That which the experimenter changes to see how it affects other factors is called the
Only twenty percent of your mail surveys are returned. Why might conclusions drawn from these surveys be flawed?
A low response rate usually indicates that your questions are not objective.
The people who returned the survey probably do not represent the general population.
A low response rate usually means your questions are outdated.
all of the above
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For the first one, maybe this will help:
"The independent variable is typically the variable representing the value being manipulated or changed and the dependent variable is the observed result of the independent variable being manipulated. "
I'm not sure about the second...I'd say the real answer is that 80% of the people are tired of taking surveys :)