def ComputeRates(hist):
    """Returns a sequence of success rates, one for each version."""
    rates = []
    for version in ['A', 'B']:
        hits = hist.Freq((version, 'Y'))
        misses = hist.Freq((version, 'N'))
        rate = float(hits) / (hits + misses)
        rates.append(rate)

    return rates