В тексте в разных местах написано по-разному, хотя смысл вроде бы один:

nBS - Number of bootstrap pseudo-data sets
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Bootstrapping involves the evaluation of a given estimator for a large number (typically 1,000 or more) of ‘‘pseudo-datasets’’ constructed by random sampling with replacement from the observed data.
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While there are many resampling schemes, in general the variability of the expected value of a bootstrapped statistic is proportional to the square-root of the number of pseudo-datasets generated.
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For this study, the same nBS = 1,000 pseudo-data sets were used for both the BSmean and BSmedian.

Вроде бы по логике "набор псевдо-данных", но pseudo-dataset смущает.