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 A4: Given an experiment with two variables changing at the same time, determine that no conclusion can be drawn regarding the effect of each individual variable.

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 A4: Given an experiment with two variables changing at the same time, determine that no conclusion can be drawn regarding the effect of each individual variable.

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
CR010003	In an experiment involving balls and ramps, if both the height of the ramp and the size of the ball are changed at the same time, no conclusion can be reached about the effect of either of the individual variables on how far the ball can roll.	25%	46%
CR022002	In an experiment involving skateboards, in which the size of the wheels and the material the skateboards are made of are both varied at the same time, it is not possible to find out if either of these variables affects the performance of the skateboard.	21%	38%
CR020003	In an experiment on the effect of temperature and the amount of water on the growth of plants, with both variables changing at the same time, no conclusion can be reached regarding the effect of either amount of water or termperature on the growth of the plants.	22%	34%
CR021003	In an experiment on the effect of temperature and soil type on the growth of corn plants, with both variables changing at the same time, no conclusion can be reached regarding the effect of either temperature or soil type on the growth of the plants.	18%	39%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Grades 6–8	Grades 9–12
CVM015	If two variables change at the same time, one can learn about the effect of each variable on the outcome (AAAS Project 2061, n.d.).	47%	41%
CVM023	If two variables change at the same time, one can learn about the effect of at least one of the variables on the outcome (AAAS Project 2061, n.d.).	32%	20%

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.

No NGSS statements are associated with this idea in the selected project.