diff --git a/src/interface.jl b/src/interface.jl
index f07de3b9..7609b65b 100644
--- a/src/interface.jl
+++ b/src/interface.jl
@@ -16,7 +16,7 @@ function has_static_variable end
     variable_index(sys, sym, [i])
 
 Return the index of the given variable `sym` in `sys`, or `nothing` otherwise. If
-[`has_static_variable`](@ref) is `false`, this accepts the current timestep as an
+[`has_static_variable`](@ref) is `false`, this accepts the current time index as an
 additional parameter `i`.
 """
 function variable_index end
@@ -57,12 +57,14 @@ Check whether the given `sym` is an observed value in `sys`.
 function is_observed end
 
 """
-    observed(sys, sym)
+    observed(sys, sym, [states])
 
 Return the observed function of the given `sym` in `sys`. The returned function should
 have the signature `(u, p) -> [values...]` where `u` and `p` is the current state and
 parameter vector. If `istimedependent(sys) == true`, the function should accept
-the current time `t` as its third parameter.
+the current time `t` as its third parameter. If `constant_structure(sys) == false`,
+accept a third parameter which can either be a vector of symbols indicating the order
+of states or a time index which identifies the order of states.
 
 See also: [`is_time_dependent`](@ref), [`has_static_variable`](@ref)
 """
diff --git a/test/example_test.jl b/test/example_test.jl
index e1e8a61f..2adccf0a 100644
--- a/test/example_test.jl
+++ b/test/example_test.jl
@@ -8,7 +8,7 @@ end
 SymbolicIndexingInterface.is_variable(sys::SymbolCache, sym) = sym in sys.vars
 function SymbolicIndexingInterface.variable_index(sys::SymbolCache, sym, t = nothing)
     if !has_static_variable(sys) && t === nothing
-        error("timestep must be present")
+        error("time index must be present")
     end
     findfirst(isequal(sym), current_state(sys, t))
 end
@@ -25,18 +25,22 @@ end
 function SymbolicIndexingInterface.is_observed(sys::SymbolCache, sym)
     is_variable(sys, sym) || is_parameter(sys, sym) || is_independent_variable(sys, sym)
 end
-function SymbolicIndexingInterface.observed(sys::SymbolCache, sym)
+function SymbolicIndexingInterface.observed(sys::SymbolCache, sym, states = nothing)
+    if !has_static_variable(sys) && states === nothing
+        error("States required")
+    end
+    states = states isa Vector ? states : current_state(sys, states)
     if is_variable(sys, sym)
         return is_time_dependent(sys) ?
-               (u, p, t) -> [u[variable_index(sys, sym, i)] * i for i in t] :
-               (u, p) -> [u[variable_index(sys, sym)] * i for i in 1:5]
+               (u, p, t) -> u[findfirst(isequal(sym), states)] :
+               (u, p) -> u[findfirst(isequal(sym), states)]
     end
     idx = parameter_index(sys, sym)
     if idx !== nothing
         return is_time_dependent(sys) ? (u, p, t) -> p[idx] : (u, p) -> p[idx]
     end
     if is_independent_variable(sys, sym)
-        return is_time_dependent(sys) ? (u, p, t) -> t : (u, p) -> 1:5
+        return is_time_dependent(sys) ? (u, p, t) -> t : (u, p) -> 1
     end
 end
 SymbolicIndexingInterface.is_time_dependent(sys::SymbolCache) = isequal(sys.indepvar, :t)
@@ -56,13 +60,13 @@ sys = SymbolCache(true, [:x, :y, :z], [:a, :b, :c], :t)
 @test is_independent_variable(sys, :t)
 @test all(.!is_independent_variable.((sys,), [:x, :y, :z, :a, :b, :c, :p, :q, :r]))
 @test all(is_observed.((sys,), [:x, :y, :z, :a, :b, :c, :t]))
-@test all(observed(sys, :x)(1:3, 4:6, 1:4) .== [1, 2, 3, 4])
-@test all(observed(sys, :y)(1:4, 4:6, 1:4) .== [2, 4, 6, 8])
-@test all(observed(sys, :z)(1:4, 4:6, 1:4) .== [3, 6, 9, 12])
-@test observed(sys, :a)(1:3, 4:6, 1:4) == 4
-@test observed(sys, :b)(1:3, 4:6, 1:4) == 5
-@test observed(sys, :c)(1:3, 4:6, 1:4) == 6
-@test observed(sys, :t)(1:3, 4:6, 1:4) == 1:4
+@test all(observed(sys, :x)(1:3, 4:6, 1.5) .== 1)
+@test all(observed(sys, :y)(1:3, 4:6, 1.5) .== 2)
+@test all(observed(sys, :z)(1:3, 4:6, 1.5) .== 3)
+@test observed(sys, :a)(1:3, 4:6, 1.5) == 4
+@test observed(sys, :b)(1:3, 4:6, 1.5) == 5
+@test observed(sys, :c)(1:3, 4:6, 1.5) == 6
+@test observed(sys, :t)(1:3, 4:6, 1.5) == 1.5
 @test is_time_dependent(sys)
 @test constant_structure(sys)
 @test has_static_variable(sys)
@@ -70,9 +74,9 @@ sys = SymbolCache(true, [:x, :y, :z], [:a, :b, :c], :t)
 sys = SymbolCache(true, [:x, :y, :z], [:a, :b, :c], nothing)
 
 @test !is_time_dependent(sys)
-@test all(observed(sys, :x)(1:3, 4:6) .== [1, 2, 3, 4, 5])
-@test all(observed(sys, :y)(1:3, 4:6) .== [2, 4, 6, 8, 10])
-@test all(observed(sys, :z)(1:3, 4:6) .== [3, 6, 9, 12, 15])
+@test all(observed(sys, :x)(1.0:3.0, 4:6) .== 1.0)
+@test all(observed(sys, :y)(1.0:3.0, 4:6) .== 2.0)
+@test all(observed(sys, :z)(1.0:3.0, 4:6) .== 3.0)
 @test observed(sys, :a)(1:3, 4:6) == 4
 @test observed(sys, :b)(1:3, 4:6) == 5
 @test observed(sys, :c)(1:3, 4:6) == 6
@@ -82,13 +86,14 @@ sys = SymbolCache(false, [:x, :y, :z], [:a, :b, :c], :t)
 @test !has_static_variable(sys)
 for variable in [:x, :y, :z, :a, :b, :c, :t]
     @test_throws ErrorException variable_index(sys, variable)
+    @test_throws ErrorException observed(sys, variable)
 end
 @test all(variable_index.((sys,), [:z, :y, :x], 1) .== [1, 3, 2])
 @test all(variable_index.((sys,), [:a, :b, :c, :t], 1) .== nothing)
-@test all(observed(sys, :x)(1:3, 4:6, 1:4) .== [2, 6, 3, 8])
-@test all(observed(sys, :y)(1:4, 4:6, 1:4) .== [3, 2, 6, 12])
-@test all(observed(sys, :z)(1:4, 4:6, 1:4) .== [1, 4, 9, 4])
-@test observed(sys, :a)(1:3, 4:6, 1:4) == 4
-@test observed(sys, :b)(1:3, 4:6, 1:4) == 5
-@test observed(sys, :c)(1:3, 4:6, 1:4) == 6
-@test observed(sys, :t)(1:3, 4:6, 1:4) == 1:4
+@test all(observed(sys, :x, 2)(1:3, 4:6, 1.5) .== 3)
+@test all(observed(sys, :y, 2)(1:3, 4:6, 1.5) .== 1)
+@test all(observed(sys, :z, 2)(1:3, 4:6, 1.5) .== 2)
+@test observed(sys, :a, 2)(1:3, 4:6, 1.5) == 4
+@test observed(sys, :b, 2)(1:3, 4:6, 1.5) == 5
+@test observed(sys, :c, 2)(1:3, 4:6, 1.5) == 6
+@test observed(sys, :t, 2)(1:3, 4:6, 1.5) == 1.5