# DP scree in dppca

This article describes the scree estimation used in the `dppca` package. The main idea is to rewrite each scree value as a mean estimation problem.

The package considers three main methods as follows.

1. Clipped mean using Gaussian noise
2. Huber GDP mean based on noisy gradient descent
3. Private modified winsorized mean (PMWM)

## Overview

Let \[ X \in \mathbb{R}^{n \times p} \]

be the preprocessed data matrix. This means that the data have been centered, and possibly standardized.

Let \[ V_k = [v_1,\ldots,v_k] \in \mathbb{R}^{p \times k} \]

be the matrix of PC directions. Depending on the analysis, \(V_k\) may be non-private or differentially private.

The goal is to estimate the scree vector \(\lambda = (\lambda_1,\ldots,\lambda_k)\) in a differentially private way.

In the sample PCA case, the \(\ell\)-th scree value is

\[ \hat\lambda_\ell = v_\ell^\top \hat\Sigma v_\ell \quad \text{where} \quad \hat\Sigma = \frac{1}{n-1}X^\top X. \]

In `dppca`, we construct private estimates \(\widetilde\lambda_1,\ldots,\widetilde\lambda_k\) and use them to build a differentially private scree plot.

### Scree Estimation in dppca

For the \(\ell\)-th principal component direction \(v_\ell\), define the score vector \(z_\ell = X v_\ell\).

The sample variance of the \(\ell\)-th score vector is

\[ \hat\lambda_\ell = \frac{1}{n-1} \sum_{i=1}^n z_{i\ell}^2 = \frac{n}{n-1} \left( \frac{1}{n}\sum_{i=1}^n w_{i\ell} \right) \quad \text{where} \quad w_{i\ell} = z_{i\ell}^2. \]

Therefore, for each component \(\ell\), scree estimation can be viewed as a mean estimation problem for \(w_{1\ell},\ldots,w_{n\ell}\).

In `dppca`, we want to construct a differentially private estimate of \(\widehat{\lambda}_\ell\), denoted by \(\widetilde{\lambda}_\ell\). So, we first privately estimate the mean of \(W_{1\ell},\ldots,W_{n\ell}\), and then rescale it by \(n/(n-1)\).

\[ \widetilde{\lambda}_\ell = \frac{n}{n-1} \cdot \operatorname{DP-Mean}\left(w_{1\ell},\ldots,w_{n\ell}\right). \]

## Method 1: Clipped scree estimation

The simplest way to privately estimate the mean of \(w_{1\ell},\ldots,w_{n\ell}\) is to first clip the values to a bounded interval and then add Gaussian noise.

### Clipping

Choose a clipping threshold \[ C > 0. \]

Define the clipped observations by \(w_{i\ell}^{\mathrm{clip}} = \min(w_{i\ell}, C)\).

Then \[ 0 \leq w_{i\ell}^{\mathrm{clip}} \leq C. \]

The clipped empirical mean is

\[ \hat\mu_\ell^{\mathrm{clip}} = \frac{1}{n} \sum_{i=1}^n w_{i\ell}^{\mathrm{clip}} = \frac{1}{n} \sum_{i=1}^n \min(w_{i\ell}, C). \]

The corresponding non-private clipped scree estimate is \[ \hat\lambda_\ell^{\mathrm{clip}} = \frac{n}{n-1} \hat\mu_\ell^{\mathrm{clip}}. \]

### Sensitivity

Because each clipped observation lies in \([0,C]\), changing one observation can change the mean by at most \(\frac{C}{n}\).

After multiplying by \(n/(n-1)\), the sensitivity of the clipped scree estimate is

\[ \Delta_\ell \leq \frac{n}{n-1} \cdot \frac{C}{n} = \frac{C}{n-1}. \]

### DP clipped scree estimate

For privacy parameters \((\epsilon_\ell,\delta_\ell)\), the Gaussian mechanism releases

\[ \widetilde\lambda_\ell^{\mathrm{clip}} = \hat\lambda_\ell^{\mathrm{clip}} + \xi_\ell, \]

where

\[ \xi_\ell \sim N(0,\sigma_\ell^2) \] and \[ \sigma_\ell = \frac{\Delta_\ell\sqrt{2\log(1.25/\delta_\ell)}}{\epsilon_\ell} = \frac{(C/(n-1))\sqrt{2\log(1.25/\delta_\ell)}}{\epsilon_\ell}. \]

### Parameter

Clipped scree estimation is simple and easy to implement, but it depends on the choice of the clipping threshold \(C\).

- If \(C\) is too small, the scree values may be underestimated.
- If \(C\) is too large, the required privacy noise may increase.

### Example usage

In the `dppca`, we set clipped-mean parameter by

\[ \texttt{scree\_clipped\_control(C\_clip = C)}. \]

    library(dppca)
    dp_scree(
      X,
      k = 3,
      method = "clipped",
      eps_total = 1,
      delta_total = 1e-6,
      center = TRUE,
      standardize = FALSE,
      control = clipped_control(
        C_clip = 3
      )
    )

## Method 2: Huber scree estimation

The Huber method is based on the differentially private robust mean estimator of Yu, Ren, and Zhou (2024). For each component \(\ell\), it estimates the mean of \(w_{1\ell},\ldots,w_{n\ell}\) using a robust loss function instead of the ordinary sample mean.

### Huber loss

For a robustification parameter \(\tau > 0\), the Huber loss is

\[ \rho_\tau(r) = \begin{cases} \frac{1}{2}r^2, & |r| \leq \tau,\\ \tau |r| - \frac{1}{2}\tau^2, & |r| > \tau. \end{cases} \]

For small residuals, the Huber loss behaves like squared error loss. For large residuals, it grows linearly, which reduces the influence of extreme observations.

The derivative of the Huber loss is the score function

\[ \psi_\tau(r) = \rho_\tau'(r) = \begin{cases} -\tau, & r < -\tau,\\ r, & |r| \leq \tau,\\ \tau, & r > \tau. \end{cases} \]

Thus, \(\psi_\tau(r)\) clips the residual \(r\) to the interval \([-\tau,\tau]\).

### Huber mean estimator

For the \(\ell\)-th component, the Huber mean estimator is defined as

\[ \hat\mu_{\tau,\ell} \in \arg\min_{\mu \in \mathbb{R}} \frac{1}{n} \sum_{i=1}^n \rho_\tau(w_{i\ell}-\mu). \]

The first-order condition is

\[ \frac{1}{n} \sum_{i=1}^n \psi_\tau(w_{i\ell}-\mu) = 0. \]

This equation shows that large residuals are clipped through the score function. As a result, the estimator is less sensitive to outliers than the ordinary mean.

### Noisy gradient descent

The Huber objective is optimized using noisy gradient descent.

Given the current estimate \(\mu_\ell^{(t)}\) at iteration \(t\), we define the corresponding residuals as

\[ r_{i\ell}^{(t)} = w_{i\ell} - \mu_\ell^{(t)}, \qquad i=1,\ldots,n. \]

The clipped score values are \(\psi_\tau(r_{i\ell}^{(t)})\),

and the average gradient type quantity is

\[ g_\ell^{(t)} = \frac{1}{n} \sum_{i=1}^n \psi_\tau(r_{i\ell}^{(t)}). \]

A non-private update would take the form

\[ \mu_\ell^{(t+1)} = \mu_\ell^{(t)} + \eta_t g_\ell^{(t)} \quad \text{where} \quad \eta_t > 0 \quad \text{is the step size.} \]

To ensure privacy, Gaussian noise is added at each iteration.

\[ \mu_\ell^{(t+1)} = \mu_\ell^{(t)} + \eta_t g_\ell^{(t)} + \xi_\ell^{(t)} \quad \text{where} \quad \xi_\ell^{(t)} \sim N(0,\sigma_{t,\ell}^2). \]

### Sensitivity of one gradient step

Since

\[ \psi_\tau(r) \in [-\tau,\tau], \]

changing one observation can change the average score by at most \(\frac{2\tau}{n}\).

After multiplying by the step size \(\eta_t\), the one-step sensitivity is

\[ \Delta_{\mathrm{step}} = \frac{2\eta_t \tau}{n}. \]

This sensitivity determines the scale of the Gaussian noise added at each gradient step.

### DP Huber scree estimate

After \(T\) noisy gradient descent steps, let the final private Huber mean estimate be

\[ \mu_\ell^{(T)}. \]

The final Huber scree estimate is

\[ \widetilde\lambda_\ell^{\mathrm{Huber}} = \frac{n}{n-1} \mu_\ell^{(T)}. \]

The Huber method is useful when the squared scores \(w_{i\ell}\) may be heavy-tailed or affected by outliers.

### Robustification parameter

The robustification parameter \(\tau\) controls the trade-off between bias, robustness, and privacy.

A small \(\tau\) gives stronger clipping and more robustness, but may increase the bias of the estimator. A large \(\tau\) behaves more like the ordinary mean, but may be less robust and may require more noise.

In the Huber DP mean, \(\tau\) is chosen using a scale quantity related to the second moment. A typical theoretical form is

\[ \tau \asymp \sqrt{m_2} \left( \frac{\epsilon n} {\sqrt{(d+\log n)\log n}} \right)^{1/2}, \]

where

\[ m_2 = \mathbb{E}\|X-\mu\|_2^2 \quad \text{is a second-moment scale.} \]

Because \(m_2\) is usually unknown and can be sensitive to outliers, it may need to be estimated robustly and privately. we use a private robust estimator for \(m_2\) described in Algorithm 2.

### Other control parameters

The Huber scree method also uses additional control parameters for noisy gradient descent and for the private scale-proxy step used to estimate \(m_2\). The default values are chosen based on the recommendations in the Yu, Ren, and Zhou (2024)

- `mu0`: Initial value for noisy gradient descent. (default: \(\mu_\ell^{(0)} = 0\))

- `eta0`: Step size for noisy gradient descent. (default: \(\eta_0 = 1\))

- `T`: Number of noisy gradient descent iterations. (default: \(T = \lfloor \log n \rfloor\))

- `M`: Number of blocks used in the private estimator for \(m_2\). (default: \(M = \lfloor \sqrt{n}/2 \rfloor\))

- `k_min_m2` and `k_max_m2`: Lower and upper dyadic bin indices used in the private histogram step for estimating \(m_2\). The histogram searches over scale levels \(2^k\) for

  \[ k_{\min} \le k \le k_{\max}. \]

- `m2_frac`: Fraction of the Huber scree privacy budget used to privately estimate \(m_2\).

  If \((\epsilon_{\mathrm{scree}},\delta_{\mathrm{scree}})\) is the budget for Huber scree estimation, then \((\epsilon_{m_2},\delta_{m_2}) = \text{m2_frac}\cdot (\epsilon_{\text{scree}},\delta_{\text{scree}})\), while the remaining budget \((\epsilon_{\text{gd}},\delta_{\text{gd}}) = (1-\text{m2_frac})\cdot (\epsilon_{\text{scree}},\delta_{\text{scree}})\) is used for Huber noisy gradient descent.

### Example usage

In `dppca`, the Huber scree estimator is controlled by

\[ \texttt{scree\_huber\_control( mu0,\ eta0,\ T,\ M,\ k\_min\_m2,\ k\_max\_m2,\ m2\_frac)}. \]

    dp_scree(
      X,
      k = 3,
      method = "huber",
      eps_total = 1,
      delta_total = 1e-6,
      center = TRUE,
      standardize = FALSE,
      control = huber_control(
        mu0 = 0,
        eta0 = 1,
        T = 50,
        M = 20,
        k_min_m2 = -40,
        k_max_m2 = 40,
        m2_frac = 1/4
      )
    )

## Method 3: Private modified winsorized mean scree estimation

The PMWM method is based on the private modified winsorized mean of Ramsay and Spicker (2025). For each component \(\ell\), it estimates the mean of \(w_{1\ell},\ldots,w_{n\ell}\) by privately estimating tail quantiles, winsorizing the data, and then adding noise to the winsorized mean.

### Non-private modified winsorized mean

The PMWM method builds on the non-private modified winsorized mean of Lugosi and Mendelson (2021).

The non-private modified winsorized mean starts with a clipping proportion \(0 < p < \frac{1}{2}\), which determines the lower and upper tail quantiles used for winsorization.

Let \(\hat\xi_p \quad\text{and}\quad \hat\xi_{1-p}\) be empirical lower and upper quantiles.

Define the function

\[ \phi_{a,b}(x) = \begin{cases} a, & x < a,\\ x, & a \leq x \leq b,\\ b, & x > b. \end{cases} \]

The non-private modified winsorized mean is

\[ \hat\mu_p = \frac{1}{n} \sum_{i=1}^n \phi_{\hat\xi_p,\hat\xi_{1-p}}(X_i). \]

The idea is to first estimate the lower and upper tail cutoffs, and then winsorize the data by replacing extreme values with the corresponding cutoffs.

### Sample splitting notation

In the theoretical description, the data may be split into two subsets:

- a quantile estimation subset with size \(n_q\),
- a mean estimation subset with size \(n_m\).

If the full sample size is \(n\), a simple split is \[ n_q = n_m = \frac{n}{2}. \]

In practice, the implementation may use all available observations at each step instead of splitting the sample.

### Private quantile estimation

PMWM uses private quantile estimates instead of non-private empirical quantiles. For component \(\ell\), let the clipping proportion be \[ p_\ell = \zeta_\ell. \]

A theoretical choice has the form

\[ \zeta_\ell = 16\eta + \frac{112}{3} \frac{ \log\left(32(\beta(u-\ell)/(\beta-1)\vee 1)/\delta\right) } {n_q}, \]

where

- \(\eta\): Contamination level,
- \(\beta > 1\): Log-grid parameter in the private quantile estimator,
- \(\ell, u\): Lower and upper search bounds used in private quantile estimation.
- \(\delta\): Confidence parameter controlling the high-probability accuracy statement of the PMWM estimator.
- \(n_q\): Number of observations used for quantile estimation.

For practical implementation, the paper suggests using the clipping proportion

\[ p = \frac{C}{n_q} \vee \eta, \]

where \(C\) is a user-chosen trimming constant. This is also the choice used in the `dppca` package.

### Winsorized mean

Let \(L_\ell\) and \(U_\ell\) be private estimates of the lower and upper quantiles.

\[ L_\ell \approx Q_{\zeta_\ell}(w_{1\ell},\ldots,w_{n\ell}), \quad \text{and} \quad U_\ell \approx Q_{1-\zeta_\ell}(w_{1\ell},\ldots,w_{n\ell}). \]

Define the winsorized observations by

\[ w_{i\ell}^{\mathrm{win}} = \min\left\{ \max(w_{i\ell},L_\ell), U_\ell \right\}. \]

Equivalently,

- if \(w_{i\ell} < L_\ell\), replace it by \(L_\ell\);
- if \(L_\ell \leq w_{i\ell} \leq U_\ell\), keep it unchanged;
- if \(w_{i\ell} > U_\ell\), replace it by \(U_\ell\).

Thus, \[ L_\ell \leq w_{i\ell}^{\mathrm{win}} \leq U_\ell. \]

Using the mean estimation subset \(I_m\), define \(\hat\mu_\ell^{\mathrm{win}} = \frac{1}{n_m} \sum_{i\in I_m} w_{i\ell}^{\mathrm{win}}\).

The corresponding non-private winsorized scree estimate is

\[ \hat\lambda_\ell^{\mathrm{PMWM,np}} = \frac{n}{n-1} \hat\mu_\ell^{\mathrm{win}}. \]

### Sensitivity

Because all winsorized observations lie in \([L_\ell,U_\ell]\), the sensitivity of the winsorized mean is

\[ \Delta_\ell^{(\mu)} = \frac{U_\ell-L_\ell}{n_m}. \]

After multiplying by \(n/(n-1)\), the scree sensitivity is \(\Delta_\ell^{(\lambda)} = \frac{n}{n-1} \cdot \frac{U_\ell-L_\ell}{n_m}\).

### DP PMWM scree estimate

The final PMWM scree estimate can be written as

\[ \widetilde\lambda_\ell^{\mathrm{PMWM}} = \hat\lambda_\ell^{\mathrm{PMWM,np}} + Z_\ell \quad \text{where} \quad Z_\ell \sim N(0,\sigma_\ell^2). \]

For privacy parameters \((\epsilon_{M,\ell},\delta_{M,\ell})\), the noise scale is

\[ \sigma_\ell = \frac{ \Delta_\ell^{(\lambda)} \sqrt{2\log(1.25/\delta_{M,\ell})} } {\epsilon_{M,\ell}} = \frac{ \left( \frac{n}{n-1} \cdot \frac{U_\ell-L_\ell}{n_m} \right) \sqrt{2\log(1.25/\delta_{M,\ell})} } {\epsilon_{M,\ell}}. \]

### Privacy budget splitting

PMWM uses privacy budget for both private quantile estimation and private mean estimation.

For component \(\ell\), let the total component-level budget be \((\epsilon_\ell,\delta_\ell)\).

This can be split as \[ \epsilon_\ell = \epsilon_{Q,\ell} + \epsilon_{M,\ell}, \qquad \delta_\ell = \delta_{Q,\ell} + \delta_{M,\ell}. \]

The quantile budget itself is used to estimate two quantiles, so it can be split again.

\[ \epsilon_{Q,\ell} = \epsilon_{q1,\ell} + \epsilon_{q2,\ell}, \qquad \delta_{Q,\ell} = \delta_{q1,\ell} + \delta_{q2,\ell}. \]

### Parameters

The PMWM scree estimator uses additional parameters for private quantile estimation and winsorization.

- `beta`: Log-binning base used in the private quantile estimator.

  It determines the spacing of the geometric search grid and must satisfy \(\beta > 1\).

- `a`, `b`: Lower and upper search bounds supplied to the private quantile. The private lower and upper clipping cutoffs are searched within this range.

- `trim_const`, `eta`: Parameters used to set the practical clipping proportion \[ p = \min\left\{ \max\left(\frac{\texttt{trim\_const}}{n_q}, \eta\right), 0.49 \right\}. \] Here, `trim_const / n_q` controls the baseline clipping level, while `eta` gives a lower bound reflecting the expected contamination level.

- `split_mode`: Logical value indicating whether the sample is split into two parts.

  If `TRUE`, one part is used for private quantile estimation and the other part is used for the winsorized mean step. If `FALSE`, all observations are used in both steps.

- `max_extra_bins`: Maximum number of additional log-grid bins searched beyond the largest occupied bin in the private quantile.

### Example usage

In `dppca`, the PMWM-specific parameters can be specified through

\[ \text{scree_pmw_control( beta = beta,a = a,b = b, trim_const = C, eta = eta split_mode = split)} \]

    dp_scree(
      X,
      k = 3,
      method = "pmwm",
      eps_total = 1,
      delta_total = 1e-6,
      center = TRUE,
      standardize = FALSE,
      control = pmwm_control(
        beta = 1.01,
        a = 0,
        b = 10,
        trim_const = 10,
        eta = 0.01,
        split_mode = TRUE
      )
    )

## Post-processing

Because of the added privacy noise, the raw DP scree estimates

\[ (\widetilde\lambda_1,\ldots,\widetilde\lambda_k) \]

may not have the usual scree shape. They may not be decreasing, and some values may be negative.

In ordinary PCA, scree values satisfy

\[ \lambda_1 \ge \lambda_2 \ge \cdots \ge \lambda_k \ge 0. \]

Therefore, `dppca` can apply post-processing to make the DP scree estimates nonnegative and decreasing.

\[ \widetilde\lambda_1^{\mathrm{mono}} \ge \widetilde\lambda_2^{\mathrm{mono}} \ge \cdots \ge \widetilde\lambda_k^{\mathrm{mono}} \ge 0. \]

If monotone post-processing is used, the PVE can be computed from the post-processed scree values.

\[ \widetilde{\operatorname{PVE}}_\ell^{\mathrm{mono}} = \frac{\widetilde\lambda_\ell^{\mathrm{mono}}} {\sum_{j=1}^k \widetilde\lambda_j^{\mathrm{mono}}}. \]

This step only modifies the already released DP estimates, so it does not use any additional privacy budget.

### Example usage

    dp_scree_plot(
      X,
      k = 3,
      dp_scree_method = "clipped",
      eps_total = 1,
      delta_total = 1e-6,
      control = clipped_control(
        C_clip = 3
      )
    )

    dp_scree_plot(
      X,
      k = 3,
      dp_scree_method = "all",
      eps_total = 1,
      delta_total = 1e-6,
      center = TRUE,
      standardize = FALSE,
      control = list(
        clipped = clipped_control(
          C_clip = 3
        ),
        pmwm = pmwm_control(
          beta = 1.01,
          a = 0,
          b = 10,
          trim_const = 10,
          eta = 0.01,
          split_mode = TRUE
        ),
        huber = huber_control(
          mu0 = 0,
          eta0 = 1,
          T = 50,
          M = 20,
          k_min_m2 = -40,
          k_max_m2 = 40,
          m2_frac = 1/4
        )
      )
    )

## References

Myeonghun Yu. Zhao Ren. Wen-Xin Zhou. “Gaussian differentially private robust mean estimation and inference”. Bernoulli 30 (4) 3059 - 3088, November 2024. https://doi.org/10.3150/23-BEJ1706

Kelly Ramsay and Dylan Spicker. (2025). “Improved subsample-and-aggregate via the private modified winsorized mean”. arXiv preprint. https://arxiv.org/abs/2501.14095

Gábor Lugosi. Shahar Mendelson. “Robust multivariate mean estimation: The optimality of trimmed mean.” Ann. Statist. 49 (1) 393 - 410, February 2021. https://doi.org/10.1214/20-AOS1961