5.4 Sample size calculation Data on the frequency of bleeding outcomes in severely thrombocytopenic neonates are available from the PlaNeT-1 survey in which 15/169 or 9% of neonates experienced a major bleed while on study.(1) However, this proportion applies to neonates of all gestational ages. Other studies show incidences in premature neonates of 7-11%, depending on the type of population studied.(3,10,11) Assuming an event rate of approximately 10%, we calculated that for testing 5 variables we will need a sample size of 500 neonates, because we will need approximately 10 events per tested variable. Each year, 4000 neonates are admitted to the neonatal intensive care unit, of which approximately 5% have severe thrombocytopenia. Therefore, we expect 200 eligible neonates each year, and a total sample size of 1000 neonates, which will allow us to include a maximum of 10 variables.