# Conjugate Prior

Python implementation of the conjugate prior table for Bayesian Statistics https://en.wikipedia.org/wiki/Conjugate_prior#Table_of_conjugate_distributions

## Installation:

`pip install conjugate-prior`

## Supported Models:

1. `BetaBinomial` - Useful for independent trials such as click-trough-rate (ctr), web visitor conversion.
2. `BetaBernoulli` - Same as above.
3. `GammaExponential` - Useful for churn-rate analysis, cost, dwell-time.
4. `GammaPoisson` - Useful for time passed until event, as above.
5. `NormalNormalKnownVar` - Useful for modeling a centralized distribution with constant noise.
6. `NormalLogNormalKnownVar` - Useful for modeling a Length of a support phone call.
7. `InvGammaNormalKnownMean` - Useful for modeling the effect of a noise.
8. `InvGammaWeibullKnownShape` - Useful for reasoning about particle sizes over time.
9. `DirichletMultinomial` - Extension of BetaBinomial to more than 2 types of events (Limited support).

## Basic API

1. `model = GammaExponential(a, b)` - A Bayesian model with an `Exponential` likelihood, and a `Gamma` prior. Where `a` and `b` are the prior parameters.
2. `model.pdf(x)` - Returns the probability-density-function of the prior function at `x`.
3. `model.cdf(x)` - Returns the cumulative-density-function of the prior function at `x`.
4. `model.mean()` - Returns the prior mean.
5. `model.plot(l, u)` - Plots the prior distribution between `l` and `u`.
6. `model.posterior(l, u)` - Returns the credible interval on `(l,u)` (equivalent to `cdf(u)-cdf(l)`).
7. `model.update(data)` - Returns a new model after observing `data`.
8. `model.predict(x)` - Predicts the likelihood of observing `x` (if a posterior predictive exists).
9. `model.sample()` - Draw a single sample from the posterior distribution.

## Coin flip example:

``````from conjugate_prior import BetaBinomial
tails = 105
prior_model = BetaBinomial() # Uninformative prior
credible_interval = updated_model.posterior(0.45, 0.55)
print ("There's {p:.2f}% chance that the coin is fair".format(p=credible_interval*100))
predictive = updated_model.predict(50, 50)
print ("The chance of flipping 50 Heads and 50 Tails in 100 trials is {p:.2f}%".format(p=predictive*100))
``````

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