There is a calculation for this which is based on an estimate:
Return on Equity %: The current fiscal years estimated earnings per share (EPS) divided by the book value per share.
which can vary significantly:
One of the most widely used measures of firm profitability and stockholder return is return-on-equity(ROE). However, depending upon an individual firm's financial decisions (for example, dividend policy) and an analyst's assumptions used for the ROE calculation, the results can vary significantly. This variation can create confusion for students when the topics of ratio and financial statement analysis are presented.
as can quote spreads:
For reporting purposes, a market center's quoted spread is calculated by taking each market center's current best offer and subtracting that market center's current best bid, and then averaging those values for the day. Quotations with a size or price of zero are ignored. The NBBO quoted spread is calculated using the difference between the National Best Offer and the National Best Bid.
and pricing models:
In the make-or-take pricing model, exchanges (and some alternative trading systems) charge an access fee for executing marketable orders that fill against (take) standing orders and provides a liquidity rebate for executed standing orders that make markets. The difference between the access fee and the liquidity rebate is the net fee that the make-or-take exchanges earn for
arranging trades. In contrast, exchanges that charge a transaction fee for arranging trades simply charge the buyer, the seller, or the member trader a fee for executed trades. The transaction fee and the net fee earned by make-or-take exchanges are of similar magnitudes so that access fees are generally greater than transaction fees. (On rare occasions, the relationship has been inverted
when an exchange runs a promotion.)
and risk of price changes between close and open:
Market orders and stop orders face a common risk: those who submit market orders, or whose
stop orders convert to market orders, anticipate that there will be robust and orderly quoting and trading activity to provide an immediate execution at a reasonable price. Market orders prioritize immediacy of execution over price protection, and, on a volatile trading day, the execution price achieved by these orders can deviate significantly from recently traded prices or from an order's stop price. Although the recently implemented Limit Up-Limit Down ("LULD") mechanism is meant to moderate excessive market volatility and places outer bounds on the potential movement of a market order, there remains substantial room for prices to move before the LULD volatility moderators are triggered. In addition, the LULD mechanism does not constrain price movements between one day's close and the next day's open. Thus, even with the protections of LULD, investors using market orders risk receiving an execution at a price substantially worse than anticipated, particularly in volatile markets. An example of such severe price volatility occurred on August 24, 2015, when, during the first 15 minutes of trading, more than 20% of the S&P 500 companies and more than 40% of the NASDAQ 100 companies traded 10% or more below their previous day's closing price.