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Table 1 Different sample strategies and characteristics to assess outcomes associated with Zika infection during pregnancy in analytical cohort studies

From: Outcomes of Zika virus infection during pregnancy: contributions to the debate on the efficiency of cohort studies

Characteristics

Sample of study population

Sample of unexposed pregnant women, and census of exposed population (from study population).

Example 1

Example 2

Example 3

Study population size of eligible pregnant women (Study population)

20.000

20.000

20.000

Sample size

2.700

800 (ratio 1:3)

1.200 (ratio 1:5)

Exposed (sample)

27 (1%)a

  

Unexposed (sample)

2673

  

Exposed (from population identified by surveillance systems)

200 (1%)a exposed

200 (1%)a exposed

Unexposed (from population identified by surveillance systems)

600–6 exposed (1%) = 594 unexposed

1.000–10 exposed (1%) = 990 unexposed

Ratio exposed:unexposed

1:99 (as the study population)

1:3 (defined by investigator)

1:5 (defined by investigator)

Expected cases among exposed (rounded numbers)

1 (5%)a

10 (5%)a

10 (5%)a

Expected cases among unexposed (round numbers)

7 (0.25%)a

1 (0.25%)a

2 (0.25%)a

Total expected cases (round numbers)

8

11

12

Type 1 error

0.05

0.05

0.05

Study Powerb

59%

95%

97%

  1. aParameters: 1% of pregnant women will be infected by the Zika virus; 5 and 0.25% of infected and non-infected pregnant women will present the outcome, respectively
  2. bStataCorp. Stata Statistical Software: Release 10. College Station, TX: StataCorp LP. 2007 [4]