2.1. The Traders’ Belief Settings
Following the literature, this model divides all traders into three groups: market makers, informed traders, and noise traders. Because market makers are just the mechanism to set price, we assume market makers have the same belief, whereas, informed traders and noise traders have different beliefs. At the beginning of trading, market makers acquire a common signal
about the firm value per share
:
,
. Therefore, the identical belief shared by all market makers is
Equation (1) writes the belief’s variance in the form of its reciprocal i.e.,
. This accords with the literature, which uses the variance’s reciprocal
as the signal/belief value. Unlike market makers who share an identical belief, informed and noise traders have differing beliefs. We use
and
to designate the informed traders and noise traders, respectively. All informed traders also know the common signal
. Besides
, each informed trader
also acquires a private signal
,
. The private signals have the same precision, i.e.,
, among informed traders. The diversity of private signals comes from
, which is only known to informed traders
. According to the Bayesian theorem, after observing signals
and
, an informed trader
believes that
Equation (2) demonstrates the useful property of Bayesian learning with normally distributed beliefs. That is, both pre-and posterior prior beliefs follow normal distributions. The posterior mean is the “precision-weighted” average of the prior belief and the new signal; and the posterior precision is the sum of the prior’s and new signal’s precisions. Vives [
10] gives a formal proof of these properties.
Having initiated the beliefs of market makers and informed traders, we start investigating the trading process. At the beginning of each round
of trading, market makers set the stock price
, which is equal to their mean belief. In reality, the price of a particular stock at a particular time is not quoted as a single value, but as a set of bid prices and a set of ask prices. However, market microstructure literature has shown that the midpoint of the best bid and ask prices reflects the current beliefs of market makers [
14,
15], in this instance, the stock price
in our model should be viewed as the midpoint of bid-ask price at time
.
After market makers set price
at the beginning of trading round
, informed and noise traders submit orders anonymously. To equate the buy and sell orders, market makers set a new price
, which starts trading round
. In other words, we do not assume the equilibrium price to be established instantly, but rather to be found in a trial and error process. The way equilibrium price is established distinguishes two schools of thought. The earlier literature adopts auction models [
14,
15,
16,
17]. In auction models, there are no rounds of trading. All traders submit orders at the same time. Then an equilibrium price is established by maximizing the transaction volume when buyers and sellers are matched. Although theoretically insightful, the auction models are distant from what happens in the real stock markets, where trades occur second by second, and no price can be nominated as being in equilibrium. To shed more light on the price dynamics in real stock markets, more recent literature begins to examine how prices, trades, and beliefs interact with each other in a sequential setting [
14,
15,
16,
17]. This recent literature is not interested in equilibrium prices, which are not empirically well defined or readily testable. On the contrary, this school of thought focuses on the trade-price process through which an equilibrium can be achieved eventually. Our model is consistent with this recent approach.
2.2. The Update of Beliefs
Since our model allows for rounds of trading, traders update their beliefs after observing each round of trading. New beliefs stimulate new trading, and so on and so forth. We now use a mathematical model to demonstrate this belief updating process.
In round , let , and denote the order size of informed trader and noise trader , respectively. Therefore, the total order size is . Notice that is known to all market participants once the orders are submitted and executed. Therefore, serves as a new signal to all traders.
Noise traders do not trade on the basis of any information, hence the name “noise”. It follows that the order size of each noise trader should be independent. Let us further assume that noise traders independently choose their order size from , i.e., .
Informed traders trade on their own beliefs. To determine how their beliefs affect their trades, we assume that informed traders are utility maximizers. Let informed traders’ utility function be
, where
is the trader’s risk aversion coefficient and
is the wealth. For informed trader
who believes
, a standard mathematical practice in asset pricing literature can show that
According to Equations (2) and (3), informed trader
’s order size in round
is
When market makers and informed traders observe the total order
, which includes
and orders from noise traders, they infer
based on
. For market makers, although they do not know
, they do know that
and consequently the following two equations:
Informed traders also want to infer
based on
because they only have their own imprecise private signals, and knowledge of other informed traders’ private signals assists in enhancing their belief precision. To show how informed traders make inferences, we first write informed trader
’s conditional belief of
as
Then we can apply the Bayesian theorem to Equations (1) and (6) and calculate the market makers’ posterior belief
at the end of round 1 as:
Given that market makers always set prices based on their own beliefs, stock price in the beginning of trading round is .
Similarly, applying the Bayesian theorem to Equations (2) and (7), we can determine the informed trader
’s posterior belief
as:
The posterior beliefs of market makers and informed traders specified by Equations (8) through (11), in turn determine
and
in round
. In round
, similar belief updating occurs, leading to the next price
at the start of the next round of trading
. By induction, at the end of round
, we have
where
and
Now considering Equations (12) and (13) in more detail; as these define the updated beliefs of market makers and informed traders, respectively. Notice that we assume all traders’ beliefs to follow normal distributions, and all normal distributions can be determined by two parameters, i.e., mean and variance. This means that we can interpret the belief updating process by looking at how the mean and precision (reciprocal of variance) of beliefs evolve over time.
At the end of round , market makers’ and informed traders’ belief precisions are and , respectively. After each round, the belief precisions of both market makers and informed traders are enhanced by the same amount .
At the end of round , market makers’ and informed traders’ belief means are the precision-weighted average of all information that they have. Specifically, Equations (12) and (14) demonstrate that market makers have two pieces of information: the initial common signal and the history of rounds of orders. The precision of is a constant . The precision of each round of order is also a constant, i.e., . Therefore, the relative precisions, hence the weights, of the common signal and of each order are and , respectively.
Equation (14) describes the history of trading and translates it into a new signal. Although the right-hand side of Equation (14) appears complex, it can be viewed as two parts. The first part consists of the terms in the round bracket. It is easy to prove that . Thus, the first part serves as a discount factor, which discounts each round of orders to its present value. This discount term gives more weight to more recent orders. In other words, the more recent an order is, the more informative it is. This property makes intuitive sense because the informativeness of orders depends on informed traders’ belief precisions. Over time, informed traders’ beliefs become more precise, leading to a more informative order flow.
The second part, in the square bracket of Equation (14) can be viewed as a
translation term, which translates the total order size
in round
into new signals. The translation term of Equation (14) is similar, with the equation defining informed trader
’s order size
in Equation (3). To show this point, let us rearrange Equation (3) in the following form:
In the above form of Equation (16), it demonstrates that Equation (3) is actually also a translation function, which translates informed trader’s order size to a mean belief. Equation (14) is much more complicated than Equation (16) simply because the former adjusts for the information precision of all rounds of orders.
2.3. Relationship Between Prices and Order Flows
As we have shown that beliefs, prices, and orders interact with each other, we will now proceed to establish the mathematical relationships between them. First, the Equations (12) and (13) can be rewritten as:
where
and
where
Rearranging Equations (17) and (19), we have
where
Equation (21) includes the price series , order series , and a zero-mean normally distributed error term , along with some scalar parameters. In this form, Equation (21) demonstrates the mathematical relation among beliefs, prices, and orders. Moreover, Equation (21) can also be viewed as a regression function of the next order on the previous orders and prices. This regression function has two main implications, which are discussed below.
First, Equation (21) illustrates a positive autocorrelation among order series
, which is consistent with empirical results in prior studies (see Van Ness et al. [
18] for a brief summary of these results). Second, the coefficient of
is
, which is strictly larger than
. However, it does not imply the order series
will explode over time. That is because the second term of the right-hand side of Equation (21) exerts an opposite effect on the next order. For example, let us assume the previous order is a buy order, i.e.,
. Apparently, this buy order moves the price up, i.e.,
. Therefore, the second term is negative, causing the next order
to be smaller.
2.4. Data and the Belief Estimation Procedure
Since Equation (21) can be viewed as a regression, and the variables and are observable, we can use this regression function to estimate its unobserved parameters: , , , and . The first three parameters are of great economic interest as they represent the precision of informed traders’ private signals, the precision of common signals, and the precision of the signal conveyed by order flows, respectively.
We apply this estimation to the data of S&P 500 index constituent stocks over the period between 1 January 2003 and 31 December 2011. To avoid the impacts of S&P 500 index constituent changes, we have excluded the first month for newly added stocks and the last month for newly removed stocks.
The estimation requires detailed series of prices and orders. This paper uses the NYSE’s Trade and Quote (TAQ) database, which provides order-by-order trades and quotes data. Before the estimation, we clean TAQ raw data in the following ways. First, trades that occur at the same time and price are aggregated as one trade. Second, earlier microstructure literature routinely adjusts the timestamps in pre-2000 TAQ data [
19]. It is unclear whether similar timestamp adjustments should be applied to post-2000 TAQ data [
20]. Specifically, Bessembinder [
21] and Boehmer and Kelley [
22] do not adjust timestamps, whereas Brennan et al. [
23] and Tannous et al. [
13] do. In this paper, we use both time-adjusted and time-unadjusted data; however, as results are quantitively similar, we only report results of time-unadjusted data.
We also removed obviously erroneous data entries from the raw data according to widely accepted principles [
24,
25]. These omitted trade data entries are: (1) out of time sequence order executions; (2) trades with an error or correction indicator; (3) trades that are not settled in standard ways; (4) trades that are associated with exchange acquisitions or distributions; (5) trades that are not preceded by a valid same-day quote; (6) trades that are executed at a price changed
% from that of the previous trade. The removed t quote data entries are: (1) quotes with negative bid or ask prices; (2) quotes with negative bid-ask spreads; (3) quotes with bid-ask spreads larger than
$; (4) quotes with the midpoint of bid and ask prices changed by
% from the previous same-day midpoint; and (5) quotes associated with trading halts or designated order imbalances.
After cleaning the raw data, we apply an estimation for each stock on each trading day, using a maximum likelihood method. To avoid local optima, we run the estimations times with different initial values and keep the estimates that have the largest likelihood.
2.5. Definitions and Estimates of Belief Parameters
The estimation generates estimates for four parameters:
,
,
, and
, although the last parameter is not of interest in this paper and is not reported. The first three parameters represent information precisions of informed traders’ private signals, common signals, and the signal conveyed by order flows. To better interpret these three parameters, we define three variables. The first variable is belief uncertainty. It is actually the price-adjusted precision. The rationale for the adjustment is as follows. Precisions are mathematical variations of prices, and thus stocks with higher prices necessarily have smaller precision values. This fact prevents comparisons of belief precisions among stocks with different price levels. Specifically, we define belief uncertainty by the following Equation:
In Equation (23), informed traders’ is and market makers’ is . Equation (23) uses average transaction price during the day to proxy for true value of the stock. As that precision is the reciprocal of belief variance and we assume the beliefs are normally distributed, then Equation (23) uses to represent the range of likely prices. Belief uncertainty is a measure of how uncertain traders are about the signals. The larger the belief uncertainty is, the more uncertain traders feel.
The second variable we define is information equality, which measures the informational advantage of informed traders over market makers. This variable is defined as:
Information equality can be viewed as an inverse measure of information asymmetry, for the following reasons. When the information asymmetry is severe, the informed traders are much more certain than market makers. In this case, the numerator of information equality is relatively smaller, leading to a smaller information equality.
The third variable we define is information incorporation rate. This variable focuses on how aggressively market makers adjust prices after observing order flows. The level of aggression of market adjustment is dependent on how much weight market makers put on the signals conveyed by order flows. If the signal conveyed by order flows is more informative relative to their own common signal
, i.e.,
is relatively larger than
, then market makers put more weight on the order flow signal and adjust prices more aggressively. Accordingly, we define information incorporation rate as follows:
In this instance, the aggressiveness of market makers’ adjustments of prices should decrease over time, because after observing more and more rounds of orders they become less and less uncertain, thus putting less and less weight on the signal of the newest orders. In this context, one should view the variable information incorporation rate as a proxy for the decreasing aggressiveness of price adjustments. Another property of this variable is that it also measures how quickly prices can fully incorporate all information in the market.