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:

A graph with 'Potassium (mmol/L)' on the vertical axis and 'Temperature (deg C)' on the horizontal axis.  Points are marked showing potassium and temperature for about 100 different days.  High temperature days tend to have low potassium, though the relationship is not close, so there is a cloud of points.  There is a downwards sloping line through the middle of the cloud of points.  On the graph are written 't=2.51', 'p<0.02', 'r=0.3'.


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.


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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