Part A
This is not an experiment.
Why not? Because the participants aren't divided into two groups of control group vs treatment group. This is an observational survey in which the researchers are asking each participant about the alcohol consumption level (independent variable) and how it may be tied to the risk of cancer (dependent variable).
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Part B
The population appears to focus on all women between the ages of 50 and 64.
The sample of the 1.28 million women is not the population, but rather, it ideally tries to approximate the population the best it can.
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Part C
The independent variable is the alcohol consumption rate, i.e. how much beer/wine/etc was consumed. Think of it like the variable x as the input of a function.
The dependent variable is the cancer rate. It's similar to the y output of a function. The dependent variable depends on the independent variable; hence the name. The study is trying to see if altering the amount of alcohol somehow alters the rate of cancer.
Keep in mind that cancer is still a very ongoing subject of research. Unfortunately we still have a lot to learn about it. Despite this, it's highly unlikely that alcohol is the direct cause of cancer. So instead, we consider that these variables are correlated in some way even if a link is found/proven. Other factors need to be considered such as
- fitness level
- diet, other than the alcohol consumption
- environmental factors other than the alcohol consumption (eg: does the woman live nearby an industrial plant perhaps?)
- genetics
- possibly other factors I haven't thought of to list here