How to model a biased coin with time varying bias? Models of biased coins typically have one parameter $\theta = P(\text{Head} | \theta)$.
One way to estimate $\theta$ from a series of draws is to use a beta prior and compute posterior distribution with binomial likelihood.
In my settings, because of some weird physical process, my coin properties are slowly changing and $\theta$ becomes a function of time $t$.
My data is a set of ordered draws i.e. $\{H,T,H,H,H,T,...\}$. I can consider that I have only one draw for each $t$ on a discrete and regular time grid.
How would you model this? I'm thinking of something like a Kalman filter adapted to the fact that hidden variable is $\theta$ and keeping the binomial likelihood. What could I use to model $P(\theta(t+1)|\theta(t))$ to keep inference tractable?
Edit following answers (thanks!): I would like to model $\theta(t)$ as a Markov Chain of order 1 like it is done in HMM or Kalman filters. The only assumption I can make is that $\theta(t)$ is smooth. I could write $P(\theta(t+1)|\theta(t)) = \theta(t) + \epsilon$ with $\epsilon$ a small Gaussian noise (Kalman filter idea), but this would break the requirement that $\theta$ must remain in $[0,1]$. Following idea from @J Dav, I could use a probit function to map the real line to $[0,1]$, but I have the intuition that this would give a non-analytical solution. A beta distribution with mean $\theta(t) $ and a wider variance could do the trick.
I'm asking this question since I have the feeling that this problem is so simple that it must have been studied before.
 A: I doubt you can come up with a model with analytic solution, but the inference can still be made tractable using right tools as the dependency structure of your model is simple. As a machine learning researcher, I would prefer using the following model as the inference can be made pretty efficient using the technique of Expectation Propagation: 
Let $X(t)$ be the outcome of $t$-th trial.
Let us define the time-varying parameter
$\eta(t+1) \sim \mathcal{N}(\eta(t), \tau^2)$ for $t \geq 0$.
To link $\eta(t)$ with $X(t)$, introduce latent variables
$Y(t) \sim \mathcal{N}(\eta(t), \beta^2)$,
and model $X(t)$ to be 
$X(t) = 1$ if $Y(t) \geq 0$, and $X(t) = 0$ otherwise. You can actually ignore $Y(t)$'s and marginalize them out to just say $\mathbb{P}[X(t)=1] = \Phi(\eta(t)/\beta)$, (with $\Phi$ cdf of standard normal) but the introduction of latent variables makes inference easy. Also, note that in your original parametrization $\theta(t) = \eta(t)/\beta$.
If you are interested in implementing the inference algorithm, take a look at this paper. They use a very similar model so you can easily adapt the algorithm. To understand EP the following page may found useful. If you are interested in pursuing this approach let me know; I can provide more detailed advice on how to implement the inference algorithm.
A: To elaborate on my comment a model such as p(t)=p$_0$ exp(-t) is a model that is simple and allows estimation of p(t) by estimating p$_0$ using maximum likelihood estimation.  But does the probability really decay exponentially.  This model would be clearly wrong if you observe time periods with high frequency of success than you observed at earlier and later times.  Oscillatory behavior could be modelled as p(t)=p$_0$ |sint|.  Both models are very tractable and can be solved by maximum likelihood but they give very different solutions.
A: Your probability changes with $t$ but as Michael said, you don't know how. linearly or not ?
It looks like a model selection problem where your probablity $p$ : 
$p=\Phi(g(t,\theta))$ may depend on a highly non linear $g(t,\theta)$ function. $\Phi$ is just a bounding function that guarantees between 0 and 1 probabilities.
A simple exploratory approach would be to try several probits for $\Phi$ with different non linear $g()$ and to perform a $g()$ model selection based on standard Information Criterias.
To answer your re-eddited question:
As you said using probit would imply numerical solutions only but you may use a logistic function instead :
Logistic function: $P[\theta(t+1)]  = \frac{1}{1+\exp{(\theta(t)+\epsilon)}}$ 
Linearized by : $ \log{\frac{P}{1-P}} = \theta(t)+\epsilon $
I'm not sure how this can work under Kalman filter approach, but still believe that a non linear   specification like $\theta(t+1)=a t^3 +bt^2+ct + d$ or many others without a random term will do the job. As you can see this function is "smoth" in the sense that it's continous and differentiable. Unfortunately adding $\epsilon$ would generate jumps of the resulting probability which is something you don't want so my advice would be to take out $\epsilon$.
Logit probablity: $P[Coin_{t+1}=H | t]  = \frac{1}{1+\exp{(\theta(t))}}$ 
You already have randomnes in the bernoulli event (Markov Chain) and you are adding an additional source of it due to $\epsilon$. Thus, your problem could be solved as a Probit or Logit estimated by Maximum likelihood with $t$ as explanatory variable. I suppose you agree that that parsimony is very important. Unless your main objective is to apply a given method (HMM and Kalman Filter) and not to give the simplest valid solution to your problem.
