The problem statement poses two research
questions: “On average do creatinine levels exceed the upper
endpoint (2 mg/dL) of the Stage 1 kidney insufficiency range?”
and “What proportion of patients have creatinine levels greater
than 2 mg/dL?” The three questions below will guide the selection
of appropriate statistical methods.
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What is the response
(Y) of interest and how is it measured? For the first research question,
the creatinine level is of interest. It is a continuous variable.
For the second research question, the nominal variable indicating
creatinine level greater than two should be analyzed.
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Are predictor variables
mentioned in the problem statement? If so, how many and what are
their measurement levels? There are no predictor variables mentioned
in the research questions.
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What are you being asked
to deliver? A data description, an interval estimate, an answer to
a question, or a predictive model? For the first research question,
we need a yes or no answer to a question. A test of hypothesis is
an appropriate method. For the second research question, a numerical
estimate of a proportion is needed. The point estimate of the proportion
of patients with creatinine level greater than 2 can be obtained from
the sample data. A confidence interval for a proportion will provide
a range of plausible values for the true proportion of patients with
creatinine levels greater than 2.