Say you're implementing basic importance sampling. To approximate a distribution $p(x)$, you utilize an importance density $q(x)$ that you can sample from. Also, assume that you can only evaluate $p_u(x) = C_p p(x)$, the target density up to a constant of proportionality.

So you sample $X^i \sim q(x)$ for $i=1,\ldots,n$ and you weight each of these samples with $W^i = p_u(X^i)/q(X^i)$, where $p_u(x) = C_p p(x)$. By the law of large numbers, a consistent estimator of $C_p$ is $\hat{C}_p = \frac{1}{N}\sum_i W^i$.

However when you can't evaluate $q(x)$ exactly either, but instead can only evaluate $q_u(x) = C_q q(x)$, your weights become $W^i_a = p_u(X^i)/q_u(X^i)$, and your average weight is no longer a consistent estimate of $C_p$

Auxiliary Particle Filter decomposition

It seems to me that with auxiliary particle filters, the weight updates are sort of like this second situation. Here's a hopefully useful decomposition of the time $t$ smoothed distribution.

\begin{align*} p(x_{1:t},k|y_{1:t}) &= C^{-1} \frac{g(y_t|x_t)f(x_t|x_{t-1},k)p(k|x_{1:t-1},y_{1:t-1}) }{q_t(x_t,k|y_{t},x_{t-1})} \\ & \hspace{20mm} \times \frac{p(x_{r+1:t-1},y_{r+:t-1}|x_r)}{q(x_{r+1:t-1}|y_{r+1:t-1})} \\ & \hspace{20mm} \times q_t(x_t,k|y_{1:t},x_{1:t-1}) q(x_{r+1:t-1}|y_{r+1:t-1})p(x_{1:r}|y_{1:r}) \end{align*}

Notation: this is a state space model with observations $y_t$, and hidden states $x_t$. $g(y_t|x_t)$ is the observation density, $f(x_t|x_{t-1})$ is the state transition density, $p(k|x_{1:t-1},y_{1:t-1})$ is the mass function of the previous time's particle index. The time $r$ is the time of the last resampling step. And $q_t(x_t,k|y_{1:t},x_{t-1})$ is your proposal distribution. Sorry that this notation isn't standard.

My question

This $q_t(x_t,k|y_{t},x_{t-1})$, it seems like everything I read describes an implementation where the user is not not evaluating this exactly, just up to proportionality. Right now I have

$$ q^{\text{apf}}_t(x_t,k|y_{1:t},x_{1:t-1}) = \frac{f(x_t|x_{t-1},k)g(y_t|\mu_t[x_{t-1},k])p(k|x_{1:t-1}, y_{1:t-1})}{\sum_{k'} g(y_t|\mu_t[x_{t-1},k'])p(k'|x_{1:t-1}, y_{1:t-1})}, $$ exactly. So the weight update at time $t$ for particle $i$ $$ \frac{g(y_t|X^i_t)f(X_t^i|x_{t-1}^i,k^i)p(k^i|x_{1:t-1},y_{1:t-1}) }{q_t(x_t^i,k^i|y_{t},x^i_{t-1})} $$ simplifies to $g(y_t|X_t^i)/g(y_t|\mu_t[x_{t-1}^i, k^i])$ if you use the non-standardized evaluation of $q_t^{\text{apf}}$. For example wiki seems to have it this way. . These are the so-called second stage weights from Pitt and Shephard's original paper. But it simplifies to

$$ \frac{ g(y_t|X^i_t)}{g(y_t|\mu_t[x^i_{t-1},k^i])} \sum_{k'} g(y_t|\mu_t[x_{t-1},k'])p(k'|x_{1:t-1}, y_{1:t-1}) , $$ if I use the exact $q_t$ and spend that extra time normalizing it.

If I want estimates of $p(y_t|y_{1:t-1})$ at every time I should use the latter, correct? Since there are only two possible answers, I could code this up and find out that way, but if anyone wants to weigh in on this in the meantime, I'm all ears.


I was right about having to normalize the function that evaluates the importance/proposal density. At least if you want to use the usual formula to estimate at each time point $p(y_t|y_{1:t-1})$. For what it's worth, here's an answer for other people to reference.


Evaluate the normalized importance density so you can get these weight updates: $$ \alpha_t^i \overset{\text{def}}{=} \frac{ g(y_t|X^i_t)}{g(y_t|\mu_t[x_{t-1},k^i])}\sum_{k} g(y_t|\mu_t[x_{t-1},k])p(k|x_{1:t-1}, y_{1:t-1}) , $$ then use that to get estimates of the conditional likelihoods with the formula $$ \hat{p}(y_t|y_{1:t-1}) = \sum_i \widetilde{W}^i_{r+1:t-1}\alpha_t^i. $$ where $\widetilde{W}^i_{r+1:t-1}$ are the normalized particle weights you had sitting in memory before you updated.

Empirical Stuff

The proof pretty much follows the first section of the question. Also I tried coding both on simulated 200 observations from the following model: $$ X_t = .91\cdot X_{t-1} + W_t $$ and $$ Y_t = X_t + V_t $$ with $\{W_t\} \overset{iid}{\sim} \text{Normal}(0, 1)$ and $\{V_t\} \overset{iid}{\sim} \text{Normal}(0, 1.5^2)$ and $X_1 \sim \text{Normal}(0, \frac{1}{1-.91^2})$. I ran an APF, a regular bootstrap version of SISR, and a Kalman filter, the simulation-based filters each with 1500 particles and resampling at every time point. Here's a plot showing that the log of the conditional likelihoods all line up.

enter image description here

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