Science Assessment ~ Ideas ~ If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables.

Key Idea: If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables.
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Item Type A2: Select an experimental setup to test the effect of a variable on the experimental outcome, when all relevant variables are provided.

Sub-Ideas
Items & Student Performance
Misconceptions
NGSS

These items have been aligned to more than one key idea. To view the sub-ideas click on a key idea below.

If more than one variable changes at the same time in an experiment, the outcome of the experiment may not be clearly attributable to any one of the variables.
Students are expected to know that:
1. By changing a single variable at a time and holding all other relevant variables constant it is possible to determine whether that single variable is correlated with the outcome or not.
2. When varying more than one variable at a time it is not possible to determine the correlation between either variable and the outcome of an experiment.
3. The reason for controlling a particular variable (holding it constant) in an experiment is because it may have an effect on what is being tested.
4. A variable is an entity that may assume different values, either quantitative or qualitative, not a particular value of a variable. For example, when “types of liquid” is defined as a variable, water and juice are “values” of the variable and not variables themselves.
Boundaries:
1. Students are not expected to know which variables, out of all possible variables, could be related to the outcome of an experiment.
2. Students are not expected to know that it may not be possible to control or even identify all relevant variables in an experiment. These ideas are included in 1B/M2b and 1B/H3. However, when given a set of variables, students are expected to know that in order to determine if there is a relationship between a particular variable and an outcome, all other variables in the set must remain constant.
3. Students are not expected to know when they can or cannot generalize the results of an experiment beyond the given experimental and control groups.
4. Students are not expected to know the terms "independent" and "dependent" variables.
Item Type A2: Select an experimental setup to test the effect of a variable on the experimental outcome, when all relevant variables are provided.

This is an item type. There are no sub-ideas.

Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID Number	Knowledge Being Assessed	Grades 6–8	Grades 9–12
CR004003	An experiment in which the number of fish in a series of fish bowls is held constant and the temperature of the water in the fish bowls is varied could tell you the effect that water temperature has on the behavior of goldfish.	66%	83%
CR006003	An experiment in which the amount of bleach and the size of a stain on a shirt were held constant, and the type of bleach varied, could tell you which of two bleaches was better at removing stains.	67%	79%
CR002003	An experiment in which the amount of sunlight is varied and the minerals in the plant's water is held constant could tell you the effect of light on plant growth.	63%	78%
CR024005	An experiment in which variable X is allowed to vary and variables Y and Z are held constant could tell you if variable X has an effect on the result of the experiment.	44%	59%
CR001003	A carts and ramp experiment in which the weight of the carts is varied and the height of the ramp is held constant could tell you the effect of the weight of the cart on the cart's speed.	43%	56%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Grades 6–8	Grades 9–12
CVM013	When testing the effect of a variable on the outcome of the experiment, it does not matter if other relevant variables change at the same time (AAAS Project 2061, n.d.)	25%	17%
CVM010	In order to determine whether a certain variable has an effect on the outcome, one must keep this variable constant and change other related variables (Tschirgi 1980, Zimmerman & Glaser 2001). (Note: This is especially true in situations where the control experiment produced a favorable outcome.)	21%	16%
CVM009	In order to determine whether a certain variable has an effect on the outcome, one must vary ALL variables at the same time (Tschirgi 1980).	19%	12%
CVM021	When testing for the effect of a particular variable on an outcome, it is important for that variable to vary, but only one other relevant variable has to be controlled and the rest can vary.	15%	10%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.

NGSS Statements
Code	Statement
SEP3 MS	Conduct an investigation and/or evaluate and/or revise the experimental design to produce data to serve as the basis for evidence that meet the goals of the investigation.