'Calculate the alpha based on the time since the last tick. This is
necessary because a single threaded Python program loses precision
under high load, so we can\'t assume a consistant I{EWMA._interval}.
:type interval: C{float}
:param interval: the interval we use to calculate the alpha'

def _alpha(self, interval):

    return (1 - math.exp(((- interval) / self.period)))
