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Table 2 Estimated sample sizes per group, based on different outcome measures. For function and symptoms, standard deviations are from the Maine Lumbar Spine study, with 3-month follow-up (roughly equal proportions treated surgically and non-surgically). Return to work proportion is based on a study by the Washington State Department of Labor and Industries.

From: A prospective cohort study of surgical treatment for back pain with degenerated discs; study protocol

Category Measure Hypothesized difference between groups N/group, 2-sided alpha = 0.05, power = 0.80
Functional Status Difference in change in Roland score (0–23 scale, SD = 7.37) 2.5 points 136
   3.0 points 95
Symptoms Improvement in pain (1 = gone, 5 = same, 7 = much worse) (SD = 1.841) 0.75 point 94
   1.0 point 53
Return to Work Proportion currently employed, if lower proportion is 0.40 0.15 173
   0.20 97