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.
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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:
 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.
 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.
 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.
 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:
 Students are not expected to know which variables, out of all possible variables, could be related to the outcome of an experiment.
 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.
 Students are not expected to know when they can or cannot generalize the results of an experiment beyond the given experimental and control groups.
 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 subideas.
Item ID Number 
Knowledge Being Assessed  Grades 6–8 
Grades 9–12 
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Misconception 
Student Misconception 
Grades 6–8 
Grades 9–12 

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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.