def uniform_disk(a):
    """Return the standard uncertainty for a uniform disk 
    
    :arg float a: the radius
    
    Convert the radius of a uniform disk distribution ``a`` 
    to a standard uncertainty.
    
    See reference: B D Hall, *Metrologia* **48** (2011) 324-332

    **Example**::

        >>> z = ucomplex( 0, type_b.uniform_disk(1) )
        >>> z
        ucomplex((0+0j), u=[0.5,0.5], r=0.0, df=inf)
        
    """
    return a / 2.0
