library(dplyr)
data(efc)
std(efc$c160age) %>% head()
std(efc, e17age, c160age) %>% head()
std(efc, e17age, c160age, append = TRUE) %>% head()
center(efc$c160age) %>% head()
center(efc, e17age, c160age) %>% head()
center(efc, e17age, c160age, append = TRUE) %>% head()
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