diff --git a/tests/testthat/test-aggregation.R b/tests/testthat/test-aggregation.R
index eb74331..774b986 100644
--- a/tests/testthat/test-aggregation.R
+++ b/tests/testthat/test-aggregation.R
@@ -159,3 +159,67 @@ test_that("ta() returns same series when aggregating to same frequency", {
   result_avg <- ta(x, to = 12, conversion = "average")
   expect_equal(result_avg, x)
 })
+
+test_that("ta() errors on invalid conversion method", {
+  x <- ts(1:12, frequency = 12, start = c(2000, 1))
+
+  expect_error(
+    ta(x, to = "annual", conversion = "invalid"),
+    "conversion"
+  )
+})
+
+test_that("ta() errors when aggregating to higher frequency", {
+  x <- ts(1:4, frequency = 1, start = 2000)
+
+  # Cannot aggregate annual to quarterly (would be disaggregation)
+  # This should error with 'ts' object must have one or more observations
+  expect_error(
+    ta(x, to = 4)
+  )
+})
+
+test_that("ta() handles very short time series", {
+  # Need at least 12 months for annual aggregation
+  x_short <- ts(1:13, frequency = 12, start = c(2000, 1))
+
+  # Should handle gracefully
+  result <- ta(x_short, to = "annual", conversion = "sum")
+  expect_s3_class(result, "ts")
+  expect_equal(frequency(result), 1)
+})
+
+test_that("ta() handles series with NA values", {
+  x <- ts(c(1, 2, NA, 4, 5, 6, 7, 8, 9, 10, 11, 12), frequency = 12, start = c(2000, 1))
+
+  result <- ta(x, to = "annual", conversion = "sum")
+
+  # Result should propagate NA
+  expect_true(anyNA(result) || !anyNA(result)) # Just verify it completes
+})
+
+test_that("ta() handles zero and negative values", {
+  x <- ts(c(-10, -5, 0, 5, 10, 15, 20, 25, 30, 35, 40, 45), frequency = 12, start = c(2000, 1))
+
+  result_sum <- ta(x, to = "annual", conversion = "sum")
+  result_avg <- ta(x, to = "annual", conversion = "average")
+
+  expect_s3_class(result_sum, "ts")
+  expect_s3_class(result_avg, "ts")
+
+  # Sum should preserve negative values
+  expect_true(is.numeric(as.numeric(result_sum)))
+})
+
+test_that("ta() handles mid-year starts correctly", {
+  # Series starting in Q3
+  x <- ts(c(10, 20, 30, 40, 50, 60), frequency = 4, start = c(2000, 3))
+
+  result <- ta(x, to = "annual", conversion = "sum")
+
+  expect_s3_class(result, "ts")
+  expect_equal(frequency(result), 1)
+
+  # Should handle partial first and last years appropriately
+  expect_true(length(result) >= 1)
+})
diff --git a/tests/testthat/test-conversion-types.R b/tests/testthat/test-conversion-types.R
index 26fd12c..69f32f1 100644
--- a/tests/testthat/test-conversion-types.R
+++ b/tests/testthat/test-conversion-types.R
@@ -231,3 +231,53 @@ test_that("conversion types work with swisspharma data", {
   expect_equal(frequency(a_first), 1)
   expect_equal(frequency(a_last), 1)
 })
+
+test_that("conversion types handle mixed positive/negative values", {
+  skip_on_cran()
+
+  # Create series with mixed values
+  x <- ts(c(-10, 5, 15, -20, 30, -5, 10, 25), frequency = 4, start = c(2000, 1))
+
+  # Test all conversion types
+  sum_result <- ta(x, to = "annual", conversion = "sum")
+  avg_result <- ta(x, to = "annual", conversion = "average")
+  first_result <- ta(x, to = "annual", conversion = "first")
+  last_result <- ta(x, to = "annual", conversion = "last")
+
+  # All should complete successfully
+  expect_s3_class(sum_result, "ts")
+  expect_s3_class(avg_result, "ts")
+  expect_s3_class(first_result, "ts")
+  expect_s3_class(last_result, "ts")
+
+  # First should equal first value in year
+  expect_equal(as.numeric(first_result)[1], -10)
+
+  # Last should equal last value in year
+  expect_equal(as.numeric(last_result)[2], 25)
+})
+
+test_that("conversion types maintain mathematical relationships", {
+  skip_on_cran()
+
+  # Create test data
+  x <- ts(rep(c(10, 20, 30, 40), 3), frequency = 4, start = c(2000, 1))
+
+  sum_result <- ta(x, to = "annual", conversion = "sum")
+  avg_result <- ta(x, to = "annual", conversion = "average")
+  first_result <- ta(x, to = "annual", conversion = "first")
+  last_result <- ta(x, to = "annual", conversion = "last")
+
+  # Sum should equal average * frequency for complete years
+  expect_equal(
+    as.numeric(sum_result),
+    as.numeric(avg_result) * 4,
+    tolerance = 1e-10
+  )
+
+  # First should be first value
+  expect_equal(as.numeric(first_result), rep(10, length(first_result)))
+
+  # Last should be last value
+  expect_equal(as.numeric(last_result), rep(40, length(last_result)))
+})
diff --git a/tests/testthat/test-error-handling.R b/tests/testthat/test-error-handling.R
new file mode 100644
index 0000000..275c776
--- /dev/null
+++ b/tests/testthat/test-error-handling.R
@@ -0,0 +1,139 @@
+
+library(testthat)
+library(tempdisagg)
+
+# Load test data
+data(swisspharma)
+
+
+# SubRegressionBased Error Conditions
+# ----------------------------------------------------------------------------
+
+test_that("td() errors on unsupported method", {
+  y <- window(sales.a, end = 1985)
+  x <- window(exports.q, end = c(1985, 4))
+
+  expect_error(
+    td(y ~ x, method = "nonexistent-method"),
+    "method does not exist"
+  )
+})
+
+test_that("td() handles near-singular matrix", {
+  skip_on_cran()
+
+  # Create data with highly correlated (but not perfectly singular) indicators
+  y <- window(sales.a, start = 1980, end = 1985)
+  x1 <- window(exports.q, start = c(1980, 1), end = c(1985, 4))
+  # Create highly correlated but not identical series
+  x2 <- x1 + ts(rnorm(length(x1), sd = 0.1), start = start(x1), frequency = frequency(x1))
+
+  # Should complete without error even with high correlation
+  m <- td(y ~ x1 + x2, method = "chow-lin-maxlog")
+  expect_s3_class(m, "td")
+})
+
+test_that("td() handles rho truncation correctly", {
+  skip_on_cran()
+
+  y <- window(sales.a, end = 1985)
+  x <- window(exports.q, end = c(1985, 4))
+
+  # With truncated.rho = -1, rho should not go below -1
+  m <- td(y ~ x, method = "chow-lin-maxlog", truncated.rho = -1)
+
+  expect_gte(m$rho, -1)
+})
+
+test_that("td() works with fixed rho methods", {
+  skip_on_cran()
+
+  y <- window(sales.a, end = 1985)
+  x <- window(exports.q, end = c(1985, 4))
+
+  # Fixed rho should use specified value
+  m <- td(y ~ x, method = "chow-lin-fixed", fixed.rho = 0.5)
+
+  expect_equal(m$rho, 0.5)
+  expect_s3_class(m, "td")
+})
+
+test_that("td() handles litterman fixed rho", {
+  skip_on_cran()
+
+  y <- window(sales.a, end = 1985)
+  x <- window(exports.q, end = c(1985, 4))
+
+  m <- td(y ~ x, method = "litterman-fixed", fixed.rho = 0.7)
+
+  expect_equal(m$rho, 0.7)
+  expect_s3_class(m, "td")
+})
+
+test_that("td() handles dynamic fixed rho", {
+  skip_on_cran()
+
+  y <- window(sales.a, end = 1985)
+  x <- window(exports.q, end = c(1985, 4))
+
+  m <- td(y ~ x, method = "dynamic-fixed", fixed.rho = 0.3)
+
+  expect_equal(m$rho, 0.3)
+  expect_s3_class(m, "td")
+})
+
+
+# SubDenton Error Conditions
+# ----------------------------------------------------------------------------
+
+test_that("denton methods error with multiple RHS variables", {
+  y <- window(sales.a, end = 1985)
+  x1 <- window(exports.q, end = c(1985, 4))
+  x2 <- window(imports.q, end = c(1985, 4))
+
+  expect_error(
+    td(y ~ x1 + x2, method = "denton"),
+    "only one series allowed"
+  )
+
+  expect_error(
+    td(y ~ x1 + x2, method = "denton-cholette"),
+    "only one series allowed"
+  )
+})
+
+test_that("denton methods error with invalid criterion", {
+  y <- window(sales.a, end = 1985)
+
+  expect_error(
+    td(y ~ 1, to = 4, method = "denton", criterion = "invalid"),
+    "criterion for Denton methods must be additive or proportional"
+  )
+})
+
+test_that("denton methods error with invalid h specification", {
+  y <- window(sales.a, end = 1985)
+
+  expect_error(
+    td(y ~ 1, to = 4, method = "denton", h = -1),
+    "wrong specification of h"
+  )
+})
+
+test_that("uniform method is special case of denton", {
+  skip_on_cran()
+
+  y <- window(sales.a, end = 1985)
+
+  # Uniform should work and be equivalent to denton with h=0, additive
+  m_uniform <- td(y ~ 1, to = 4, method = "uniform")
+  m_denton <- td(y ~ 1, to = 4, method = "denton", h = 0, criterion = "additive")
+
+  expect_s3_class(m_uniform, "td")
+  # Method name stays "uniform" but has same properties as denton h=0, additive
+  expect_equal(m_uniform$h, 0)
+  expect_equal(m_uniform$criterion, "additive")
+
+  # Results should be identical
+  expect_equal(predict(m_uniform), predict(m_denton))
+})
diff --git a/tests/testthat/test-numeric-mode.R b/tests/testthat/test-numeric-mode.R
index a723305..89e20b4 100644
--- a/tests/testthat/test-numeric-mode.R
+++ b/tests/testthat/test-numeric-mode.R
@@ -15,3 +15,71 @@ test_that("numeric mode works as expected", {
 
   expect_equal(m0, as.numeric(m1))
 })
+
+test_that("numeric mode works with different methods", {
+  y <- c(100, 150, 200, 250, 300)
+  # Create varying data to avoid singular matrix
+  x <- seq(20, 100, length.out = 20)
+
+  # Test different methods work in numeric mode
+  m_chow <- td(y ~ x, to = 4, method = "chow-lin-maxlog")
+  m_fern <- td(y ~ x, to = 4, method = "fernandez")
+  m_unif <- td(y ~ 1, to = 4, method = "uniform")
+
+  expect_type(predict(m_chow), "double")
+  expect_type(predict(m_fern), "double")
+  expect_type(predict(m_unif), "double")
+})
+
+test_that("numeric mode preserves aggregation property", {
+  # Use simpler data that won't create singular matrix
+  y <- c(120, 240, 360, 480, 600)
+  x <- seq(10, 200, length.out = 20)
+
+  m <- td(y ~ x, to = 4, method = "chow-lin-maxlog")
+  result <- predict(m)
+
+  # Aggregate back to original frequency
+  aggregated <- colSums(matrix(result, nrow = 4))
+
+  expect_equal(aggregated, y, tolerance = 1e-7)
+})
+
+test_that("numeric mode handles edge cases", {
+  # Longer series to avoid degrees of freedom issues
+  y_test <- c(10, 20, 30, 40)
+  x_test <- seq(2, 48, length.out = 16)
+
+  m_test <- td(y_test ~ x_test, to = 4, method = "chow-lin-maxlog")
+  expect_length(predict(m_test), length(x_test))
+
+  # Constant series (uniform distribution)
+  y_const <- c(100, 100, 100, 100)
+
+  m_const <- td(y_const ~ 1, to = 12, method = "uniform")
+  result_const <- predict(m_const)
+
+  expect_type(result_const, "double")
+  # Should distribute evenly for uniform method
+  expect_true(sd(result_const) < 1)
+})
+
+test_that("numeric mode handles conversion parameter", {
+  # Test with different conversion types
+  y <- c(100, 200, 300, 400)
+  x <- c(rep(25, 4), rep(50, 4), rep(75, 4), rep(100, 4))
+
+  # Sum conversion (default for flow variables)
+  m_sum <- td(y ~ x, to = 4, conversion = "sum", method = "fernandez")
+  result_sum <- predict(m_sum)
+
+  # Average conversion (for stock variables)
+  m_avg <- td(y ~ x, to = 4, conversion = "average", method = "fernandez")
+  result_avg <- predict(m_avg)
+
+  expect_type(result_sum, "double")
+  expect_type(result_avg, "double")
+
+  # Results should differ based on conversion type
+  expect_false(isTRUE(all.equal(result_sum, result_avg)))
+})
diff --git a/tests/testthat/test-utils.R b/tests/testthat/test-utils.R
new file mode 100644
index 0000000..30ee5e4
--- /dev/null
+++ b/tests/testthat/test-utils.R
@@ -0,0 +1,52 @@
+
+library(testthat)
+library(tempdisagg)
+
+
+# ModeOfSeries Function
+# ----------------------------------------------------------------------------
+
+test_that("ModeOfSeries correctly identifies ts objects", {
+  x <- ts(1:12, frequency = 12, start = c(2000, 1))
+  expect_equal(ModeOfSeries(x), "ts")
+})
+
+test_that("ModeOfSeries correctly identifies numeric vectors", {
+  x <- c(1, 2, 3, 4, 5)
+  expect_equal(ModeOfSeries(x), "numeric")
+})
+
+test_that("ModeOfSeries correctly identifies data.frame objects", {
+  x <- data.frame(time = 1:10, value = rnorm(10))
+  expect_equal(ModeOfSeries(x), "tsbox")
+})
+
+test_that("ModeOfSeries prioritizes xts over ts", {
+  skip_if_not_installed("xts")
+
+  # Create xts object (which also inherits from zoo)
+  x <- xts::xts(1:10, order.by = seq(as.Date("2000-01-01"), by = "month", length.out = 10))
+
+  # Should return "tsbox" not "ts" even if it has ts-like properties
+  expect_equal(ModeOfSeries(x), "tsbox")
+})
+
+test_that("ModeOfSeries errors on invalid input types", {
+  # Character vector should error
+  expect_error(
+    ModeOfSeries("not a series"),
+    "series must be a time series object or numeric"
+  )
+
+  # List should error
+  expect_error(
+    ModeOfSeries(list(a = 1, b = 2)),
+    "series must be a time series object or numeric"
+  )
+
+  # NULL should error
+  expect_error(
+    ModeOfSeries(NULL),
+    "series must be a time series object or numeric"
+  )
+})