Experimental Design: An Overview Mr. Reissfelder Lewiston High School

What is Experimental Design?  The way that an experiment is set up to ensure that all variables in all groups are controlled (held constant) EXCEPT one.

What does Experimental Design allow us to do?  Test for cause and effect relationships between two things being studied.

What does Experimental Design allow us to do?  Show what happened between the things being studied by obtaining and analyzing numbers (data) from each group in the experiment.

What does Experimental Design allow us to do? MMMMake predictions about what might happen in similar situations, then test them experimentally.

Experimental Design- Parts  An experiment is composed of VARIABLES & GROUPS:  VARIABLES refer to the way each group is treated and how they react.  GROUPS refer to the conditions subjects are kept in.

Experimental Design- Variables  Variables are the many conditions found in groups in an experiment.  There are three types:  DEPENDENT Variable  INDEPENDENT Variable  STANDARDIZED Variables

Experimental Design- Variables  DEPENDENT VARIABLE  This is what is measured in the experiment.  Technically, it is the effect of the independent variable acting on the experimental group, measured in numbers (Data).

Experimental Design- Variables  INDEPENDENT VARIABLE This is the one thing that is different between your control & experimental groups. This is the one thing that is different between your control & experimental groups.

Experimental Design- Variables  STANDARDIZED VARIABLES  These are the conditions that stay the same (held constant) in your control group & experimental group or groups.

Experimental Design- Standardized Variables  Example: In a study of different plant foods on growth of radishes, some typical standardized variables would be the…  Amount / type of soil used  Amount of light given to plants  Amount of water given to plants  Size / type of containers grown in  Temperature / humidity of room

Experimental Design- Groups TTTThere are two types of groups: AAAA. CONTROL group BBBB. EXPERIMENTAL groups SSSSometimes there is only one experimental group; sometimes there are many!

 CONTROL GROUPS usually have the normal, unchanged conditions (variables).  There are two types of control groups:  NEGATIVE CONTROL  NORMAL CONTROL

Experimental Design- Types of Controls  NEGATIVE CONTROL- This is when conditions lack the one thing that is being studied.  This allows the researcher (you!) to find out if something other than your independent variable is actually causing the effect that you’re seeing in your data!

Experimental Design- Types of Controls  NORMAL CONTROL- This is when the conditions are kept the way they normally are.  This is often used in animal studies to not harm them!

Experimental Design- Controls  Why use a normal control?  For example, you can’t study the effect of temperature on goldfish respiration rates with a negative control. That would mean the lack of temperature (freezing water!) & that’s unethical!!

Experimental Design- Groups  EXPERIMENTAL GROUPS have all the same variables as controls, but ONE variable differs from the control.  The one variable that makes the experimental group (s) different from the control group is the independent variable!

Experimental Design- Levels of treatment  The number of experimental groups you will be studying is called the levels of treatment.  It is when you use differing amounts or strengths of the Independent variable.