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Table 2 Design of the flexible two-phase study for Example 1

From: Flexible Two-Phase studies for rare exposures: Feasibility, planning and efficiency issues of a new variant

Disease Status

(D)

Metal-workers

Z (Ï„ i j )

Fixed number of subjects to be included in Phase Two

(n ij )

Expected Phase One numbers of subjects to be screened

(N ij )

Expected Proportion of MWF exposure within strata §

X(Ï€ i jk )

Expected distribution of subjects by MWF in Phase Two

   

N0 = Max(160/20%, 40/80%) = 800

  

Control

No (80%*)

40

800*80% = 640

No (97.5%*)

40*97.5% = 39

    

Yes (2.5%*)

40*2.5% = 1

 

Yes (20%*)

160

800*20% = 160

No (75%*)

160*75% = 120

    

Yes (25%*)

160*25% = 40

   

N1 = Max(85/23.4%, 20/76.6%) = 364

  

Case

No (76.6%#)

20

364*76.6% = 278.8

No (95.1%#)

20*95.1% = 19.02

    

Yes (4.9%#)

20*4.9% = 0.98

 

Yes (23.4%#)

85

364*23.4% = 85

No (60%#)

85*60% = 51

    

Yes (40%#)

85*40% = 34

  1. * Values of parameters fixed in Table 1
  2. # Values of parameters computed from parameter values fixed in Table 1 (see Appendix 2)
  3. § In Phase One controls, the overall expected percentage of MWF exposure is equal to 7%, that is, 2% = 2.5% of 80% non-metal-workers plus 5% = 25% of 20% metal-workers. Similar computations lead to 13% MWF exposure in cases.