test_that("SI returns a data.frame with correct columns and lengths", {
  set.seed(1)
  data <- as.ts(rnorm(12, 100, 10))
  sl <- 4
  result <- SI(data, sl)
  expect_s3_class(result, "data.frame")
  expect_named(result, c("Season", "Seasonal_Index"))
  expect_equal(nrow(result), sl)
  expect_true(all(result$Seasonal_Index > 0))
})