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 A1: Given an idea to be tested (hypothesis) and an experimental setup, explain why certain variables are (or should be) kept constant.

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 A1: Given an idea to be tested (hypothesis) and an experimental setup, explain why certain variables are (or should be) kept constant.

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
CR015005	To find out which car gets better gas mileage, it is important that both cars get the same grade of gas because if they didn't, it wouldn't be possible to tell which car gets the better gas mileage.	53%	66%
CR014003	To find out if type of soil affects the growth of carrot plants, it is important that all of the plants get the same amount of water because if they didn't, it wouldn't be possible to tell which soil type was better.	51%	69%
CR005004	To find out the effect of paper type on how well paper airplanes fly, it is important that all the airplanes tested use the same design because if they didn't, it wouldn't be possible to tell what effect the type of paper had on how well the paper airplanes fly.	53%	66%
CR013003	To find out if the type of swimsuit swimmers wear affects a swim team's performance, it is important that everyone wear the same kind of swim cap because otherwise it wouldn't be possible to find out the effect of the swimsuits on the team's performance.	49%	65%
CR012003	To find out if the shape of objects has an effect on how fast the objects sink, it is important that all the objects tested have the same mass because if they didn't have the same mass, it wouldn't be possible to learn about the effect of shape on how fast they sink.	45%	62%
CR016003	To determine the effect of water temperature on getting stains out of shirts, it is important for all shirts to be washed for the same length of time because if they weren't, it wouldn't be possible to find out the effect of temperature.	44%	54%
CR015006	To determine the effect of speed on a truck's gas mileage, is important that trucks of the same weight be compared because if the trucks didn't have the same weight, it wouldn't be possible to learn about the effect of speed on gas mileage.	41%	54%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Grades 6–8	Grades 9–12
CVM026	A given experiment can test for the effects of everything that is included in the study, whether these variables are allowed to vary or are held constant.	26%	19%
CVM011	A given experiment tests for the effects of ALL related variables, regardless of whether they are allowed to vary or are held constant (AAAS Project 2061, n.d.).	26%	19%
CVM018	A given experiment tests for the effect of a variable that remains constant while other variables change (AAAS Project 2061, n.d.).	18%	13%
CVM027	It is not important to hold a variable constant in an experimental study if the purpose of the study is not to find out the effect of that variable on the outcome of the study.	8%	6%
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.)	8%	6%

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.