tag:blogger.com,1999:blog-5337555368793819627.post2374765692612829776..comments2023-06-07T09:04:36.390-04:00Comments on More Grumbine Science: How can annual average temperatures be so precise?Robert Grumbinehttp://www.blogger.com/profile/10783453972811796911noreply@blogger.comBlogger7125tag:blogger.com,1999:blog-5337555368793819627.post-60099955450945524742010-03-19T17:19:25.996-04:002010-03-19T17:19:25.996-04:00Michael:
Random vs. systematic error is part of wh...Michael:<br />Random vs. systematic error is part of why I mentioned in the article that nature is not as nice as pictured. The distinction is one that seemed worth a full post of its own. <br /><br />On the other hand, it seems strikingly unrealistic to assume that <i>all</i> error in measurement might be systematic. You'd have to believe that your instruments had absolutely nothing that could contribute to random errors, which is unheard of for any real system. <br /><br />In terms of effects on trends, a systematic bias disappears since the trend will be the difference between the true temperature in 1900 (plus its systematic bias) and the true temperature in 2000 (plus the systematic bias). When we do the subtraction, the systematic bias cancels.<br /><br />At least it does, as you note, if the systematic bias is unchanged. Not so much if there's a systematic bias in 1900, and a different one in 2000. In that case, the change in bias itself could start to produce an apparent (but false). But then you go back to the instruments themselves, their usage, and their calibration (and re-calibration), and examine how large the systematic biases are or can be versus the size of signal you are trying to examine. <br /><br />This concern is very old, and had much to do with international standardization of the surface thermometer networks and their exposures in the 1800s and since. Instrument systematic biases are well under 1 degree and have been for a long time. Changes in biases are even smaller. Played a role, a friend told me in graduate school, in selecting 30 years as the standard climate averaging period -- long enough that instrument system changes weren't a significant part of your signals. (But I never asked him for the reference, and I've never found it myself.)<br /><br />In other news, the SST problem you mentioned was fixed a couple weeks ago. I hope that things look more sensible. If not, please email the addresses on the RTG web page.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-2735762659506782702010-03-14T22:22:17.470-04:002010-03-14T22:22:17.470-04:00I think its worthwhile delving into the topic of s...I think its worthwhile delving into the topic of systematic vs random errors. Random errors are purely random, and if you know the actual error for one measurement, that will not give you any information on the error of the next measurement. Systematic errors are not random, and the same error can be repeated over many measurements.<br /><br />If the error is purely random, then the overall error reduces as the sample number increases. But if the error is systematic, then the overall error does not aways reduce as the sample size decreases.<br /><br />For example a thermometer may have a callibration error so that the scale is slightly off, and always reads 0.5 degree to high. This error repeates for every single measurement, and if you took 10,000 measurements with the same thermometer, averaged them all, the actual error would be quite close to 0.5 degrees, and would not become small.<br /><br />If you know nothing else, but the accuracy of a measurement, then it is not a bad rule of thumb to be pessimistic, and assume that the error could be 100% systematic. So if thermometers have an accuracy of +/- 1 degree, applying this rule of thumb would say that the error cannot be assumed to be any smaller than +/- 1 degree.<br /><br />Trouble arises when someone is taught this reasonable rule of thumb, and then insists that this rule of thumb is an unalterable law of measurement. Without considering why this rule of thumb was a good idea in the first place, and the difference between good places to apply this rule of thumb and bad places.<br /><br />In the case of global temperature measurements the primary issue is an error in trend. So if the error is systematic it may not matter at all - if the thermometer is callibrated wrong, or there is a microsite bias, the temperature will be too high now, and too high 100 years ago, so the trend is still the same and has no error (due to this cause).<br /><br />It is when a systematic error in temperature measurement changes, that we could be in trouble. So if thermometers were all callibrated to give a cool reading 50 years ago, and are all callibrated differently today to give a warmer reading we would have a big problem. It seems a reasonable assumption that with many different thermometers that the bias between different thermometers will tend to act like a random error and at least partially cancel out.Michael Haubernoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-39060753868655841902010-03-12T19:21:00.537-05:002010-03-12T19:21:00.537-05:00Chad:
Sorry. It's what I get for reading too ...Chad:<br />Sorry. It's what I get for reading too casually (just checking that your note was on point). Updated mention now.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-20370718745563085542010-03-12T01:39:09.489-05:002010-03-12T01:39:09.489-05:00One note: my post doesn't use the US surface t...One note: my post doesn't use the US surface temperature observing network to make the point about false precision. It uses a climate model and looks at global averages.Chadhttp://treesfortheforest.wordpress.comnoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-76481178752487703202010-03-11T17:15:42.040-05:002010-03-11T17:15:42.040-05:00Chad:
Nice article, I'll update my main note ...Chad:<br /><br />Nice article, I'll update my main note with your link.<br /><br />Carrot:<br />Also noted in email by Hank. D'Oh. Correcting the note now.<br /><br />And thank you.Robert Grumbinehttps://www.blogger.com/profile/10783453972811796911noreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-7498323196409466362010-03-11T15:54:19.720-05:002010-03-11T15:54:19.720-05:00"The real world is quite as friendly as this ..."The real world is quite as friendly as this example,"<br /><br />Is a 'not' missing in there?<br /><br /><br />By the way, I very much appreciate the concept of your blog.carrot eaternoreply@blogger.comtag:blogger.com,1999:blog-5337555368793819627.post-39569067872585107642010-03-11T14:52:21.762-05:002010-03-11T14:52:21.762-05:00I've taken up this matter of false precision o...I've taken up this matter of false precision on my blog: http://treesfortheforest.wordpress.com/2009/12/14/false-precision-it-doesnt-matter/Chadhttp://treesfortheforest.wordpress.comnoreply@blogger.com