18 Beliefs Across Time

Author

Rahul Shah, Jean-Stanislas Denain, Jacob Steinhardt

In this lecture, we will think of beliefs as probabilities, and we will show that well-calibrated beliefs must obey certain mathematical rules across time. This can be useful in two ways:

It allows us to assess our own calibration even before observing the outcomes of our predictions
It makes it possible to evaluate prediction platforms, e.g. to tell whether Metaculus is behaving irrationally

As a preview, consider the following evolution over time of the probabilility assigned to an event:

We can tell that this evolution is not good:

Intuitively, the probability oscillates too much, which suggests that the forecaster is at times overconfident and at times underconfident.
A bookie could easily pump money out of the forecaster if they knew in advance that the market would oscillate, by buying equal odds contracts at low price when the probability is low, and selling these contracts at a high price when the probability is high.
- You are paying $0.60 to get much more than \$0.60 in expectation. The value of a yes contract + a no contract is \$1 but you are paying \$1.20 for something worth \$1, so the bookie can pump money from you – and you’d be happy to make the trade, even though you will end up losing money.

18.1 An example: will it rain on Friday?

Every week, on Monday and Tuesday, you are asked to predict whether it will rain on this week’s Friday. We will see how certain series of predictions cannot be well-calibrated for any possible outcome.

18.1.1 Example 1: 100% and 0%

Suppose every week, your probabilities are 100% every Monday, and 0% every Tuesday.

Monday	Tuesday	…	Friday
100%	0%	…

Exercise

With these beliefs, can you be well-calibrated?

(whitespace to avoid spoilers)

…

Suppose it rains on a fraction $\pi$ of all weeks.

Monday	Tuesday	…	Friday
100%	0%	…	$\pi$

Then, for your Monday predictions to be well-calibrated, you need to have $\pi = 1$, but for your Tuesday predictions to be well-calibrated, you need to have $\pi = 0$. This is impossible, therefore these predictions cannot be well-calibrated for any possible outcome.

This aligns with your intuition, since you oscillate from 100% to 0% which is a large deviation, going back to the previous diagram.

18.1.2 Example 2: 80% and 70%

Suppose instead that every week, your probabilities are 80% every Monday, and 70% every Tuesday.

Monday	Tuesday	…	Friday
80%	70%	…

Exercise

With these beliefs, can you be well-calibrated?

(whitespace to avoid spoilers)

…

Once again, suppose it rains on a fraction $\pi$ of all weeks.

Monday	Tuesday	…	Friday
80%	70%	…	$\pi$

Then, the same reasoning shows that these predictions cannot be well-calibrated. Indeed, for your Monday predictions to be well-calibrated, you would need to have $\pi = 0.8$, but for your Tuesday predictions to be well-calibrated, you would need to have $\pi = 0.7$. This is impossible, therefore these predictions cannot be well-calibrated for any possible outcome.

These two examples showed that some changes in beliefs over time cannot occur if you are well-calibrated. One property these examples have in common is that your probability consistently decreases from Monday to Tuesday. After a few weeks, a better-calibrated forecaster would notice this pattern, and adjust their Monday probability accordingly.

New information can change your beliefs but on average, our beliefs should stay the same.

18.1.2.1 Example

Will Kanye release his Vultures album?

Your probability that it happens decreases over time (after hearing nothing from him about it) but then once an announcement comes out (say they say they will release it next week) then your probability shoots up.

18.1.3 Example 3: two kinds of weeks

Now, suppose that there are two kinds of weeks:

on half of the weeks, you forecast 50% on Mondays and 60% on Tuesdays
on the other half of the weeks, you forecast 50% on Mondays and 40% on Tuesdays

	Monday	Tuesday	…	Friday
1/2 of weeks	50%	60%	…
1/2 of weeks	50%	40%	…

For a concrete example, suppose that after predicting on Mondays, you watch a weather forecast on TV. On half of the weeks, the TV’s forecast says that it will rain on Fridays, in which case you increase your probability to 60% on Tuesday. On the other weeks, you lower your probability to 40% on Tuesday.

Exercise

With these beliefs, can you be well-calibrated?

(whitespace to avoid spoilers)

…

Suppose it rains on a fraction $\pi_1$ of Fridays for weeks in which the TV forecast predicts rain, and a fraction $\pi_2$ of Fridays when the TV forecast predicts no rain.

	Monday	Tuesday	…	Friday
1/2 of weeks	50%	60%	…	$\pi_1$
1/2 of weeks	50%	40%	…	$\pi_2$

Then, if $\pi_1 = 0.6$ and $\pi_2 = 0.4$, these predictions are well-calibrated. Indeed, in that case:

$\mathbb{P}(\text{rain on Friday} \mid \text{you predicted 60\%}) = \pi_1 = 0.6$
$\mathbb{P}(\text{rain on Friday} \mid \text{you predicted 40\%}) = \pi_2 = 0.4$
$\mathbb{P}(\text{rain on Friday} \mid \text{you predicted 50\%}) = \frac 1 2 \pi_1 + \frac 1 2 \pi_2 = 0.5$

Notice that this would not work if you had predicted 55% instead of 50% on Mondays. If that was the case, then $\mathbb{P}(\text{rain on Friday} \mid \text{you predicted 55\%}) = \frac 1 2 \pi_1 + \frac 1 2 \pi_2 = 0.5$ but we predicted $55\%$ so we are not well-calibrated then.

18.2 Well-calibrated beliefs must be a Martingale

We will now describe the key property that Example 3 verified, but that Examples 1 and 2 did not: well-calibrated beliefs must be a Martingale.

Let $p_1, ..., p_t, ...$ be the probabilities you assign to an event at different points in time. For example, in the past example, $p_1$ would be your Monday probability of rain on Friday, and $p_2$ would be your Tuesday probability of rain on Friday.

Qualitatively, we say that the sequence of random variables $\left(p_t\right)_{t\in\mathbb{N}}$ is a Martingale when the $p_t$ stay the same in expectation. Formally, for every time step $t \in \mathbb{N}$,

\[ \mathbb{E}[p_{t+1} \mid p_1, ..., p_t] = p_t. \]

“Beliefs do not change in expectation”

For some extra intuition, suppose you are at time step $t$: up to now, you have made predictions $p_1, ..., p_t$. Between time steps $t$ and $t+1$, you will observe some extra information that will allow you to adjust your probability to $p_{t+1}$. If you thought that on average this extra information would lead to a higher value of $p_{t+1}$, i.e. $\mathbb{E}[p_{t+1} \mid p_1, ..., p_t] > p_t$, then you should already have set $p_t$ to this higher value! If you don’t, then a bookie will exploit this for you and happily make money from you.

Because well-calibrated beliefs must be a Martingale, if you find a sequence of probabilities that violates the Martingale property, you can tell that these probabilities are uncalibrated. This is useful to decide how much to trust the probabilities of forecasting platforms such as Metaculus before a question has even resolved.

Even though the Martingale property allows you to notice that something is wrong with a sequence of beliefs, it does not tell you what to do to fix it. Sometimes the beliefs alternate between extreme overconfidence and extreme underconfidence, in which case moderating the probabilities can be useful, but that is not always the only underlying issue.

18.3 Properties of Martingales

In this section, we will describe some properties of Martingales, that will make it easier for you to notice that a sequence of probabilities likely violates the Martingale property. We will also help you see how Martingales are so useful – one of the proofs for SGD (Stochastic Gradient Descent) uses Martingales.

For example, in the motivating example at the beginning of these notes, we observed probabilities that oscillated a lot between very high and very low values. The following theorems will make precise the idea that Martingales, and therefore sequences of well-calibrated beliefs, “cannot oscillate too much”.

Our first theorem, which we present without proof, upper bounds the expected amount of variation between consecutive probabilities:

Theorem 1: Suppose $p_1, p_2, ..., p_t, ... \in [0,1]$ is martingale. Then:

\[ \mathbb{E}\left[ \sum\limits_{t=1}^{\infty} (p_{t+1} - p_t)^2 \right] \leq p_1 (1 - p_1) \leq \frac 1 4 \]

The LHS represents a measure of variance (total squared distance).

In particular, applying Markov’s inequality allows us to upper bound the probability of large summed squared differences: for every $x > 0$,

\[ \mathbb{P}\left[ \sum\limits_{t=1}^{\infty} (p_{t+1} - p_t)^2 \geq \frac x 4 \right] \leq \frac 1 x \]

However, a low sum of squared deviation does not preclude large oscillations. Therefore, we introduce the notion of a crossing.

Definition: Let $p_1, p_2, ..., p_t, ... \in [0,1]$ be a sequence of probabilities, and $0 < a < b < 1$. We call a pair of time steps $s < t$ a crossing if $p_s \leq a$ and $p_t \geq b$, or $p_t \leq a$ and $p_s \geq b$. A pair $(s, t)$ can only be involved in at most one upcrossing pair and at most one downcrossing pair.

For example, in the image below, $(s_1, t_1)$, $(s_2, t_2)$, and $(s_3, t_3)$ are all the examples of upcrossings in the following image. Note that $(s_1, t_2)$ is not considered an upcrossing as we already crossed up with $t_1$. Furthermore, there are also downcrossings!

Our second theorem formalizes the intuition that a martingale can’t have too many crossings.

Theorem 2 (Doob’s upcrossing lemma): Suppose $p_1, p_2, ..., p_t, ... \in [0,1]$ is martingale. Let $K$ be the number of crossings of $(p_t)_{t\geq 1}$. This is defined as the number of upcrossing + the number of down-crossings. Then for every $n \geq 1$,

\[\mathbb{P}\left[ K \geq n \right] \leq \max\left(\frac a b, \frac {1-b} {1-a}\right)^n.\]

For example, if $a = 0.2$ and $b = 0.8$, then $\frac a b = \frac {1-b} {1-a} = \frac 1 4$. Therefore, the probability of having 10 or more crossings is at most $\left(\frac 1 4\right)^{10}$, which is less than one in a million.

Proof: Suppose we start at $a$ and we look at some specific timestep $s$ and what is the probability of being bigger than $b$ at time $t$ is: \[ \mathbb{P}[p_t \geq b \mid p_s = a] \leq \frac{a}b \] via Markov’s Inequality.

We can prove this using the Martingale property: \[ \mathbb{E}[p_t \mid p_s=a]=a \leq \frac{1-b}{1-a}. \]

18.4 Conclusion

In this lecture, we saw how well calibrated beliefs must verify certain conditions across time. In particular:

Well-calibrated beliefs are martingales, which means that they stay the same in expectation.
Because of Doob’s upcrossing lemma, well-calibrated beliefs cannot oscillate too much between extreme values.
Therefore, if you notice a lot of oscillations over time in your probability or the market price of a prediction market, this suggests that these predictions are uncalibrated. This can be useful to spot potential errors in forecasts before a question has resolved.