1. The table below displays data taken from a study comparing self-reported smoking status with measured serum cotinine level. As part of the study, cotinine level was used as a diagnostic tool for predicting smoking status; the self-reported status was considered to be true. For a number of different cutoff points, the observed sensitivities and specificities are given below.

Cotinine Level (ng/ml)	Sensitivity	Specificity
5	0.971	0.898
7	0.964	0.931
9	0.960	0.946
11	0.954	0.951
13	0.950	0.954
14	0.949	0.956
15	0.945	0.960
17	0.939	0.963
19	0.932	0.965

1. As the cutoff point is raised, how does the probability of a false positive result change? How does the probability of a false negative result change?
2. Use these data to construct a receiver operator characteristic curve.
3. Based on this graph, what value of serum cotinine level would you choose as an optimal cutoff point for predicting smoking status? Why?