Another question; in the wiki, they write the following about the independent variable
Independent Variable (IV)
This is the variable that is changed to examine its effect on the dependent variable. There should only be one IV, which should be listed with units. The IV must be operationally defined (in terms of the experiment) and empirically defined (in general for future variations of the experiment), and a minimum of 3 different levels must be listed excluding the control level.
what are the "3 different levels" that are required to be defined?
I think the writer of that section means that you need at least three manipulations excluding the control. So if your independent variable is length of string, you might test 10cm, 20cm, 30cm, and 40cm. Good experiments tend to have many manipulations and a wide range of manipulations tested. So an experiment that tests 10cm, 20cm, 30cm, and 40cm, might be better than one that tests 10cm, 11cm, 12cm, and 13cm.
This range of manipulations and the number of manipulations affects how well you can draw conclusions from your data. If you have only three values of your independent variable tested (say 10cm, 20cm, 30cm), you might not be able to tell if your graphed data is linear or quadratic or exponential, etc. Perhaps it should be a linear relationship but you have a lot of error in your measurements and it appears exponential.
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