summarySE2

summarySE2 <- function (data = NULL, measurevar, groupvars = NULL, na.rm = TRUE, conf.interval = 0.95) {
    library(data.table)
    data <- data.table(data)
    
    length2 <- function(x, na.rm = FALSE) {
        if (na.rm) 
            sum(!is.na(x))
        else length(x)
    }
    
    datac <- data[, .(lapply(.SD, length2, na.rm = na.rm), 
                      lapply(.SD, mean, na.rm = na.rm),
                      lapply(.SD, sd, na.rm = na.rm)),
                  by = groupvars, .SDcols = measurevar]
    names(datac) <- c(groupvars, "N", measurevar, "sd")
    setkeyv(datac, groupvars)
    
    datac[, se := unlist(sd) / sqrt(unlist(N))] #compute standard error
    
    ciMult <- qt(conf.interval / 2 + 0.5, unlist(datac$N) - 1)
    datac[, ci := se * ciMult]
    datac <- data.frame(datac)
    return(datac)
}