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Exercise: Serum potassium and ambient temperature

In a study of the relationship between serum potassium concentration
and ambient temperature, investigators obtained mean daily temperatures
recorded in central Oxford between April 1, 1996, and Dec 31, 1997
(Ulahannan *et al.*, 1998).
They also obtained details of mean daily potassium concentrations
from six hospital sites and family physicians in Oxfordshire for the same time.
They produced the following figure:

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Questions

1. What kind of diagram is this?

Check suggested answer 1.

2. What is the sloping straight line on the diagram?

Check suggested answer 2.

3. What principle is used to calculate the line which best fits the data?

Check suggested answer 3.

4. What is meant by the symbol '*r*'?

Check suggested answer 4.

5. Why is the value of '*r*' wrong on the diagram?

Check suggested answer 5.

6. What assumption is required for the tests of significance for *r* to be valid?

Check suggested answer 6.

7. How would you describe the evidence for a relationship between
ambient temperature and serum potassium based on this graph?

Check suggested answer 7.

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Reference

Ulahannan TJ, McVittie J, Keenan J. (1998)
Ambient temperatures and potassium concentrations.
*Lancet* **352**, 1680-1.

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Last updated: 31 July, 2006.

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