Experimental Design
Controlled Experiments
Observations
Scientific research begins with making observations. Observations give rise to questions. Some questions can be answered simply
through observation. Ex: what do crayfish eat?
Other questions can be answered only through experimentation. Do crayfish prefer to eat animal matter or plant matter?
Hypotheses
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: Either,
Does not get the treatment, or
Is followed under “normal” conditions.
Ex: Eats a diet that meets the daily recommended value for sodium.
Experimental Group: Either,
Gets the treatment, or
Is followed under altered conditions.
Ex: Eats a diet that exceeds the recommended value for sodium.
Can have more than 1 group. Different groups can eat varying amounts of sodium, all exceeding the recommendation.
Variables of a Controlled Experiment
Independent Variable: the variable that differs between the control and experimental groups. It is what is purposely changed.
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 groups.
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