I am currently trying to critique some orthopaedic research papers using the Downs and Black (1998) critiquing tool, but I am completely confused by the final question. It asks:

“Did the study have sufficient power to detect a clinically important effect where the probability value for a difference being due to chance is less than 5%?

Sample sizes have been calculated to detect a difference of x% and y%.”

-It then gives you a table with options on how to score each paper:

Size of smallest intervention group

A: <n1 (= paper scores 0)

B: n1-n2 (= paper scores 1)

C: n3-n4 (= paper scores 2)

D: n5-n6 (= paper scores 3)

E: n7-n8 (= paper scores 4)

F: n8+ (= paper scores 5)

I cannot work out what the ‘n’ values are and therefore I cannot calculate the score. I also do not know what it means by “calculated to detect a difference of x% and y%.”

Below is exactly what the 4 papers state regarding power:

- Moseley et al 2005: “It was estimated that a 150 patients (50 in each group) would provide an 80% probability of detecting differences between pairs of group means of 10 poinys on the LEFS (assuming a SD of 16 points)….a correlation of 0.6 between pre- and posttest measures, an alpha of .05…” (They achieved 150 patients at baseline, 139 at 4 weeks and 134 at 3 months)

- Wilson 1991: This is a pilot study therefore only 10 patients and no power calculation

- Lin et al 2008: “We decided on a priori sample of 90 patients. This was sufficient to provide a 90% probability of detecting a difference between group means of 10 points on the LEFS…assuming a SD of 13, a correlation of 0.6 between pre- and post-test measures, and an alpha of 0.05…” (They achieved 94 patients at baseline, 92 at 4 weeks, 92 a 12 weeks and 91 at 24 weeks).

- Nilsson et al 2009: “Sample size calculation regarding the main outcome measure OMAS showed a sample size of 51 individuals per group was needed to detect a difference of 10 points between groups with an alpha level of 0.05 and 80% power.” (They achieved 110 patients at baseline, 105 at 6 months and 96 at 12 months).

Any help would be gratefully received!!

Thanks