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Consider a random variable following an AR(1) model:

$$x_t = \mu+\rho x_{t-1} + \epsilon_t$$

The unconditional expected value of this proces is:

$$E(x)=\mu/(1-\rho)$$

Now take a rolling mean of $x_t$ of n periods including the current: $$\bar{x} = \frac{1}{n}\sum_{i=0}^{n-1} x_{t-i}$$

Based on Wiki: Sum of normally distributed random variables the expected value is still just the sum of the expected value for each $x_t$ which is given above, hence the expected value of $\bar{x}_t$ is:

$$E(\bar{x}_t)=1/n \sum_{i=0}^{n-1} E(x_{t-i})=n/n E(x_t)=E(x_t)$$

However, another approach is to roll back each observation in the summand to a common starting point using (this is with thanks to Aleksej at this post here): $$x_t = \mu \sum_{i=0}^{k-1} \rho^i + \sum_{i=0}^{k-1} \rho^i \epsilon_{t-i} + \rho^k x_{t-k}$$

Inserting this into the expression for $\bar{x}_t$ yields: $$\bar{x}_t=1/n\sum_{i=0}^{n-1}(\rho^{n-i}x_{t-n}+\mu\sum_{j=0}^{k-i-1}\rho^j+\sum_{j=0}^{k-i-1}\rho^{j}\epsilon_{t-i-j})$$

$$=1/n(\sum_{i=0}^{n-1}x_{t_n}\rho^{n-i}+ \mu\sum_{i=0}^{n-1}\sum_{j=0}^{n-i-1}\rho^{j}+\sum_{i=0}^{n-1}\sum_{j=0}^{n-i-1}\rho^j\epsilon_{t-i-j})$$

Taking the expection to this yields: $$E(\bar{x})=1/n(E[x_{x-t}]\sum_{i=0}^{n-1}\rho^{n-i}+\mu\sum_{i=0}^{n-1}\sum_{j=0}^{n-i-1}\rho^{j})$$

$$=\frac{1}{n}(\frac{\mu}{1-\rho}\sum_{i=0}^{n-1}\rho^{n-i}+\mu\sum_{i=0}^{n-1}\sum_{j=0}^{n-i-1}\rho^{j})$$ But this does not coincide with the previous results using the sum of normal distributed variables.

Can someone point out, if I have made a mistake and which method is the correct one?

EDIT: Simulation study

Based on the comment from mlofton, I have conducted a simulation study, and I find that the expected difference and the simulated difference are indeed very close. Using the formula from the model yields the same as average of the sum of the unconditional means for $$x_t$$

import matplotlib.pyplot as plt
import numpy as np


def simulate_ar_process(initial_value=None, intercept=-0.01, slope=0.5, noise_vol=0.005, obs=100):
    # If initial value is None use the unconditional mean
    if initial_value is None:
        initial_value = intercept / (1-slope)

    errors = np.random.normal(0, noise_vol, obs)
    pd_levels = []

    # Estimate PD-levels
    for err in errors:
        if len(pd_levels) == 0:
            pd_levels.append(initial_value + err)
        else:
            new_obs = intercept + pd_levels[-1] * slope + err
            pd_levels.append(new_obs)

    return pd_levels


def calculate_rolling_mean(pd_levels, look_back=20, burn_in=0):
    mas = []
    for i in range(look_back+burn_in, len(pd_levels)):
        current_range = pd_levels[i-look_back:i]
        mas.append(np.mean(current_range))

    return np.mean(mas)


intercept = -0.01
slope = 0.9
noise_vol = 0.005
obs = 1000
look_back = 20

average_of_rolling_means = []
average_of_x = []
for i in range(100):
    x = simulate_ar_process(intercept=intercept, slope=slope, noise_vol=noise_vol, obs=obs)
    average_of_rolling_means.append(calculate_rolling_mean(x, look_back=look_back))
    average_of_x.append(np.mean(x[20:]))

plt.hist(average_of_x, label="Average of x", alpha=0.3)
plt.hist(average_of_rolling_means, label="Average of rolling mean", alpha=0.3)
plt.legend()

# Calcualte difference
diffs = [x-y for x, y in zip(average_of_x, average_of_rolling_means)]
plt.hist(diffs)
np.average(diffs)

# Theoretical diff
unconditional_mean = intercept / (1-slope)
unconditional_mean_mva = unconditional_mean * np.sum([slope**(look_back-i) for i in range(0, look_back)])
unconditional_mean_mva += intercept * np.sum([slope ** j for i in range(0, look_back) for j in range(0, look_back-i)])
unconditional_mean_mva /= look_back

print("Expected difference: {}".format(unconditional_mean - unconditional_mean_mva))
print("Found difference: {}".format(np.average(diffs)))
#Expected difference: 0.0
#Found difference: 9.324487397407016e-06

However, I cannot see from the formula above, that $$E(\bar{x})=E(x)$$ Can someone point me in the right direction in the deviation of this equality?

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    $\begingroup$ Hi: I wouldn't expect them to be the same because, for the one where you use the result from the Wiki, you're not taking the model into account. The Wiki result would be true for any iid random variable which is normally distributed. The second result-derivation applies due to the model specifics for $x_t$ and $x_t$ is not iid given the model. $\endgroup$ – mlofton Feb 24 at 18:00
  • $\begingroup$ Hi: Thank you for your comment. I have updated my answer with a simulation study. If you post your original comment and perhaps can give a pointer or two with the remaining part, I'll mark it as the answer. $\endgroup$ – RVA92 Feb 25 at 7:47
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    $\begingroup$ @mlofton: the linearity of expectation always hold, regardless of the dependence structure between the random variables. $\endgroup$ – Dayne Feb 26 at 5:50
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    $\begingroup$ Nice derivation and you are absolutely correlation about the correlation structure. My mistake. But when the OP wrote $E(\bar x) = E(x_{i})$, the expectation of the $x_t$ in an AR(1) are not the same unless one refers to the long term unconditional mean which is exactly what you used in your nice derivation. @RVA92: When you wrote the expression for $\bar{x}$, I didn't know that you were referring to the unconditional mean of $x$. The expression does hold in that case. After I send this, I delete my other comments and answer since I was totally confused by the equality. $\endgroup$ – mlofton Feb 27 at 18:18
  • $\begingroup$ In case, above confuses things even more, I was referring to E(x_t) given the previous $x_t$. That expectation is not constant. because of the dependence structure in the AR(1). For the long term mean, you're absolutely correct. $\endgroup$ – mlofton Feb 27 at 18:21
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There seems to be an error in your calculations. Using a slightly different notation for clarity.

Let the AR(1) process be: $$X_t = \mu + \phi X_{t-1} + e_t$$

Define mean of $n$ periods be:

$$\mu_n \equiv \frac{1}{n}\sum\limits_{t=1}^n X_t$$

The expectation operator is linear regardless of dependence structure between the random variables. So, your first expression is correct:

$$\mathbb E(\mu_n) = \mathbb E(X_t) = \frac{\mu}{1-\phi}$$

Second approach, for $t>1$:

\begin{align} X_t &= \mu + \phi X_{t-1} + e_t \\ &= \phi^{t-1}X_1+ \sum\limits_{i=0}^{t-2} \Big(\phi^{i}\mu +\phi^{i}e_{t-i}\Big) \tag{1} \end{align}

Using $(1)$: \begin{align} n\mu_n &= \sum\limits_{t=1}^n\phi^{t-1}X_1 +\sum\limits_{t=2}^n \sum\limits_{i=0}^{t-2} \Big(\phi^{i}\mu +\phi^{i}e_{t-i}\Big) \\ &= \sum\limits_{t=1}^n\phi^{t-1}X_1 +\sum\limits_{t=2}^n \sum\limits_{i=0}^{t-2} \phi^{i}\mu +\sum\limits_{t=2}^n \sum\limits_{i=0}^{t-2}\phi^{i}e_{t-i} \tag{2} \end{align}

In equation (2) the middle term is non-random and the last term has expected value of $0$. So, \begin{align} n\mathbb E(\mu_n) &= \sum\limits_{t=1}^n\phi^{t-1}\mathbb E(X_1) + \sum\limits_{t=2}^n \sum\limits_{i=0}^{t-2} \phi^{i}\mu \\ &= \frac{\mu}{1-\phi}\sum\limits_{t=1}^n\phi^{t-1} + \sum\limits_{t=2}^n \sum\limits_{i=0}^{t-2} \phi^{i}\mu \\ &= \frac{\mu}{1-\phi}\sum\limits_{t=1}^n\phi^{t-1} + \sum\limits_{t=2}^n \bigg( \frac{\mu(1-\phi^{t-1})}{1-\phi}\bigg) \\ &= \frac{\mu}{1-\phi} \bigg( \sum\limits_{t=1}^n\phi^{t-1} + \sum\limits_{t=2}^n (1-\phi^{t-1})\bigg) \\ &=n \frac{\mu}{1-\phi} \end{align}

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    $\begingroup$ Hi Dayne: I got it to match using your approach, so thank you. $\endgroup$ – RVA92 Feb 26 at 7:46

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