This Science quiz is called 'Key/Independent Variable' and it has been written by teachers to help you if you are studying the subject at middle school. Playing educational quizzes is a fabulous way to learn if you are in the 6th, 7th or 8th grade - aged 11 to 14.
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The key independent variable is the one variable that you change in your investigation or experiment and it is the basis of your hypothesis. All other variables in your investigation should be kept the same, after all, if you changed two variables, how would you know which one has had the effect you observed? The dependent variable is the variable you hope will be changed as you alter the values of the key independent variable. It is the one that you will be measuring.
When you write out your hypothesis, most people will naturally place the key independent variable first. For example, "I think that as the light intensity is increased, the seedlings will grow taller because ...". It could have also been written as I think that the seedlings will grow taller as the light intensity increases because ..." so it doesn't happen all the time. When deciding on the key independent variable, the most reliable way of working out what it is, is by checking your hypothesis for the variable that you think will cause something else to change in a predictable way. In our example, you can see that the light intensity is the key independent variable because it is the one that you think will cause the seedlings to grow taller.
As with all variables, the data can be continuous or discrete. Continuous data can have any value and so you must decide on the range and number of readings that you will take. You must consider how many readings you think are practical, for instance, if you have temperature as the key independent variable, in a school laboratory, -5oC would be the lowest practical temperature you could achieve (using a mixture of water, ice and salt) and would you really need to take readings at 1oC intervals? Of course not! Every 5 or 10 degrees would give you sufficient results from which to draw a conclusion. You also need to take into account safety factors, so using concentrated hydrochloric acid would not be possible as it is not allowed to be used in middle school science experiments because it is dangerously corrosive. You are more limited in your choices if your key independent variable has discrete values, but you will probably still need to make decisions based on practicality and safety.