A function that calculates the first difference in a panel data setting. If the data.frame that is passed in has nxT rows, the resulting vector will also have nxT elements with one element for each unit set to be NA.
get_first_difference(df, idname, yname, tname)the data.frame used in the function
name of column that holds the unit id
name of column containing the outcome (or other variable) for which to calculate its outcome in the immediate pre-treatment period
name of column that holds the time period