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    Re: Round-off
    From: Frank Reed
    Date: 2009 May 22, 12:33 -0700

    John, you wrote:
    "But remember, that's only one of many different probability estimates.  The max error is still L*N/2."
    
    You're right that there are many different ways of describing probability 
    limits and estimating the odds of extreme outcomes. But the key is that you 
    should always compare apples with apples. If we're looking at ONE standard 
    deviation error in one variable, then we should never be comparing that to a 
    TWO standard deviation measure of error, or yet some other measure of error 
    (like max error), in another variable. 
    
    So what about that maximum potential error in round-off? If we're dropping 
    tenths, each round-off could be as large as 0.5 minutes of arc, and if there 
    are a dozen round-offs, then that might lead to an error as large as +6.0 or 
    -6.0, equal to the L*N/2 you gave. But this just isn't relevant in practice 
    because we have no means of distinguishing round-off error from observational 
    error (except, of course, by the obvious approach of running the calculation 
    again without rounding off). To put it differently, we already should 
    consider the possibility, rather low in probability, that our observations 
    are off by +/-6.0 minutes of arc, and unlike round-off error, this is not an 
    upper limit. Then we should ask whether the probability of being that far off 
    is significantly increased by the possibility of round-off error. And that 
    all depends on how the standard deviation of observational error compares 
    with the standard deviation of round-off error. As noted in my post a little 
    earlier today, if the standard deviation of the observational error is just a 
    little above 1 minute of arc, the increase in the net standard deviation from 
    rounding off (dropping the tenths) is not that great, less than 33%. It is 
    nearly identical to assuming that your observational error is moderately 
    higher and the round-off error is zero. 
    
    -FER
    
    
    
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