Experimental Design

Controlled Experiments

Biologists use controlled experiments to answer many questions

Controlled Experiment: An experiment in which two or more groups are set up for comparison; only one variable is different between the groups. Example:

Question:“Does eating high-sodium foods cause high blood pressure”?

What two groups would be compared in an experiment to answer this question?

Sodium Experiment

One group eats the “normal” amount of sodium. (Diet meets the daily recommended value).

One group eats a high-sodium diet (exceeds the recommended value.)

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.

Hypotheses

Controlled experiments are designed to answer a question, but more specifically, are designed to determine the validity of a hypothesis.

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.

Variables of a Controlled Experiment

Independent Variable: the variable that differs between the control and experimental groups.

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 the 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…