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