diff --git a/NAMESPACE b/NAMESPACE
index e372811..7d8f2c0 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -1,17 +1,19 @@
 # Generated by roxygen2: do not edit by hand
 
 S3method(as.character,pool_strat)
+S3method(design_effect,fixed_design)
+S3method(design_effect,variable_design)
 S3method(format,pool_strat)
 S3method(print,pool_strat)
 S3method(print,power_size_results)
 export(design_effect)
-export(design_effect_random)
 export(detection_errors)
 export(detection_errors_cluster)
 export(fi_pool)
 export(fi_pool_cluster)
 export(fi_pool_cluster_random)
 export(fi_pool_random)
+export(fixed_design)
 export(nb_catch)
 export(optimise_random_prevalence)
 export(optimise_sN_prevalence)
@@ -24,3 +26,4 @@ export(power_pool_random)
 export(power_size_results)
 export(sample_size_pool)
 export(sample_size_pool_random)
+export(variable_design)
diff --git a/R/check_input.R b/R/check_input.R
index 3e68ebf..da72ac3 100644
--- a/R/check_input.R
+++ b/R/check_input.R
@@ -1,3 +1,5 @@
+# NOTE: Will discontinue - use check_input2.R 
+
 check_input <- function(argument_name, input_value) {
   # Wrapper function to triage arguments, so one function can be used for all
   # inputs instead of remembering which one to use.
diff --git a/R/check_input2.R b/R/check_input2.R
new file mode 100644
index 0000000..65eba67
--- /dev/null
+++ b/R/check_input2.R
@@ -0,0 +1,20 @@
+check_geq2 <- function(val, min) {
+  name <- deparse(substitute(val)) # get name of variable
+  if (!is.numeric(val)) {
+    stop(glue::glue("{name} must be numeric, not {class(val)}."))
+  }
+  if (val < min) {
+    stop(glue::glue("{name} must be >= {min}."))
+  }
+}
+
+check_in_range2 <- function(val) {
+  name <- deparse(substitute(val)) # get name of variable
+  if(!is.numeric(val)) {
+    stop(glue::glue("{name} must be numeric, not {class(val)}."))
+  }
+  if (val < 0 | val > 1) {
+    message(glue::glue("{name} must be a numeric value between 0 and 1, inclusive."))
+    stop(glue::glue("{name} = {val}"))
+  }
+}
\ No newline at end of file
diff --git a/R/design_effect.R b/R/design_effect.R
index a02aec3..5b55648 100644
--- a/R/design_effect.R
+++ b/R/design_effect.R
@@ -9,22 +9,11 @@
 #' to be multiplied by a factor of D to achieve the same degree of precision in
 #' estimating prevalence as a simple random survey with individual tests. The
 #' functions support cluster and simple random sampling with perfect or
-#' imperfect tests, and either fixed sample sizes (`design_effect()`) or random
-#' sample sizes (`design_effect_random()`).
+#' imperfect tests, and either fixed sample sizes 
+#' (`design_effect(fixed_design, ...)`) or variable sample sizes 
+#' (`design_effect(variable_design, ...)`).
 #'
-#' @param pool_size numeric The number of units per pool. Must be a numeric
-#'   value greater than or equal to 0.
-#' @param pool_number numeric The number of pools per cluster. Must be a numeric
-#'   value greater than or equal to 0.
-#' @param catch_dist An object of class `distribution` (e.g. produced by
-#'   `nb_catch()`) defining the distribution of the possible catch. If
-#'   `correlation = 0` the catch is for the whole survey. For `correlation > 0`
-#'   the catch is per cluster (i.e. cluster size).
-#' @param pool_strat function Defines a rule for how a number of units will be
-#'   divided into pools. Must take a single numeric argument and return a named
-#'   list of pool sizes and pool numbers. `pool_max_size()` and
-#'   `pool_target_number` provide convenience functions for defining common
-#'   pooling strategies.
+#' @param x a sample_design object 
 #' @param prevalence numeric The proportion of units that carry the marker of
 #'   interest (i.e. true positive). Must be be a numeric value between 0 and 1,
 #'   inclusive of both.
@@ -35,63 +24,51 @@
 #'   (there are no differences units within a single cluster). A value of 0
 #'   indicates that units within clusters are no more correlated than units in
 #'   different clusters.
-#' @param sensitivity numeric The probability that the test correctly identifies
-#'   a true positive. Must be a numeric value between 0 and 1, inclusive of
-#'   both. A value of 1 indicates that the test can perfectly identify all true
-#'   positives.
-#' @param specificity numeric The probability that the test correctly identifies
-#'   a true negative. Must be a numeric value between 0 and 1, inclusive of
-#'   both. A value of 1 indicates that the test can perfectly identify all true
-#'   negatives.
 #' @param form string The distribution used to model the cluster-level
 #'   prevalence and correlation of units within cluster. Select one of "beta",
-#'   "logitnorm" or "cloglognorm". See details.
+#'   "logitnorm" or "cloglognorm".
 #'
 #' @return A numeric value of the design effect `D`.
 #' @export
-#'
+#' 
 #' @examples
-#' design_effect(
-#'   pool_size = 5, pool_number = 10, prevalence = 0.01,
-#'   correlation = 0.05, sensitivity = 0.99, specificity = 0.95
-#'   )
-design_effect <- function(pool_size,
-                          pool_number,
-                          prevalence,
-                          correlation,
-                          sensitivity,
-                          specificity,
-                          form = "beta"){
-
-  check_input("pool_size", pool_size)
-  check_input("pool_number", pool_number)
-  check_input("prevalence", prevalence)
-  check_input("correlation", correlation)
-  check_input("sensitivity", sensitivity)
-  check_input("specificity", specificity)
-  check_input("form", form)
+#' design_effect(fixed_design(10, 2), prevalence = 0.01, correlation = 0.05)
+#' 
+#' vd <- variable_design(nb_catch(10, 13), pool_target_number(20))
+#' design_effect(vd, prevalence = 0.01, correlation = 0.05)
+design_effect <- function(x, prevalence, correlation, form) {
+  check_in_range2(prevalence)
+  check_in_range2(correlation)
+  # No input check for form as done in downstream functions/methods
+  UseMethod("design_effect")
+}
 
-  pool_number * pool_size * fi_pool(pool_size = 1, prevalence, sensitivity, specificity) *
+#' @rdname design_effect
+#' @method design_effect fixed_design
+#' @export
+design_effect.fixed_design <- function(x,
+                                       prevalence,
+                                       correlation,
+                                       form = "beta") {
+  
+  x$pool_number * x$pool_size * fi_pool(pool_size = 1, prevalence, x$sensitivity, x$specificity) *
     solve(fi_pool_cluster(
-      pool_size, pool_number, prevalence,
-      correlation, sensitivity, specificity, form)
+      x$pool_size, x$pool_number, prevalence,
+      correlation, x$sensitivity, x$specificity, form)
     )[1, 1]
 }
 
-
 #' @rdname design_effect
+#' @method design_effect variable_design
 #' @export
-design_effect_random <- function(catch_dist,
-                                 pool_strat,
-                                 prevalence,
-                                 correlation,
-                                 sensitivity,
-                                 specificity,
-                                 form = "beta") {
+design_effect.variable_design <- function(x,
+                                          prevalence,
+                                          correlation,
+                                          form = "beta") {
   
-  mean(catch_dist) * fi_pool(pool_size = 1, prevalence, sensitivity, specificity) *
+  mean(x$catch_dist) * fi_pool(pool_size = 1, prevalence, x$sensitivity, x$specificity) *
     solve(fi_pool_cluster_random(
-      catch_dist, pool_strat, prevalence,
-      correlation, sensitivity, specificity, form)
+      x$catch_dist, x$pool_strat, prevalence,
+      correlation, x$sensitivity, x$specificity, form)
     )[1, 1]
 }
diff --git a/R/sample_design.R b/R/sample_design.R
new file mode 100644
index 0000000..dc65c41
--- /dev/null
+++ b/R/sample_design.R
@@ -0,0 +1,103 @@
+#' S3 sample_design constructors
+#'
+#' Stores parameters related to the sampling design. Aims to reduce having to
+#' input each param separately across functions (e.g. power/optimise). Can
+#' either be of class `fixed_design` or `variable_design`.
+#'
+#' @param pool_size numeric/NULL The number of units per pool. Must be a numeric
+#'   value greater than 0. `fixed_design` only.
+#' @param pool_number numeric/NULL The number of pools per cluster. Numeric
+#'   inputs must be an integer greater than or equal to 1. `fixed_design` only.
+#' @param catch_dist An object of class `distribution` (e.g. produced by
+#'   `nb_catch()`) defining the distribution of the possible catch. If
+#'   `correlation = 0` the catch is for the whole survey. For `correlation > 0`
+#'   the catch is per cluster (i.e. cluster size). `variable_design` only.
+#' @param pool_strat function Defines a rule for how a number of units will be
+#'   divided into pools. Must take a single numeric argument and return a named
+#'   list of pool sizes and pool numbers. `pool_max_size()` and
+#'   `pool_target_number` provide convenience functions for defining common
+#'   pooling strategies. `variable_design` only.
+#' @param sensitivity numeric The probability that the test correctly identifies
+#'   a true positive. Must be a numeric value between 0 and 1, inclusive of
+#'   both. A value of 1 indicates that the test can perfectly identify all true
+#'   positives.
+#' @param specificity numeric The probability that the test correctly identifies
+#'   a true negative. Must be a numeric value between 0 and 1, inclusive of
+#'   both. A value of 1 indicates that the test can perfectly identify all true
+#'   negatives.
+#'
+#' @return An object of class \code{sample_design}
+#' @export
+#'
+#' @examples
+#' fd_perfect <- fixed_design(pool_size = 10)
+#' 
+#' fd_imperfect <- fixed_design(
+#'   pool_size = 10, pool_number = NULL, sensitivity = 0.95, specificity = 0.99
+#' )
+#' 
+#' vd_target <- variable_design(
+#'   catch_dist = nb_catch(10, 11),
+#'   pool_strat = pool_target_number(20)
+#' )
+#' 
+#' vd_max <- variable_design(
+#'   catch_dist = nb_catch(10, 11),
+#'   pool_strat = pool_max_size(20)
+#' )
+#' 
+#' vd_max_imperfect <- variable_design(
+#'   catch_dist = nb_catch(10, 11),
+#'   pool_strat = pool_max_size(20),
+#'   sensitivity = 0.95,
+#'   specificity = 0.98
+#' )
+fixed_design <- function(pool_size = NULL,
+                         pool_number = NULL,
+                         sensitivity = 1,
+                         specificity = 1) {
+
+  # allow NULLs for optimise functions to identify which 
+  # variable should be optimised
+  if (!is.null(pool_size)) {
+    check_geq2(pool_size, 0)
+  }
+  if (!is.null(pool_number)) {
+    check_geq2(pool_number, 0)
+  }
+  # sens and spec cannot be NULL
+  check_in_range2(sensitivity)
+  check_in_range2(specificity)
+
+  structure(
+    list(
+      pool_size = pool_size,
+      pool_number = pool_number,
+      sensitivity = sensitivity,
+      specificity = specificity
+    ),
+    class = c("fixed_design", "sample_design")
+  )
+}
+
+#' @rdname fixed_design
+#' @export
+variable_design <- function(catch_dist, 
+                            pool_strat,
+                            sensitivity = 1,
+                            specificity = 1) {
+
+  # sens and spec cannot be NULL
+  check_in_range2(sensitivity)
+  check_in_range2(specificity)
+
+  structure(
+    list(
+      catch_dist = catch_dist,
+      pool_strat = pool_strat,
+      sensitivity = sensitivity,
+      specificity = specificity
+    ),
+    class = c("variable_design", "sample_design")
+  )
+}
diff --git a/man/design_effect.Rd b/man/design_effect.Rd
index b9e2aa4..fd6de9d 100644
--- a/man/design_effect.Rd
+++ b/man/design_effect.Rd
@@ -2,35 +2,18 @@
 % Please edit documentation in R/design_effect.R
 \name{design_effect}
 \alias{design_effect}
-\alias{design_effect_random}
+\alias{design_effect.fixed_design}
+\alias{design_effect.variable_design}
 \title{Calculate the design effect for pooled testing.}
 \usage{
-design_effect(
-  pool_size,
-  pool_number,
-  prevalence,
-  correlation,
-  sensitivity,
-  specificity,
-  form = "beta"
-)
+design_effect(x, prevalence, correlation, form)
 
-design_effect_random(
-  catch_dist,
-  pool_strat,
-  prevalence,
-  correlation,
-  sensitivity,
-  specificity,
-  form = "beta"
-)
+\method{design_effect}{fixed_design}(x, prevalence, correlation, form = "beta")
+
+\method{design_effect}{variable_design}(x, prevalence, correlation, form = "beta")
 }
 \arguments{
-\item{pool_size}{numeric The number of units per pool. Must be a numeric
-value greater than or equal to 0.}
-
-\item{pool_number}{numeric The number of pools per cluster. Must be a numeric
-value greater than or equal to 0.}
+\item{x}{a sample_design object}
 
 \item{prevalence}{numeric The proportion of units that carry the marker of
 interest (i.e. true positive). Must be be a numeric value between 0 and 1,
@@ -44,30 +27,9 @@ A value of 1 indicates that units within clusters are perfectly correlated
 indicates that units within clusters are no more correlated than units in
 different clusters.}
 
-\item{sensitivity}{numeric The probability that the test correctly identifies
-a true positive. Must be a numeric value between 0 and 1, inclusive of
-both. A value of 1 indicates that the test can perfectly identify all true
-positives.}
-
-\item{specificity}{numeric The probability that the test correctly identifies
-a true negative. Must be a numeric value between 0 and 1, inclusive of
-both. A value of 1 indicates that the test can perfectly identify all true
-negatives.}
-
 \item{form}{string The distribution used to model the cluster-level
 prevalence and correlation of units within cluster. Select one of "beta",
-"logitnorm" or "cloglognorm". See details.}
-
-\item{catch_dist}{An object of class `distribution` (e.g. produced by
-`nb_catch()`) defining the distribution of the possible catch. If
-`correlation = 0` the catch is for the whole survey. For `correlation > 0`
-the catch is per cluster (i.e. cluster size).}
-
-\item{pool_strat}{function Defines a rule for how a number of units will be
-divided into pools. Must take a single numeric argument and return a named
-list of pool sizes and pool numbers. `pool_max_size()` and
-`pool_target_number` provide convenience functions for defining common
-pooling strategies.}
+"logitnorm" or "cloglognorm".}
 }
 \value{
 A numeric value of the design effect `D`.
@@ -82,12 +44,13 @@ effect `D>1` (`D<1`) indicates that the pooling/sampling strategy reduces
 to be multiplied by a factor of D to achieve the same degree of precision in
 estimating prevalence as a simple random survey with individual tests. The
 functions support cluster and simple random sampling with perfect or
-imperfect tests, and either fixed sample sizes (`design_effect()`) or random
-sample sizes (`design_effect_random()`).
+imperfect tests, and either fixed sample sizes 
+(`design_effect(fixed_design, ...)`) or variable sample sizes 
+(`design_effect(variable_design, ...)`).
 }
 \examples{
-design_effect(
-  pool_size = 5, pool_number = 10, prevalence = 0.01,
-  correlation = 0.05, sensitivity = 0.99, specificity = 0.95
-  )
+design_effect(fixed_design(10, 2), prevalence = 0.01, correlation = 0.05)
+
+vd <- variable_design(nb_catch(10, 13), pool_target_number(20))
+design_effect(vd, prevalence = 0.01, correlation = 0.05)
 }
diff --git a/man/fixed_design.Rd b/man/fixed_design.Rd
new file mode 100644
index 0000000..db24e45
--- /dev/null
+++ b/man/fixed_design.Rd
@@ -0,0 +1,76 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/sample_design.R
+\name{fixed_design}
+\alias{fixed_design}
+\alias{variable_design}
+\title{S3 sample_design constructors}
+\usage{
+fixed_design(
+  pool_size = NULL,
+  pool_number = NULL,
+  sensitivity = 1,
+  specificity = 1
+)
+
+variable_design(catch_dist, pool_strat, sensitivity = 1, specificity = 1)
+}
+\arguments{
+\item{pool_size}{numeric/NULL The number of units per pool. Must be a numeric
+value greater than 0. `fixed_design` only.}
+
+\item{pool_number}{numeric/NULL The number of pools per cluster. Numeric
+inputs must be an integer greater than or equal to 1. `fixed_design` only.}
+
+\item{sensitivity}{numeric The probability that the test correctly identifies
+a true positive. Must be a numeric value between 0 and 1, inclusive of
+both. A value of 1 indicates that the test can perfectly identify all true
+positives.}
+
+\item{specificity}{numeric The probability that the test correctly identifies
+a true negative. Must be a numeric value between 0 and 1, inclusive of
+both. A value of 1 indicates that the test can perfectly identify all true
+negatives.}
+
+\item{catch_dist}{An object of class `distribution` (e.g. produced by
+`nb_catch()`) defining the distribution of the possible catch. If
+`correlation = 0` the catch is for the whole survey. For `correlation > 0`
+the catch is per cluster (i.e. cluster size). `variable_design` only.}
+
+\item{pool_strat}{function Defines a rule for how a number of units will be
+divided into pools. Must take a single numeric argument and return a named
+list of pool sizes and pool numbers. `pool_max_size()` and
+`pool_target_number` provide convenience functions for defining common
+pooling strategies. `variable_design` only.}
+}
+\value{
+An object of class \code{sample_design}
+}
+\description{
+Stores parameters related to the sampling design. Aims to reduce having to
+input each param separately across functions (e.g. power/optimise). Can
+either be of class `fixed_design` or `variable_design`.
+}
+\examples{
+fd_perfect <- fixed_design(pool_size = 10)
+
+fd_imperfect <- fixed_design(
+  pool_size = 10, pool_number = NULL, sensitivity = 0.95, specificity = 0.99
+)
+
+vd_target <- variable_design(
+  catch_dist = nb_catch(10, 11),
+  pool_strat = pool_target_number(20)
+)
+
+vd_max <- variable_design(
+  catch_dist = nb_catch(10, 11),
+  pool_strat = pool_max_size(20)
+)
+
+vd_max_imperfect <- variable_design(
+  catch_dist = nb_catch(10, 11),
+  pool_strat = pool_max_size(20),
+  sensitivity = 0.95,
+  specificity = 0.98
+)
+}
diff --git a/tests/testthat/test-check_input2.R b/tests/testthat/test-check_input2.R
new file mode 100644
index 0000000..9f09b51
--- /dev/null
+++ b/tests/testthat/test-check_input2.R
@@ -0,0 +1,18 @@
+test_that("check_geq2", {
+  x <- 1
+  expect_silent(check_geq2(x, 1))
+  expect_error(check_geq2(x, 2), "x must be >= 2.")
+  y <- "a"
+  expect_error(check_geq2(y, 1), "y must be numeric, not character.")
+})
+
+test_that("check_in_range2", {
+  x <- 0.01
+  expect_silent(check_in_range2(x))
+  x <- -1
+  expect_error(check_in_range2(x), "x = -1")
+  x <- 2
+  expect_error(check_in_range2(x), "x = 2")
+  y <- "a"
+  expect_error(check_in_range2(y), "y must be numeric, not character.")
+})
\ No newline at end of file
diff --git a/tests/testthat/test-design_effect.R b/tests/testthat/test-design_effect.R
index a7d89cf..adff430 100644
--- a/tests/testthat/test-design_effect.R
+++ b/tests/testthat/test-design_effect.R
@@ -1,52 +1,30 @@
-### design_effect() -----------------------------------------------------------
-test_that("design_effect() gives consistent output for basic tests", {
+# fixed_design ----
+fd <- fixed_design(
+  pool_size = 5, pool_number = 10, sensitivity = 0.99, specificity = 0.95
+)
+
+test_that("fixed design_effect() gives consistent output for basic tests", {
   # This one has reasonable inputs
   expect_equal(
-    design_effect(
-      pool_size = 5,
-      pool_number = 10,
-      prevalence = 0.01,
-      correlation = 0.05,
-      sensitivity = 0.99,
-      specificity = 0.95),
+    design_effect(fd, prevalence = 0.01, correlation = 0.05),
     0.7240988, tolerance = 1e-7
   )
   expect_equal(
-    design_effect(
-      pool_size = 10,
-      pool_number = 10,
-      prevalence = 0.9,
-      sensitivity = 1,
-      specificity = 1,
-      correlation = 0.9,
-      form = "beta"
-    ),
-    118.9243,
-    tolerance = 1e-4
+    design_effect(fixed_design(10, 10), prevalence = 0.9, correlation = 0.9),
+    118.9243, tolerance = 1e-4
   )
   expect_equal(
     design_effect(
-      pool_size = 10,
-      pool_number = 10,
-      prevalence = 0.2,
-      sensitivity = 0.9,
-      specificity = 0.8,
-      correlation = 0.2,
-      form = "beta"
-    ),
-    26.50055,
-    tolerance = 1e-5
+      fixed_design(10, 10, 0.9, 0.8), prevalence = 0.2, correlation = 0.2), 
+      26.50055, tolerance = 1e-5
   )
 })
 
 test_that("design_effect() fails for some very unusual parameters because integral in call to fi_pool_cluster() appears starts to have numeric issues", {
   expect_error(
     design_effect(
-      pool_size = 100,
-      pool_number = 10,
+      fixed_design(100, 10, 1, 0.7),
       prevalence = 0.8,
-      sensitivity = 1,
-      specificity = 0.7,
       correlation = 0.8,
       form = "cloglognorm"
     ),
@@ -54,13 +32,18 @@ test_that("design_effect() fails for some very unusual parameters because integr
   )
 })
 
+### variable_design() ----
+vd_target <- variable_design(nb_catch(5, 7), pool_target_number(10))
+vd_max <- variable_design(nb_catch(5, 7), pool_max_size(10))
 
-test_that("bad inputs caught in design_effect()", {
-  expect_error(design_effect(pool_size = TRUE), "TRUE is a logical")
-  expect_error(design_effect(pool_size = 5, pool_number = 10, prevalence = 10), "10 is > 1")
-  expect_error(design_effect(pool_size = 5, pool_number = 10, prevalence = 0.01, correlation = 0.1, sensitivity = 1, specificity = 1, form = "binomal"),
-               "form must be one of 'beta', 'logitnorm', 'cloglognorm', or 'discrete'.")
+test_that("variable design_effect()", {
+  expect_equal(
+    design_effect(vd_target, prevalence = 0.01, correlation = 0.05),
+    1.256262, tolerance = 1e-6
+  )
+  expect_equal(
+    design_effect(vd_max, prevalence = 0.01, correlation = 0.05),
+    3.726256, tolerance = 1e-6
+  )
 })
 
-### design_effect_random() ----------------------------------------------------
-
diff --git a/tests/testthat/test-sample_design.R b/tests/testthat/test-sample_design.R
new file mode 100644
index 0000000..a547daf
--- /dev/null
+++ b/tests/testthat/test-sample_design.R
@@ -0,0 +1,59 @@
+# fixed_design ----
+## fixtures ----
+fixed_perfect <- fixed_design(
+  pool_size = 10, pool_number = NULL, sensitivity = 1, specificity = 1
+)
+
+fixed_null <- fixed_design() # sens/spec == 1, pool_size/num == NULL
+
+## test ----
+test_that("fixed_design constructor", {
+  expect_equal(class(fixed_perfect), c("fixed_design", "sample_design"))
+  expect_equal(fixed_perfect$pool_size, 10)
+  expect_equal(fixed_perfect$pool_number, NULL)
+  expect_equal(fixed_perfect$sensitivity, 1)
+  expect_equal(fixed_perfect$specificity, 1)
+})
+
+test_that("fixed_design default", {
+  expect_equal(class(fixed_perfect), c("fixed_design", "sample_design"))
+  expect_equal(fixed_null$pool_size, NULL)
+  expect_equal(fixed_null$pool_number, NULL)
+  expect_equal(fixed_null$sensitivity, 1)
+  expect_equal(fixed_null$specificity, 1)
+}) 
+
+test_that("fixed_design bad inputs caught", {
+  expect_error(fixed_design(pool_size = -1))
+  expect_error(fixed_design(pool_number = -1))
+})
+
+# variable_design ----
+## fixtures ----
+var_target <- variable_design(
+  catch_dist = nb_catch(5, 10), 
+  pool_strat = pool_target_number(20)
+)
+
+var_max <- variable_design(
+  catch_dist = nb_catch(5, 10), 
+  pool_strat = pool_max_size(20)
+)
+
+## tests ----
+test_that("variable_design constructor (target_size)", {
+  expect_equal(class(var_target), c("variable_design", "sample_design"))
+  expect_equal(var_target$catch_dist, nb_catch(5, 10))
+  expect_equal(var_target$pool_strat, pool_target_number(20))
+  expect_equal(var_target$sensitivity, 1)
+  expect_equal(var_target$specificity, 1)
+})
+
+test_that("variable_design constructor (max_size)", {
+  expect_equal(class(var_max), c("variable_design", "sample_design"))
+  expect_equal(var_max$pool_strat, pool_max_size(20))
+})
+
+test_that("null variable_design", {
+  expect_error(variable_design())
+})