# Is the variance calculation correct in the book?

I'm reading the book "Financial Markets Under the Microscope" for my market microstructure studies. In the book, the variance of the market maker's gain is calculated as follows:

Assume that with probability $$φ$$, an arriving trade is informed, and correctly predicts a price jump of size $$±J$$. Otherwise (with probability $$1 − φ$$), the arriving trade is uninformed and does not predict a future price change. The variance $$σ^2$$ of the market-maker’s gain per trade is then given by

$$σ^2 = (1 − φ) × 0 + φ × J^2$$

But I calculate the variance as $$σ^2 = φ × J^2 - φ^2 × J^2$$ which is different from the book.

Do I miss something?

• Ok I get what you are saying. You have basically put brackets around the second half of the equation and then expanded it out. Your calculations seem correct if the book left out brackets. Either the book forgot to put brackets in its equation or there is no (1-symbol I don’t have) because it is multiplied by 0 so the true equation you would be left with is ...symbol I don’t have * J^2... May 26, 2020 at 0:07

So if the random outcome is $$X \in \{-J, 0, J\}$$ with
$$\mathbb{P}(+J) = \mathbb{P}(-J) = \varphi /2 \;$$ such that $$\mathbb{P}(\pm J) = \varphi$$,
then it follows that $$\mathbb{E}[X] = 0$$, and $$\mathbb{V}[X] = \varphi \times J^2$$. Q.E.D.