As Dr Epstein pointed out in this letter, 24-hour blood pressure readings of one person are not independent of each other. Consequently, the t test with sample sizes of 234 and 259, which we used in our recent study (J Occup Environ Med. 1996;38:1007-1011), might be rough.
If we use a conservative t test with sample sizes of 10 and 11 (Ao and Ac groups) and adopt the means of 24-hour blood pressure readings of each person as his only reading, we will get P = 0.04 in the comparison of SBP and P = 0.08 in that of DBP. Between Bo and Bc groups, we will not get any significant difference. Between busy and control periods of the C group, we will get P < 0.01 in both the comparisons of SBP and DBP, and P < 0.05 in the comparison of heart rate. Therefore, we can still suggest that overtime work increases blood pressure in white-collar workers in Japan.
Besides devising statistical methods more appropriate for the complicated comparisons such as 24-hour blood pressure readings, we plan to increase the number of workers involved in our studies.
Takeshi Hayashi, MD; Yasuki Kobayashi, MD; Kazue Yamaoka, PhD; Eiji Yano, MD
Hitachi Health Care Center; Ibaraki, Japan; Institute of Community Medicine; University of Tsukuba; Ibaraki, Japan; Department of Hygiene and Public Health; Teikyo University School of Medicine; Tokyo, Japan