group_exports_g.rate <- baci_exports %>% ungroup() %>%
  group_by(group,sna,t) %>% summarise(exports = sum(exports),
                                      n= sum(n)) %>% ungroup() %>%
  group_by(group,sna,t) %>% arrange(group,sna,t) %>%
  mutate(gr.exports = 100*((exports/lag(exports,1))-1))

dput(baci_exports)

structure(list(t = c(1995, 1995, 1995, 1995, 1995, 1995, 1995,
1995, 1995, 1995, 1995, 1995, 1995, 1995, 1995, 1995, 1995, 1995,
1995, 1995), i = c(4, 4, 4, 4, 4, 4, 4, 8, 8, 8, 8, 8, 8, 8,
8, 12, 12, 12, 12, 12), sna = c("Capital goods", "Consumer goods",
"Fuels and lubricants", "Intermediate goods", "Parts and accessories of capital goods",
"Passenger motor cars", "Primary goods", "Capital goods", "Consumer goods",
"Fuels and lubricants", "Goods not elsewhere specified", "Intermediate goods",
"Parts and accessories of capital goods", "Passenger motor cars",
"Primary goods", "Capital goods", "Consumer goods", "Fuels and lubricants",
"Intermediate goods", "Parts and accessories of capital goods"
), exports = c(1195.2624224154, 22997.0533036558, 5.3693833059,
9720.564817782, 183.9809856813, 111.2556313572, 47976.2051093033,
5711.1685900189, 120659.569187797, 2264.5410811663, 75.806, 60184.2530566294,
2249.821, 280.439, 36458.4499193458, 30316.017597217, 150361.471507771,
8686043.01945958, 448718.59940394, 6633.9115459232), n = c(36L,
207L, 1L, 137L, 30L, 5L, 58L, 143L, 654L, 5L, 3L, 322L, 57L,
12L, 143L, 270L, 364L, 106L, 563L, 150L), name_baci = c("Afghanistan",
"Afghanistan", "Afghanistan", "Afghanistan", "Afghanistan", "Afghanistan",
"Afghanistan", "Albania", "Albania", "Albania", "Albania", "Albania",
"Albania", "Albania", "Albania", "Algeria", "Algeria", "Algeria",
"Algeria", "Algeria"), cntry = c("Afghanistan", "Afghanistan",
"Afghanistan", "Afghanistan", "Afghanistan", "Afghanistan", "Afghanistan",
"Albania", "Albania", "Albania", "Albania", "Albania", "Albania",
"Albania", "Albania", "Algeria", "Algeria", "Algeria", "Algeria",
"Algeria"), group = c("Afghanistan", "Afghanistan", "Afghanistan",
"Afghanistan", "Afghanistan", "Afghanistan", "Afghanistan", "SE",
"SE", "SE", "SE", "SE", "SE", "SE", "SE", "Algeria", "Algeria",
"Algeria", "Algeria", "Algeria")), row.names = c(NA, -20L), class = c("grouped_df",
"tbl_df", "tbl", "data.frame"), groups = structure(list(t = c(1995,
1995, 1995), i = c(4, 8, 12), .rows = list(1:7, 8:15, 16:20)), row.names = c(NA,
-3L), class = c("tbl_df", "tbl", "data.frame"), .drop = TRUE))
>