To the Editor.— Dr Howard and colleagues1 report that exposure merely to ETS increases carotid intimal-medial thickening (IMT) by 20% in 3 years, but the 3-year increase (7 microns) is only the width of a red blood corpuscle. The measuring techniques actually used could not detect changes 30 times as large, and the reported results are, in my opinion, a statistical fiction.
First, measurements with any medium (light, x-ray, or sound) are not accurate beyond a half wavelength. The shortest wavelength feasible for this study is 150 µm, and more common use is 210 µm. Second, variability from one scan to the next, done a few days apart in the same patient, is commonly more than 100 µm.2 Third, measurements are made after the technician places a cursor on the apparent margins of IMT. The cursor must be set on one of the screen's pixels, which have a finite number and a fixed position regardless of the image. Even if the technician could see the exact micrometer margin of the IMT, the cursor may lie only in the nearest pixel. The pixel measures 67 µm,2 so that it always misses the true border by an average of 17 µm, and sometimes much more. Two pixels must be hand-marked for each measurement. Fourth, the Cardiovascular Health Study, in reviewing protocols for carotid ultrasound, concluded that a measured change of IMT must be at least 1400 µm to be certain of a significant difference in 1 individual. In large series, a difference of more than 200 µm could be significant.3