by_group: Peptide average or standard deviation
In tsoleary/proteomixr: Label-free quantitative proteomics & D3-Leu turnover

Peptide level average or sd of a group of samples

1	by_group(dat, col, FUN = median)

`dat`	a data frame with peptide level data as rows and samples in columns
`col`	a numeric vector containing the column numbers of samples within the group

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# selects the columns containing a string that identifies the group
group1 <- grep("Sample_norm", colnames(data))
group1_sd <- by_group(df, group1, FUN = sd)