Inflammatory biological indicators are clinically valuable in a variety of ways, including determining disease activity in disorders such as systemic lupus erythematosus (SLE). In individuals with active SLE flares, a novel inflammatory marker is being studied. When the new marker's blood level (in mg/L) is plotted against the C-reactive protein (CRP) level (also in mg/L), the following plot results: Based on the plot, the correlation coefficient between the 2 variables is closest to which of the following values?

Question

Inflammatory biological indicators are clinically valuable in a variety of ways, including determining disease activity in disorders such as systemic lupus erythematosus (SLE). In individuals with active SLE flares, a novel inflammatory marker is being studied. When the new marker's blood level (in mg/L) is plotted against the C-reactive protein (CRP) level (also in mg/L), the following plot results:  Based on the plot, the correlation coefficient between the 2 variables is closest to which of the following values?

Quizplus · Accepted Answer

Eductional Objective: Scatter plots are useful for crude data analysis. If a linear association is present between 2 variables, a correlation coefficient (r) mathematically describes how well a "line of best fit" (blue line in Figure) would correspond to the data points plotted. The value of r ranges from -1 to +1 and describes 2 important characteristics of an association: the strength and the polarity. The closer the r value is to its margins (-1, 1), the stronger the association.
An increase in C-reactive protein (CRP) level is associated with a decrease in the new marker's level; therefore, r <0. Although the scatter plot does not demonstrate a perfect linear arrangement, it does show a reasonably strong linear association as there is minimal variation along the line of best fit (points are relatively close to the line). Therefore, the most appropriate answer among the options given is r = -0.8.

Inflammatory Biological Indicators Are Clinically Valuable in a Variety of Ways