses

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