Introduction to The Scientific Method

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Introduction to The Scientific Method

• Biology 155
• Fall 2009
• B. L. Krilowicz

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What is the Scientific Method?

• A system of investigation that consists of three
  major steps
• Step 1 gathering facts or making an
  observation
• Step 2 developing a hypothesis (explanation)
  to explain your observation
• Step 3 testing your hypothesis
• (Discovery versus hypothesis based science)

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Step 1 for Primrose Example

• Observation Plants on the northern side of your
  house are large and blooming profusely, while
  those on the southern side of the house are
  small, with few blooms

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Step 2 for Primrose Example

• Note the following
• Soil is dry on the southern side of the house,
  but moist on the northern side of the house,
  thus,
• Hypothesis The primroses on the southern side
  of the house show poor growth because the soil
  there is too dry.

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Step 3 Testing Your Hypothesis Requires Four
Sub-steps

• Designing an experiment
• Making a prediction about the outcome of the
  experiment (based on the assumption that your
  hypothesis is correct)
• Conducting the experiment and collecting data
  (results)
• Drawing a conclusion interpreting the results
  of the experiment

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What is an Experiment?

• A scientific investigation carried out under
  controlled conditions.
• Controlled conditions refers to the attempt to
  regulate all variables that could influence the
  outcome of the experiment except the variable
  under investigation
• Examples of important variables for the primrose
  example?

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Experimental Design for the Primrose Example

• Give the plants on the southern side of the house
  enough water to keep the soil as moist as it is
  on the northern side of the house

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Prediction for the Primrose Example

• Predictions are If, then statements and look
  like -
• If my hypothesis is correct, then the
  following should happen during my experiment.
• If the plants on the southern side of the house
  show poor growth because the soil there is too
  dry, then?

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Conducting the Experiment for the Primrose Example

• Add additional water to the plants on the
  southern side of the house so that the soil
  moisture is /- equal to that on the northern
  side of the house.
• Conduct experiment for (how long?)
• Result (data) ?

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Conclusion for the Primrose Example

• Differences in soil moisture are NOT responsible
  for the differences in plant growth observed
  between those primroses planted on the northern
  versus southern sides of the house.

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What Do We Do Now?

• Modify our original hypothesis based on the
  outcome of our first experiment (return to step 2
  and repeat process)
• Modified Hypothesis The primroses on the
  southern side of the house show poor growth
  because they receive too much sunlight.

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Step 3 for Modified Hypothesis

• Experimental Design Use shade cloth to reduce
  the amount of sunlight received by the primroses
  on the southern side of the house to an amount
  equal to that received by plants on the northern
  side of the house.

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Step 3 for Modified Hypothesis (continued)

• Prediction If the primroses on the southern
  side of the house show poor growth because they
  receive too much sunlight, then...?

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Step 3 for Modified Hypothesis (continued)

• Conducting the Experiment
• Shade plants of the southern side of the house
  such that they receive /- the same amount of
  sunlight as those on the northern side of the
  house
• Conduct experiment for (how long?)
• Results (data) ?

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Step 3 for Modified Hypothesis (continued)

• Conclusion Differences in sunlight experienced
  by plants on the northern versus southern side of
  the house led to the original difference in
  growth noted between the two groups of plants

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A Note of Caution

• The scientific method never allows us to
  conclusively prove an hypothesis.
• Instead, the scientific method is designed to
  disprove an hypothesis. (For example, our
  original hypothesis in todays example.) The
  original hypothesis is then modified and tested
  until a hypothesis is developed that cannot be
  proven incorrect.

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What is a Scientific Theory?

• An hypothesis that has been tested in many
  different ways (i.e. with different experimental
  designs), many times and by many different
  scientists, but can not be proven incorrect
• So much evidence has been assembled in favor of
  an hypothesis that the scientific community is
  reasonably certain (but never absolutely sure)
  that the hypothesis is correct
• The scientific community is still open to
  modification of the theory, if appropriate
  evidence is produced