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Table 4 Fitted mean of sample size and 95% confidence interval (CI), type III F statistic and the corresponding p value of the effect of the covariate on sample size adjusted by class of prevalence, phase of trial and interaction between class and phase for parallel 2-arm trials only

From: Does the low prevalence affect the sample size of interventional clinical trials of rare diseases? An analysis of data from the aggregate analysis of clinicaltrials.gov

Characteristics

No. of trials, m

Fitted mean

95% CI

F test

p value

Inclusion criteria

 Gender

354

  

4.66

0.0101

 Both

 

64.21

(51.30–80.36)

  

 Female

 

105.11

(70.26–157.26)

  

 Male

 

38.08

(15.93–91.00)

  

Age categories a

354

  

2.32

0.043

 Children only

 

44.44

(29.90–66.05)

  

 Adults only

 

75.84

(53.16–108.21)

  

 Elderly only

 

100.50

(56.18–179.79)

  

 Children and adults

 

73.17

(48.83–109.64)

  

 Children to elderly

 

56.93

(42.19–76.84)

  

 Adults to elderly

 

70.83

(55.34–90.66)

  

Study designs

 Has DMC? b

321

  

8.39

0.004

 No

 

53.90

(41.26–70.41)

  

 Yes

 

72.29

(57.83–90.38)

  

FDA regulated intervention? c

347

  

2.31

0.1293

 No

 

71.04

(55.51–90.92)

  

 Yes

 

61.37

(48.04–78.39)

  

Interventions, facilities and authorities

     

 Lead sponsor

354

  

1.18

0.3179

 US Federal

 

15.15

(2.72–84.37)

  

 Industry

 

62.54

(48.26–81.04)

  

 NIH

 

69.66

(28.97–167.50

  

 Other

 

67.80

(53.39–86.09)

  

Trial location

354

  

4.53

0.0039

 US only

 

55.61

(43.34–71.36)

  

 EU only

 

100.14

(69.12–145.08)

  

 US and EU

 

76.58

(51.07–114.83)

  

 Single European country

 

69.98

(54.48–89.90)

  

No. of countries involved in the trial

354

1.07 d

(1.01–1.13)

5.86

0.016

Year that enrolment to the protocol begins

354

  

0.83

0.5084

 1990–1999

 

62.53

(28.01–139.61)

  

 2000–2004

 

89.96

(59.68–135.61)

  

 2005–2009

 

64.28

(50.05–82.56)

  

 2010–2014

 

65.55

(51.14–84.04)

  

 2015 and after

 

64.43

(39.24–105.79)

  

Types of arms

351

  

18.27

<.0001

 Experimental vs. Placebo

 

52.82

(41.54–67.17)

  

 Experimental vs. Standard

 

106.78

(83.63–136.33)

  

 Experimental vs. Experimental

 

53.44

(39.86–71.63)

  

 Non-experimental vs. Non-experimental

 

59.78

(45.57–78.43)

  
  1. a Minimum and maximum age groups are mutually exclusive. Children, term new born infants to adolescents up to 18 years; adults, 18–64 years old; and elderly, 65 years or older
  2. b Indicate whether or not a data monitoring committee (DMC) has been appointed for this study
  3. c Indicate whether or not the trial includes an intervention subject to US Food and Drug Administration regulation under section 351 of the Public Health Service Act or any of the following sections of the Federal Food, Drug and Cosmetic Act: 505, 510(k), 515, 520(m), and 522
  4. d Estimated coefficient, the estimated increase of sample size for every unit increase of the covariate