Introduction to The Scientific Method
Biology 155 Fall 2009 B. L. Krilowicz
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)
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
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.
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
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?
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
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…?
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) = ?
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.
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.
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.
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...?
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) = ?
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
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 today’s example.) The original hypothesis is then modified and tested until a hypothesis is developed that cannot be proven incorrect.
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