Fig. 3. Correlation between electroencephalographic parameters and remifentanil concentration within 60 min after the beginning of remifentanil infusion: approximate entropy (ApEn; A ), 95% spectral edge frequency (SEF95; B ), and canonical univariate parameter (CUP; C ). The overall number of paired concentration and electroencephalographic data were 1,024, and the mean (± SD) number per individual was 37 ± 5. Locally weighted scatterplot smoothing curve of each electroencephalographic parameter was shown, in which 4,168 segments were calculated with the remifentanil concentration ranging from 0 to 171.3 ng/ml, and the number of points in smoothing window was 10 (Prism 4 for Windows version 4.03; Graphpad Software Inc., San Diego, CA). Time delay between remifentanil concentration and electroencephalographic parameters was less than 10 s. The 99% confidence interval of R was −0.6919 to −0.5960 for ApEn, −0.7056 to −0.6131 for SEF95, and 0.5786 to 0.6777 for CUP, respectively.