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Averaging temperature data

  1. Dec 8, 2009 #1
    I've just been looking at a climate change thread (comments on George Monbiot's Guardian column), some one has posted some average temperatures giving figures to two decimal places. Now I doubt that the met office measure temperature to two decimal places so this is probably the result of averaging. To the meat of it, I always understood that you can not average to a greater resolution then the raw data, i.e. if you measure temperature to one decimal place you cannot then quote averaged temperatures to more then one decimal place. Have I misunderstood this for years or is there some dodgy data floating around?
     
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  3. Dec 8, 2009 #2

    Mark44

    Staff: Mentor

    Yes, it's meaningless to end up with computed values that have more precision than the values used in the computation.
     
  4. Dec 8, 2009 #3
    Actually it is meaningful and it is the point of doing multiple measurements. Obviously if you do 100 measurements of the same quantity and average, you get a much more precise estimate than if you had done just one measurement.
     
  5. Dec 8, 2009 #4

    Mark44

    Staff: Mentor

    I disagree. If you measure the temperature as 17.6 deg. C, what you are effectively saying is that the actual temp could have been anything between 17.55 and 17.65 degrees. If all your measurements are to one decimal place, a value computed from them cannot be more precise than one decimal place.

    There's a difference between accuracy and precision. To use an analogy, if a rifle marksman puts all his shots within a circle of radius 20 cm from the bullseye, his shooting is accurate, but not precise. If a different marksman puts all his shots within a circle of radius 5 cm whose center is 20 cm from the bullseye, his shooting is precise but not accurate. A third marksman who puts all his shots in a 5 cm circle around the bullseye is accurate and precise.
     
  6. Dec 8, 2009 #5
    That is the point. The average value is not a measured value. It is an estimate for a real value which is obscured by imprecise measurements.

    I'd like to use your analogy of the rifle marksman to explain this. The marksman shoots and someelse sees the hits only - without knowing where the marksman aimed at. If he sees only one shot then he will be very uncertain about the aiming position. If he see many hits he can make a very confident estimate about the position the marksman was aiming for. This estimate will be much more precise than any single of the hits could have told him.
     
  7. Dec 8, 2009 #6

    Mark44

    Staff: Mentor

    Clearly the computed average value is not a measured value, nor did I say it was.
    Suppose I measure the length of something over the course of several days using a stick with no markings on it, and get these measurements: 2, 4, 5. If I take the average of these numbers, I get 11/3 = 3.7 rounding to 1 decimal place. How confident can I be in that .7 in my average?
     
  8. Dec 8, 2009 #7
    That is true. However, we were actually talking about high precision numbers that came from a calculation which averaged measurements.

    One can actually give an answer here.
    I'm not an expert in statistics but here we go:
    http://en.wikipedia.org/wiki/Standard_error_(statistics)
    So the standard deviation of the mean is 0.7
    Which tells me that with a chance of 95% the real mean is between 2.3 and 5.1
    Which isn't great, but still better than one measurement alone. Here the last digit is indeed useless.

    However if you do many more measurements you might find than the estimated mean is between 3.6849 and 3.6836. In that case you can write that the mean is 3.68 (skipping uncertain digits). That's what the scientists did with many measurements (I hope!).
     
  9. Dec 8, 2009 #8

    Mark44

    Staff: Mentor

    And these estimated means and other measures are given with a certain probability.

    I learned in physics and chemistry a long time ago that if you measure the dimensions of some object and then compute the area, volume, whatever, your computed value should not have more precision than your measured values.
     
  10. Dec 8, 2009 #9

    Integral

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    There is a big difference between errors propagating through a computation (computing the area of a rectangular volume) and taking the average of multiple measurements. I would assume that you can recall that from those same chem and physics classes.
     
  11. Dec 8, 2009 #10
    I assume that most people take the standard deviation as their "error" which means their result have a 60% probabilities.

    If from one value you computed another value then - you are right - you won't be able to improve on accuracy. However if you repeat the same measurement many times, then the accuracy of your estimation does improve with every new measurement.
     
  12. Dec 8, 2009 #11

    Integral

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    Of course this assumes that you have consistent enough measurements to keep the standard deviation small. It is possible to get data that is unusable due to a large st. dev.
     
  13. Dec 8, 2009 #12
    Not true. Take sports, for example. If a basketball player scores 20 points in the 1st game and 19 points in the 2nd, his average after 2 games would be 19.5 points per game. If we didn't allow for greater accuracy than we started with, we would have to say that his average is 20 points per game which, of course, is not accurate.

    Suppose, over the next 4 games, he scores 27, 17, 24, and 26 points.
    His overall average after 6 games would then be (20+19+27+17+24+26)/6 = 133/6 = 22.16666666, so we would say 22.17 or 22.167 is his average.
     
  14. Dec 8, 2009 #13

    Mark44

    Staff: Mentor

    I'm willing to buy the arguments about repeated measurements, but your example is not germane in this discussion. We can assume that the points made by the BB player are exact and not subject to measurement error, so the computed average can have as much precision as you want.
     
  15. Dec 8, 2009 #14
    Again, I disagree...

    Temperature is an exact measurement as well. Just because the person taking the temperature reading may misread the value (or the equipment is out of calibration), doesn't mean that you aren't working with exact values.

    Here's a "more precise" example:

    Suppose you take 6 temperature readings, each to the nearest 1/100th of a degree, and add up to 237.39 degrees. Divide that value by 6 and you get 41.23166666...
    Therefore the average should be given as 41.2317 or 41.23167 degrees.

    In this case, only showing 1/100ths of a degree (41.23 degrees) is not enough precision, since 237.38/6 = 41.23 (exactly) and 237.37/6 is approximately equal to 41.23, as is 237.40/6. We need the extra precision to be accurate about our averages.

    As for a basketball player's score being "exact and not subject to measurement error," that is not entirely true: the scorekeeper could miss or forget to count a basket, he could write the score down incorrectly, or he could misinterpret the value of the shot (was it a 3-point basket or a 2-point basket?)
     
  16. Dec 8, 2009 #15
    Good debate so far but reading through this lot has raised another twist. Could it be that if you perform repeated readings with the same instrument then it is reasonable to take the average to a higher order but if you are using a series of different instruments then it is not?
     
  17. Dec 9, 2009 #16

    Mark44

    Staff: Mentor

    You can't mean this. There is some number that exactly represents the temperature, but there is NO way that we can measure it with infinite precision.
    I'm not talking about operator error here - I'm saying that no such instrument exists that can measure the temperature (or length, or weight, or any other continously distributed quantity) exactly.
    Each of your temperature readings, being accurate to the nearest .01 degree, is really some number in a band .005 on either side of the nominal measurement. This means that if you measure the temperature as 42.64 degrees, you're really saying that it is somewhere between 42.635 and 42.645 degrees, so there is some uncertainty in the second decimal place. To say that your computed average now has two or three more decimal places of precision -- you're going to have to justify that to make be believe it.
    Sure, that could happen, but it seldom does, because there are other people there who can see the incorrect score on the board, and who are invested enough in the game to let that slide. I think you're missing my point, which is basically the difference between integers (which are discrete) and the real numbers. It's a lot harder to screw up the integer count of something than it is to measure something whose values are distributed along a continuum.
     
  18. Dec 9, 2009 #17
    I can see that if you are taking multiple measurements of the same thing with the same instrument that it might be OK to take the average out to a higher order then the original measurement. But if you go back to the original question, if your taking mutiple measurements using different devices and getting a large spread of results, i.e temperature recordings from around the country, at different times of the year can you then take results to an order better then the original data?
     
  19. Dec 9, 2009 #18
    Either way, my examples have shown that you need to show MORE degrees of accuracy when dealing with averages, no matter how accurate your original data is. After all, if your not assuming that your original data is as accurate as it can be, then what good is it anyway?

    Sure, if I take a temperature measurement to the nearest 1/xth of a degree, one could always make the argument that it could have been taken more accurately to the nearest 1/(x+1)th degree, if we had better, more accurate equipment. But reading only to the 1/xth degree, doesn't take away from the fact that it is as accurate as you can be with what you've got. All it means is that your averages will be taken to less precision than if the original readings were more accurate.
     
  20. Dec 9, 2009 #19
    Even if you're using different instruments, aren't you assuming that each measurement is as accurate as it can be?

    Suppose you took 999 measurements to the nearest 0.01 degree and one measurement to the nearest 0.00001 degree using a different measuring device. This last measurement, being more accurate than the previous ones, increases the accuracy of your average. If you were to round that last measurement to the nearest 0.01 degree, your accuracy of THAT measurement decreases, as does your average.
     
  21. Dec 9, 2009 #20
    Any physical variable will have two error bars, statistical and systematic.

    The statistical error bar depends on how many measurements you make and on the distribution of results. If you make 5 random measurements of a variable that ranges from -20 C to 50 C, your estimate of its mean on the basis of these 5 measurements will have an extremely high value of statistical error.

    The systematic error bar has to do with how you measure the thing. If all your measurements were done with thermometers that are only accurate up to 0.5 C, that means systematic error bar +/-0.5 C, no matter how many measurements there were.
     
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