A type A uncertainty estimate is derived from repeated measurements. For example, I may estimate the uncertainty on a measurement by repeating the measurement $N$ times and then calculating some measure of its dispersion, such as the standard deviation or, as below, the Allan deviation (suitable when $y$ might vary slowly over time):
$$u_y = \sqrt{\frac{1}{2(N-1)} \sum_{i=1}^{N-1} (y_{i+1} - y_i)^2}$$
If circumstances prevent me from taking any more measurements, how can I use the distribution of $y$ to estimate the meta-uncertainty $u_u$, (i.e. the uncertainty of the uncertainty)?