diff --git a/Complexity.lean b/Complexity.lean
index bcc308a876..cdc4d5311c 100644
--- a/Complexity.lean
+++ b/Complexity.lean
@@ -3039,6 +3039,7 @@ public import Complexity.Foundations.BooleanAnalysis.Hypercube
 public import Complexity.Foundations.BooleanAnalysis.IncidenceExpansion
 public import Complexity.Foundations.BooleanAnalysis.Influence
 public import Complexity.Foundations.BooleanAnalysis.KahnKalaiLinial
+public import Complexity.Foundations.BooleanAnalysis.LevelWeight
 public import Complexity.Foundations.BooleanAnalysis.LightPatterns
 public import Complexity.Foundations.BooleanAnalysis.Linearity
 public import Complexity.Foundations.BooleanAnalysis.LongCode
@@ -3063,6 +3064,7 @@ public import Complexity.Foundations.BooleanAnalysis.SparseEquations.Degree
 public import Complexity.Foundations.BooleanAnalysis.SparseHermiteRecovery
 public import Complexity.Foundations.BooleanAnalysis.SparseIncidence
 public import Complexity.Foundations.BooleanAnalysis.SparseReplacementDesign
+public import Complexity.Foundations.BooleanAnalysis.SpectralSample
 public import Complexity.Foundations.BooleanAnalysis.Symmetrization
 public import Complexity.Foundations.BooleanAnalysis.Tribes
 public import Complexity.Foundations.BooleanAnalysis.Variance
diff --git a/Complexity/Foundations/BooleanAnalysis/LevelWeight.lean b/Complexity/Foundations/BooleanAnalysis/LevelWeight.lean
new file mode 100644
index 0000000000..5f51e2338c
--- /dev/null
+++ b/Complexity/Foundations/BooleanAnalysis/LevelWeight.lean
@@ -0,0 +1,160 @@
+/-
+Copyright (c) 2026 Vincent Liew. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Vincent Liew
+-/
+
+module
+-- upstream: candidate
+
+public import Complexity.Foundations.BooleanAnalysis.Degree
+import Mathlib.Algebra.BigOperators.Ring.Finset
+
+/-!
+# Fourier weight by degree
+
+The *degree-`k` part* of a real function `f` on the cube is `f^{=k} = ∑_{|S| = k} f̂(S) χ_S`
+(`BitString.levelPart`), and its *Fourier weight at degree `k`* is
+`W^k[f] = ∑_{|S| = k} f̂(S) ^ 2` (`BitString.levelWeight`)
+[O'Donnell, Definition 1.19][ODonnell2014]. The degree-`k` part keeps exactly the Fourier
+coefficients on the sets of size `k` (`BitString.fourierCoeff_levelPart`), has degree at most `k`
+(`BitString.levelPart_mem_lowDegree`), and the parts sum to `f` (`BitString.sum_levelPart`). The
+parts of different degrees are orthogonal, and the part of degree `k` has squared norm `W^k[f]`
+(`BitString.expect_levelPart_mul_levelPart`) [O'Donnell, Exercise 1.18][ODonnell2014], so the
+weights sum to `𝔼 f ^ 2` (`BitString.sum_levelWeight`). The weight at degree zero is the squared
+mean (`BitString.levelWeight_zero`), and the weights above a level `t` are the Fourier weight
+`∑_{t < |S|} f̂(S) ^ 2` that the library uses for spectral concentration
+(`BitString.sum_levelWeight_Ioc`).
+
+## Main definitions
+
+* `Cslib.BitString.levelPart`: the degree-`k` part `f^{=k}` of a real function.
+* `Cslib.BitString.levelWeight`: the Fourier weight `W^k[f]` at degree `k`.
+
+## Main results
+
+* `Cslib.BitString.fourierCoeff_levelPart`: the Fourier coefficients of `f^{=k}`.
+* `Cslib.BitString.sum_levelPart`: `f` is the sum of its parts of each degree.
+* `Cslib.BitString.expect_levelPart_mul_levelPart`: the parts are orthogonal, with
+  `‖f^{=k}‖² = W^k[f]`.
+* `Cslib.BitString.sum_levelWeight`: the weights sum to `𝔼 f ^ 2`, and to one for a Boolean
+  function read as signs (`Cslib.BitString.sum_levelWeight_signVal`).
+* `Cslib.BitString.levelWeight_zero`, `Cslib.BitString.sum_levelWeight_Ioc`: the weight at degree
+  zero and the weight above a level.
+
+## References
+
+* [R. O'Donnell, *Analysis of Boolean Functions*][ODonnell2014]
+-/
+
+public section
+
+namespace Cslib.BitString
+
+open Finset
+open scoped BigOperators
+
+variable {ι : Type*} [Fintype ι] [DecidableEq ι]
+
+/-- The degree-`k` part `f^{=k} = ∑_{|S| = k} f̂(S) χ_S` of a real function on the cube
+[O'Donnell, Definition 1.19][ODonnell2014]. -/
+@[expose] noncomputable def levelPart (k : ℕ) (f : (ι → Bool) → ℝ) (x : ι → Bool) : ℝ :=
+  ∑ S with #S = k, fourierCoeff f S * chi ℝ S x
+
+/-- The Fourier weight `W^k[f] = ∑_{|S| = k} f̂(S) ^ 2` of a real function at degree `k`
+[O'Donnell, Definition 1.19][ODonnell2014]. -/
+@[expose] noncomputable def levelWeight (k : ℕ) (f : (ι → Bool) → ℝ) : ℝ :=
+  ∑ S with #S = k, fourierCoeff f S ^ 2
+
+/-- The degree-`k` part as a combination of all the characters, with weight zero off size `k`. -/
+theorem levelPart_eq_sum_ite (k : ℕ) (f : (ι → Bool) → ℝ) (x : ι → Bool) :
+    levelPart k f x = ∑ S, (if #S = k then fourierCoeff f S else 0) * chi ℝ S x := by
+  simp only [levelPart, sum_filter, ite_mul, zero_mul]
+
+/-- The degree-`k` part keeps the Fourier coefficients on the sets of size `k` and no others. -/
+@[simp] theorem fourierCoeff_levelPart (k : ℕ) (f : (ι → Bool) → ℝ) (S : Finset ι) :
+    fourierCoeff (levelPart k f) S = if #S = k then fourierCoeff f S else 0 := by
+  rw [funext (levelPart_eq_sum_ite k f), fourierCoeff_sum_mul_chi]
+
+/-- The degree-`k` part has degree at most `k`. -/
+theorem levelPart_mem_lowDegree (k : ℕ) (f : (ι → Bool) → ℝ) :
+    levelPart k f ∈ lowDegree ℝ ι k := by
+  have h : levelPart k f = ∑ S with #S = k, fourierCoeff f S • chi ℝ S := by
+    funext x
+    simp [levelPart]
+  rw [h]
+  exact Submodule.sum_mem _ fun S hS =>
+    Submodule.smul_mem _ _ ((mem_filter.1 hS).2 ▸ chi_mem_lowDegree S)
+
+/-- **A function is the sum of its parts of each degree.** -/
+theorem sum_levelPart (f : (ι → Bool) → ℝ) (x : ι → Bool) :
+    ∑ k ∈ range (Fintype.card ι + 1), levelPart k f x = f x := by
+  simp only [levelPart]
+  rw [sum_fiberwise_of_maps_to (g := card)
+    (fun S _ => mem_range.2 (Nat.lt_succ_of_le (card_le_univ S))), sum_fourierCoeff_mul_chi]
+
+/-- **The parts of different degrees are orthogonal**, and the part of degree `k` has squared
+norm `W^k[f]` [O'Donnell, Exercise 1.18][ODonnell2014]. -/
+theorem expect_levelPart_mul_levelPart (j k : ℕ) (f : (ι → Bool) → ℝ) :
+    𝔼 x, levelPart j f x * levelPart k f x = if j = k then levelWeight k f else 0 := by
+  rw [expect_mul_eq_sum_fourierCoeff_mul]
+  simp only [fourierCoeff_levelPart]
+  split_ifs with h
+  · subst h
+    rw [levelWeight, sum_filter]
+    refine sum_congr rfl fun S _ => ?_
+    split_ifs <;> ring
+  · refine sum_eq_zero fun S _ => ?_
+    split_ifs with hj hk
+    · exact absurd (hj.symm.trans hk) h
+    all_goals simp
+
+/-- The part of degree `k` has squared norm `W^k[f]`. -/
+theorem expect_levelPart_sq (k : ℕ) (f : (ι → Bool) → ℝ) :
+    𝔼 x, levelPart k f x ^ 2 = levelWeight k f := by
+  simp only [sq, expect_levelPart_mul_levelPart, ↓reduceIte]
+
+/-- The Fourier weight at each degree is nonnegative. -/
+theorem levelWeight_nonneg (k : ℕ) (f : (ι → Bool) → ℝ) : 0 ≤ levelWeight k f :=
+  sum_nonneg fun _ _ => sq_nonneg _
+
+/-- There is no Fourier weight above the number of coordinates. -/
+theorem levelWeight_eq_zero_of_card_lt {k : ℕ} (hk : Fintype.card ι < k) (f : (ι → Bool) → ℝ) :
+    levelWeight k f = 0 := by
+  refine sum_eq_zero fun S hS => ?_
+  have := card_le_univ S
+  have := (mem_filter.1 hS).2
+  omega
+
+/-- **Parseval's identity by degree**: the Fourier weights sum to `𝔼 f ^ 2`. -/
+theorem sum_levelWeight (f : (ι → Bool) → ℝ) :
+    ∑ k ∈ range (Fintype.card ι + 1), levelWeight k f = 𝔼 x, f x ^ 2 := by
+  simp only [levelWeight]
+  rw [sum_fiberwise_of_maps_to (g := card)
+    (fun S _ => mem_range.2 (Nat.lt_succ_of_le (card_le_univ S))),
+    expect_sq_eq_sum_fourierCoeff_sq]
+
+/-- The Fourier weights of a Boolean function read as signs sum to one. -/
+theorem sum_levelWeight_signVal (f : (ι → Bool) → Bool) :
+    ∑ k ∈ range (Fintype.card ι + 1), levelWeight k (fun x => signVal (f x)) = 1 := by
+  rw [sum_levelWeight, expect_sq_eq_sum_fourierCoeff_sq, sum_fourierCoeff_sq_signVal]
+
+/-- The Fourier weight at degree zero is the squared mean. -/
+theorem levelWeight_zero (f : (ι → Bool) → ℝ) : levelWeight 0 f = (𝔼 x, f x) ^ 2 := by
+  have h : (univ.filter fun S : Finset ι => #S = 0) = {∅} := by
+    ext S
+    simp
+  rw [levelWeight, h, sum_singleton, fourierCoeff_empty]
+
+/-- The Fourier weights above level `t` are the Fourier weight on the sets of more than `t`
+coordinates. -/
+theorem sum_levelWeight_Ioc (f : (ι → Bool) → ℝ) (t : ℕ) :
+    ∑ k ∈ Ioc t (Fintype.card ι), levelWeight k f = ∑ S with t < #S, fourierCoeff f S ^ 2 := by
+  rw [← sum_fiberwise_of_maps_to (g := card) (t := Ioc t (Fintype.card ι))
+    (fun S hS => mem_Ioc.2 ⟨(mem_filter.1 hS).2, card_le_univ S⟩)]
+  refine sum_congr rfl fun k hk => ?_
+  rw [levelWeight, filter_filter]
+  refine sum_congr (filter_congr fun S _ => ?_) fun _ _ => rfl
+  exact ⟨fun h => ⟨h ▸ (mem_Ioc.1 hk).1, h⟩, fun h => h.2⟩
+
+end Cslib.BitString
diff --git a/Complexity/Foundations/BooleanAnalysis/SpectralSample.lean b/Complexity/Foundations/BooleanAnalysis/SpectralSample.lean
new file mode 100644
index 0000000000..b238198ee8
--- /dev/null
+++ b/Complexity/Foundations/BooleanAnalysis/SpectralSample.lean
@@ -0,0 +1,70 @@
+/-
+Copyright (c) 2026 Vincent Liew. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Vincent Liew
+-/
+
+module
+-- upstream: candidate
+
+public import Complexity.Foundations.BooleanAnalysis.LevelWeight
+public import Mathlib.Probability.ProbabilityMassFunction.Constructions
+import Mathlib.Basic.ENNReal.BigOperators
+
+/-!
+# The spectral sample
+
+By Parseval's identity, the squared Fourier coefficients of a Boolean function read as signs sum
+to one, so they are the probabilities of a distribution on sets of coordinates: the *spectral
+sample* `𝒮_f` (`BitString.spectralSample`), which draws `S` with probability `f̂(S) ^ 2`
+[O'Donnell, Definition 1.18][ODonnell2014]. The size of the spectral sample is `k` with probability
+the Fourier weight `W^k[f]` at degree `k` (`BitString.spectralSample_map_card_apply`).
+
+## Main definitions
+
+* `Cslib.BitString.spectralSample`: the spectral sample of a Boolean function.
+
+## Main results
+
+* `Cslib.BitString.spectralSample_apply`: the spectral sample draws `S` with probability
+  `f̂(S) ^ 2`.
+* `Cslib.BitString.spectralSample_map_card_apply`: its size is `k` with probability `W^k[f]`.
+
+## References
+
+* [R. O'Donnell, *Analysis of Boolean Functions*][ODonnell2014]
+-/
+
+public section
+
+namespace Cslib.BitString
+
+open Finset
+
+variable {ι : Type*} [Fintype ι] [DecidableEq ι]
+
+/-- The spectral sample of a Boolean function `f` read as signs: the distribution on sets of
+coordinates that draws `S` with probability `f̂(S) ^ 2` [O'Donnell, Definition 1.18][ODonnell2014].
+-/
+noncomputable def spectralSample (f : (ι → Bool) → Bool) : PMF (Finset ι) :=
+  PMF.ofFintype (fun S => ENNReal.ofReal (fourierCoeff (fun x => signVal (f x)) S ^ 2)) <| by
+    rw [← ENNReal.ofReal_sum_of_nonneg fun _ _ => sq_nonneg _, sum_fourierCoeff_sq_signVal,
+      ENNReal.ofReal_one]
+
+/-- The spectral sample draws `S` with probability `f̂(S) ^ 2`. -/
+theorem spectralSample_apply (f : (ι → Bool) → Bool) (S : Finset ι) :
+    spectralSample f S = ENNReal.ofReal (fourierCoeff (fun x => signVal (f x)) S ^ 2) := by
+  rw [spectralSample, PMF.ofFintype_apply]
+
+/-- **The size of the spectral sample** is `k` with probability the Fourier weight `W^k[f]` at
+degree `k` [O'Donnell, Definition 1.19][ODonnell2014]. -/
+theorem spectralSample_map_card_apply (f : (ι → Bool) → Bool) (k : ℕ) :
+    (spectralSample f).map card k = ENNReal.ofReal (levelWeight k fun x => signVal (f x)) := by
+  rw [PMF.map_apply, tsum_fintype, levelWeight,
+    ENNReal.ofReal_sum_of_nonneg fun _ _ => sq_nonneg _, sum_filter]
+  refine sum_congr rfl fun S _ => ?_
+  by_cases h : #S = k
+  · simp [h, spectralSample_apply]
+  · simp [h, Ne.symm h]
+
+end Cslib.BitString
diff --git a/ComplexityTests/BooleanAnalysisLevelWeight.lean b/ComplexityTests/BooleanAnalysisLevelWeight.lean
new file mode 100644
index 0000000000..68f3c67483
--- /dev/null
+++ b/ComplexityTests/BooleanAnalysisLevelWeight.lean
@@ -0,0 +1,54 @@
+/-
+Copyright (c) 2026 Vincent Liew. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Vincent Liew
+-/
+
+import Complexity.Foundations.BooleanAnalysis.SpectralSample
+import Complexity.Foundations.Data.Finset.Expect
+
+/-! # Fourier weight by degree on a small cube
+
+The conjunction of two bits, read as signs, has mean `1 / 2`, so its Fourier weight at degree
+zero is `1 / 4`; the weights at degrees one and two make up the remaining `3 / 4`, and its
+spectral sample is empty with probability `1 / 4`. A character's degree part is itself at its own
+degree and zero at every other.
+-/
+
+namespace ComplexityTests.BooleanAnalysisLevelWeight
+
+open Cslib Cslib.BitString Finset
+
+/-- The conjunction of two bits. -/
+def and₂ (x : Fin 2 → Bool) : Bool :=
+  x 0 && x 1
+
+/-- The conjunction, read as signs, has Fourier weight `1 / 4` at degree zero. -/
+theorem levelWeight_zero_and₂ : levelWeight 0 (fun x => signVal (and₂ x)) = 1 / 4 := by
+  rw [levelWeight_zero]
+  simp only [Fin.expect_univ_cons, Fintype.expect_bool, Fintype.expect_unique]
+  norm_num [and₂]
+
+-- The weights at degrees one and two make up the rest of the unit total weight.
+example : levelWeight 1 (fun x => signVal (and₂ x)) + levelWeight 2 (fun x => signVal (and₂ x)) =
+    3 / 4 := by
+  have h := sum_levelWeight_signVal and₂
+  simp only [Fintype.card_fin, sum_range_succ, sum_range_zero, zero_add,
+    levelWeight_zero_and₂] at h
+  linarith
+
+-- The spectral sample of the conjunction is empty with probability `1 / 4`.
+example : (spectralSample and₂).map card 0 = ENNReal.ofReal (1 / 4) := by
+  rw [spectralSample_map_card_apply, levelWeight_zero_and₂]
+
+-- A character is its own degree part at its degree, and has no part at any other degree.
+example (S : Finset (Fin 3)) (T : Finset (Fin 3)) :
+    fourierCoeff (levelPart #S (chi ℝ S)) T = fourierCoeff (chi ℝ S) T ∧
+    ∀ k ≠ #S, fourierCoeff (levelPart k (chi ℝ S)) T = 0 := by
+  refine ⟨?_, fun k hk => ?_⟩
+  · rw [fourierCoeff_levelPart, fourierCoeff_chi]
+    split_ifs with h₁ h₂ <;> simp_all
+  · rw [fourierCoeff_levelPart, fourierCoeff_chi]
+    split_ifs with h₁ h₂ <;> simp_all
+
+end ComplexityTests.BooleanAnalysisLevelWeight