The most common treatment for breast cancer discovered in its earlystages was on

Question

The most common treatment for breast cancer discovered in its earlystages was once removal of the breast. It is now usual to removeonly the tumor and nearby lymph nodes, followed by radiation. Tostudy whether these treatments differ in their effectiveness, amedical team examines the records of 25 large hospitals andcompares the survival times after surgery of all women who have hadeither treatment.

(a) What are the explanatory and response variables?

(b) Explain carefully why this study is not an experiment.

(c) Explain why confounding will prevent this study fromdiscovering which treatment is more effective. (The currenttreatment was in fact recommended after several large randomizedcomparative experiments.)

Explanation / Answer

A. In a statistical model you have two kinds of variable.Response variables are the "outputs" of your model. Explanatoryvariables, on the other hand, are the "inputs" of your model.Response variables are dependent on the explanatory variables.Explanatory variable are independent of theresponse variables.

Imagine you were trying to formulate a statistical model of yourcar's fuel economy. The "output" of your model is miles per gallon(or kilometres per litre). That's your response variable. "Inputs"into your model might be (for example) engine capacity, number ofcylinders, tyre pressure, etc. These are your explanatoryvariables. That is, fuel economy may be, or is,(to be determined by the modelling) dependent on engine capacityand/or number of cylinders and/or tyre pressure, etc.

B. In an experiment, you manipulate the explanatory variable tosee the how it affects the response variable. If the researchesmanipulate what treatment the women get, they are preforming anexperiment. If they observe the results of the treatments, it is astudy.

C. Confounding occurs when it is impossible to determine whatexplanatory variable is responsible for the response variable. Inthis study, 25 different large hospitals were sampled. Could havingan operation at a certain location influence your chances ofsuccess? Maybe better doctors save more people?

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