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Table 6 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 single-assignment 1-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

527

  

2.58

0.0769

 Both

 

33.38

(25.76–43.24)

  

 Female

 

24.84

(15.86–38.90)

  

 Male

 

20.84

(11.72–37.07)

  

Age categoriesa

527

  

1.18

0.3184

 Children only

 

27.82

(18.67–41.46)

41.46

 

 Adults only

 

35.18

(23.20–53.36)

53.36

 

 Elderly only

 

56.05

(31.69–99.15)

99.15

 

 Children and adults

 

30.30

(19.48–47.12)

47.12

 

 Children to elderly

 

33.60

(24.87–45.40)

45.4

 

 Adults to elderly

 

32.37

(24.50–42.78)

42.78

 

Study designs

     

Has DMC?b

450

  

0.01

0.9328

 No

 

31.87

(24.29–41.82)

  

 Yes

 

32.11

(24.24–42.54)

  

FDA regulated intervention?c

514

  

1.94

0.1647

 No

 

35.79

(27.16–47.16)

  

 Yes

 

31.87

(24.33–41.75)

  

Interventions, facilities and authorities

Lead sponsor

527

  

4.49

0.004

 US Federal

 

125.86

(49.53–319.80)

  

 Industry

 

31.99

(24.19–42.31)

  

 NIH

 

55.42

(31.66–97.02)

  

 Other

 

31.85

(24.44–41.50)

  

Trial location

527

  

5.14

0.0017

 US only

 

27.35

(20.71–36.11)

  

 EU only

 

50.43

(35.66–71.32)

  

 US and EU

 

34.32

(22.45–52.48)

  

 Single European country

 

27.81

(20.89–37.03)

  

No. of countries involved in the trial

527

1.12d

(1.07–1.18)

19.58

<.0001

Year that enrolment to the protocol begins

523

  

4.90

0.0002

  < 1990

 

313.70

(53.14–1852.05)

  

 1990–1999

 

26.44

(13.53–51.68)

  

 2000–2004

 

50.47

(36.25–70.28)

  

 2005–2009

 

29.75

(22.55–39.25)

  

 2010–2014

 

31.15

(23.99–40.46)

  

 2015 and after

 

35.79

(21.68–59.08)

  
  1. aMinimum 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. bIndicate whether or not a data monitoring committee (DMC) has been appointed for this study
  3. cIndicate 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. dEstimated coefficient, the estimated increase of sample size for every unit increase of the covariate