def calculate_change(old_val, new_val):
    """
    Return a float representing the decimal change between old_val and new_val.
    """
    if old_val == 0 and new_val == 0:
        return 0.0
    if old_val == 0:
        return float(new_val - old_val) / (float(old_val + new_val) / 2)
    return float(new_val - old_val) / abs(old_val)