'Simple rate'

@classmethod
def _rate(cls, sample1, sample2):

    try:
        interval = (sample2[0] - sample1[0])
        if (interval == 0):
            raise Infinity()
        delta = (sample2[1] - sample1[1])
        if (delta < 0):
            raise UnknownValue()
        return (sample2[0], (delta / interval), sample2[2], sample2[3])
    except Infinity:
        raise
    except UnknownValue:
        raise
    except Exception as e:
        raise NaN(e)
