this is a quick question. The professor gave me this formula for calculating the Allan variance of a discrete function.
$$(\Delta_{\mathrm{Allan}} y)^2 = \frac{1}{N/n} \sum_{i=1}^{N/n}\frac{\Big(\frac{1}{n} \sum_{j=1}^ny_{(i-1)n+j} - \langle y \rangle \Big)^2}{(\Delta y)^2/n}$$
Is $(\Delta y)^2$ the ordinary variance of all the values of $y$, or just the variance of the subset of that specific summand?