designing an experiment

A variable is something that changes.

4 concepts of experimental design independent variable dependent variable Constants/control repeated trials © 2004 Plano ISD, Plano, TX

the variable that is changed on purpose by the experimenter independent variable the variable that is changed on purpose by the experimenter aka cause, stimulus, reason, manipulated variable, etc. This is the ‘’if” part of a hypothesis.

the variable that responds dependent variable the variable that responds aka effect(s), result(s), responding variable, etc. This is the ‘’then” part of a hypothesis.

all factors which are NOT allowed to change during the experiment constants all factors which are NOT allowed to change during the experiment

the group (standard) to which everything is compared control group the group (standard) to which everything is compared

the number of times the experiment is repeated repeated trials the number of times the experiment is repeated The more times you repeat the experiment, the more VALID your results are.

6 drops fertilizer 4 drops fertilizer 2 drops fertilizer no fertilizer

EXPERIMENTAL GROUP CONTROL 2 drops fertilizer 4 drops fertilizer What is this called? What are these called? CONTROL EXPERIMENTAL GROUP no fertilizer

What are these called? CONSTANTS

dependent variable What is this called? The plant growth that you observe here can be called . . .  the results (of adding fertilizer)  the response (to adding fertilizer) dependent variable  the effects (of adding fertilizer) 2 4 6

These are the fertilizer amounts varied by the experimenter. iv constants dv fertilizer The variable (factor) that you will change is the independent variable. 0 drops These are the fertilizer amounts varied by the experimenter. 2 drops 4 drops 6 drops

iv constants dv fertilizer plant growth 0 drops The variable (factor) resulting from the independent variable is the dependent variable. 2 drops 4 drops 6 drops

iv constants dv fertilizer amt. of water plant growth type of soil 0 drops These are the factors that must NOT change during experimentation. They must remain constant. amt. of soil 2 drops type of plant 4 drops type of planter 6 drops size of planter type of light location

Type of soil becomes the IV. What do you do if you want to test the type of soil instead of fertilizer? iv dv constants plant growth fertilizer amt. of water type of plant type of planter size of planter amt. of soil type of light location type of soil type of soil fertilizer Type of soil becomes the IV. Fertilizer becomes a constant.

In the following two slides, identify the independent variable constants dependent variable.