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Table 1 Sample size and power calculations for reduction in hypertension under different assumptions of baseline prevalence, intervention effect and sample size (per arm per survey)

From: Evaluation of a community-based hypertension improvement program (ComHIP) in Ghana: data from a baseline survey

Baseline prevalence

30%

30%

30%

35%

35%

35%

Improvement in control (A)

−3.0%

−3.0%

−3.0%

−3.5%

−3.5%

−3.5%

Improvement in intervention (B)

−9.0%

−10.5%

−12.0%

−10.5%

−12.3%

−14%

Intervention effect size (B – A):

-6%

−8%

−9%

−7%

−9%

−11%

Power if sample size = 800

51%

71%

86%

61%

81%

93%

Power if sample size = 900

56%

76%

90%

66%

85%

95%

Power if sample size = 1000

60%

80%

92%

71%

88%

97%

Power if sample size = 1100

64%

84%

94%

75%

91%

98%

Power if sample size = 1200

68%

87%

96%

78%

93%

99%

Using type I error in the analysis: alpha=

0.05

0.05

0.05

0.05

0.05

0.05

Expected baseline prevalence in both groups: A=

30%

30%

30%

35%

35%

35%

Expected improvement in control group: B=

−3.0%

−3.0%

−3.0%

−3.5%

−3.5%

−3.5%

Expected improvement in intervention group: C=

−9.0%

−10.5%

−12.0%

−10.5%

−12.3%

−14.0%

Therefore the effect of Intervention is: C-B =

-6%

−8%

−9%

−7%

−9%

−11%

Sample per group per period

1000

1.2

833

53%

72%

87%

63%

82%

94%

Sample per group per period

1100

1.2

917

57%

76%

90%

67%

86%

96%

Sample per group per period

1200

1.2

1000

60%

80%

92%

71%

88%

97%

Sample per group per period

1000

1.4

714

47%

66%

82%

56%

76%

90%

Sample per group per period

1100

1.4

786

50%

70%

85%

60%

80%

92%

Sample per group per period

1200

1.4

857

54%

74%

88%

64%

83%

94%

 

(A)

(B)

(C)

      
  1. (A) = Number sampled; (B) = Desgin effect; (C) = Efective sample size