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    Re: Rejecting outliers
    From: John Huth
    Date: 2011 Jan 1, 17:51 -0500


     I had the benefit of looking over tons of sights done by students this last semester.   I think I became pretty good at catching obvious errors in their work just from the way the points were distributed.     In a lot of cases, they were simple transcription errors.  


    I've spoken with a number of surveyors who will attest to this problem - they'll place an object to within a few mils, but will be off by 1.00 inches.  

    In a lot of cases, I only had to look at the students' data to spot them.   In one exercise, they had to take a series of Sun shots over the course of a day and then we cooked up a parabolic fit to the data to extract the meridian altitude and also the time of meridian passage.   I think I looked at 100 fits, and could spot a transcription error a mile away by the time I was done.  

    I did my own version of this exercise the week before, so I could make sure the assignment was correct.   I found that things worked pretty well, except at the end of the day and one of my points looked like it was way off.   I was feeling uneasy about it at the time, but didn't know why.   I suspect it was because there was a haze that had drifted over the Sun.    I still used it in my fit, however. 

    By the way, I know it's not common usage, but I had a professor who insisted on making a distinction between "uncertainties" and "errors".   He said "errors are mistakes, we correct errors, uncertainties are part of the process of measurement."    When I was teaching statistics to the students I was fairly careful to not use the word "error" unless I was specifically talking about a "mistake".

    I believe that there are a number of physicists on this board, so I'll mention an experiment done at Stanford, looking for fractionally charged quarks.   The group hovered a superconducting sphere, and tried to measure residual charges.   They claimed evidence for fractionally charged particles.   In looking over their technique, they rejected some data points because a truck passed by or something strange happened.   The only problem was that they knew the value of the charge when they accepted or rejected the data.   A famous physicist suggested that the numbers be randomized by a cypher so that they could accept or reject the data and then remove the cypher later on.   This should convince the skeptics.   They did this, but never reported the results.   I managed to get hold of a copy of the thesis written by the student who did the measurements.   In an appendix he described the result of the cypher version of the experiment.   They didn't see *any* fractional charges - BUT, they didn't retract the original result, which I found irritating.   That's a good example of the dangers of rejecting data points, but I don't know of a 100% foolproof way to do this, because there are so many variables at play.   E.g. you find that a low-lying fog bank was distorting your horizon 10 minutes after taking a sighting - you already see that the shot seems off, and you have a good rationale for rejecting it, but you already know the number.  


       
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