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Table 2 Occupational exposure data and number of cases and controls in each exposure group.

From: Analysis of polymorphisms in the circadian-related genes and breast cancer risk in Norwegian nurses working night shifts

Maximum number of consecutive night shifts in all jobs that nurse has workeda No. of casesb No. of controlsb
Only day work or one or two consecutive night shifts (reference) 170 (227) 186 (275)
Three consecutive night shifts 156 (181) 189 (272)
Four or more consecutive night shifts 239 (291) 244 (348)
Total 563 (699) 619 (895)
Intensity of night work: Maximum number of consecutive night shifts the nurse has ever worked for 5 or more years c   
Only day work or one or two consecutive nights for 5 or more years (reference) 326 (421) 362 (524)
Three consecutive night shifts for 5 or more years 125 (147) 154 (227)
Four or more consecutive night shifts for 5 or more years 112 (131) 103 (144)
Total 563 (699) 619 (895)
  1. aMaximum number of consecutive night shifts in all jobs: 0 to 2, 3 or ≥4. bNumber of nurses who had consented to send saliva for genetic analysis (numbers in parentheses show the total number of cases and controls interviewed in each exposure group). cMaximum number of consecutive night shifts the nurse has ever worked for 5 or more years.