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Defense Mapping Agency |

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Lunar Topographic Orthophotomap (LTO) Series.

A compilation and extension of essays and research bibliography

Publisher: |
Defense Mapping Agency |

Scale: |
1:250,000 |

Projection: |
Transverse Mercator |

Lunar Topographic Orthophotomap (LTO) Series.

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Since Hosni Mubarak’s government fell last year, many of Egypt’s museums have been looted.

And the looting has gone beyond museums — now criminals are digging up archaeological sites and stealing their treasures.

Anchor Marco Werman speaks with Carol Redmount, an archaeologist at the University of California, about her efforts to stop the looting in Egypt.

Redmount described the looters stealing from El-Hibeh’s archaeological dig as “essentially a gang of criminals, headed by a master criminal, who escaped from jail after the revolution.”

via Egypt Looters Ransack Archaeological Sites | @pritheworld.

In clinical measurement comparison of a new measurement technique with an established one is often needed to see whether they agree sufficiently for the new to replace the old. Such investigations are often analysed inappropriately, notably by using correlation coefficients. The use of correlation is misleading. An alternative approach, based on graphical techniques and simple calculations, is described, together with the relation between this analysis and the assessment of repeatability.

INTRODUCTION

Clinicians often wish to have data on, for example, cardiac stroke volume or blood pressure where direct measurement without adverse effects is difficult or impossible. The true values remain unknown. Instead indirect methods are used, and a new method has to be evaluated by comparison with an established technique rather than with the true quantity. If the new method agrees sufficiently well with the old, the old may be replaced. This is very different from calibration, where known quantities are measured by a new method and the result compared with the true value or with measurements made by a highly accurate method. When two methods are compared neither provides an unequivocally correct measurement, so we try to assess the degree of agreement. But how?

The correct statistical approach is not obvious. Many studies give the product-moment correlation coefficient (r) between the results of the two measurement methods as an indicator of agreement. It is no such thing. In a statistical journal we have proposed an alternative analysis, [1] and clinical colleagues have suggested that we describe it for a medical readership.

Most of the analysis will be illustrated by a set of data (Table 1) collected to compare two methods of measuring peak expiratory flow rate (PEFR).

INAPPROPRIATE USE OF CORRELATION COEFFICIENT

The second step is usually to calculate the correlation coefficient (r) between the two methods. For the data in fig 1, r = 0.94 (p < 0.001). The null hypothesis here is that the measurements by the two methods are not linearly related. The probability is very small and we can safely conclude that PEFR measurements by the mini and large meters are related. However, this high correlation does not mean that the two methods agree:

(1) r measures the strength of a relation between two variables, not the agreement between them. We have perfect agreement only if the points in fig 1 lie along the line of equality, but we will have perfect correlation if the points lie along any straight line.

(2) A change in scale of measurement does not affect the correlation, but it certainly affects the agreement. For example, we can measure subcutaneous fat by skinfold calipers. The calipers will measure two thicknesses of fat. If we were to plot calipers measurement against half-calipers measurement, in the style of fig 1, we should get a perfect straight line with slope 2.0. The correlation would be 1.0, but the two measurements would not agree — we could not mix fat thicknesses obtained by the two methods, since one is twice the other.

(3) Correlation depends on the range of the true quantity in the sample. If this is wide, the correlation will be greater than if it is narrow. For those subjects whose PEFR (by peak flow meter) is less than 500 l/min, r is 0.88 while for those with greater PEFRs r is 0.90. Both are less than the overall correlation of 0.94, but it would be absurd to argue that agreement is worse below 500 l/min and worse above 500 l/min than it is for everybody. Since investigators usually try to compare two methods over the whole range of values typically encountered, a high correlation is almost guaranteed.

(4) The test of significance may show that the two methods are related, but it would be amazing if two methods designed to measure the same quantity were not related. The test of significance is irrelevant to the question of agreement.

(5) Data which seem to be in poor agreement can produce quite high correlations. For example, Serfontein and Jaroszewicz [2] compared two methods of measuring gestational age. Babies with a gestational age of 35 weeks by one method had gestations between 34 and 39.5 weeks by the other, but r was high (0.85). On the other hand, Oldham et al. [3] compared the mini and large Wright peak flow meters and found a correlation of 0.992. They then connected the meters in series, so that both measured the same flow, and obtained a “material improvement” (0.996). If a correlation coefficient of 0.99 can be materially improved upon, we need to rethink our ideas of what a high correlation is in this context. As we show below, the high correlation of 0.94 for our own data conceals considerable lack of agreement between the two instruments.

MEASURING AGREEMENT

It is most unlikely that different methods will agree exactly, by giving the identical result for all individuals. We want to know by how much the new method is likely to differ from the old: if this is not enough to cause problems in clinical interpretation we can replace the old method by the new or use the two interchangeably. If the two PEFR meters were unlikely to give readings which differed by more than, say, 10 l/min, we could replace the large meter by the mini meter because so small a difference would not affect decisions on patient management. On the other hand, if the meters could differ by 100 l/min, the mini meter would be unlikely to be satisfactory. How far apart measurements can be without causing difficulties will be a question of judgment. Ideally, it should be defined in advance to help in the interpretation of the method comparison and to choose the sample size.

The first step is to examine the data. A simple plot of the results of one method against those of the other (fig 1) though without a regression line is a useful start but usually the data points will be clustered near the line and it will be difficult to assess between-method differences. A plot of the difference between the methods against their mean may be more informative. Fig 2 displays considerable lack of agreement between the large and mini meters, with discrepancies of up to 80 l/min, these differences are not obvious from fig 1. The plot of difference against mean also allows us to investigate any possible relationship between the measurement error and the true value. We do not know the true value, and the mean of the two measurements is the best estimate we have. It would be a mistake to plot the difference against either value separately because the difference will be related to each, a well-known statistical artefact. [4]

via Statistical methods for assessing agreement between two methods of clinical measurement.

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My reasons for jumping into stats was to directly compare two measurement methods… with multiple trials, on multiple ILDs (inter-landmark distances). I don’t really go for “funny name, lol” things, but when Bland and Borg are cited in the same paper on stats (which I long thought of [cluelessly/ignorantly] as boring). Eponysterical.

But getting real, the issues raised by Bland and Altman sound pretty interesting, and they raise the issue that many tests of this sort may be using misleading information… I have tried to duplicate their methods in my own little H.T.-UGR/Inquiry Study.

Summary

When comparing a new method of measurement with a standard method, one of the things we want to know is whether the difference between the measurements by the two methods is related to the magnitude of the measurement. A plot of the difference against the standard measurement is sometimes suggested, but this will always appear to show a relationship between difference and magnitude when there is none. A plot of the difference against the average of the standard and new measurements is unlikely to mislead in this way. This is shown theoretically and illustrated by a practical example using measurements of systolic blood pressure.

Introduction

In earlier papers [1,2] we discussed the analysis of studies of agreement between methods of clinical measurement. We had two issues in mind: to demonstrate that the methods of analysis then in general use were incorrect and misleading, and to recommend a more appropriate method. We saw the aim of such a study as to determine whether two methods agreed sufficiently well for them to be used interchangeably. This led us to suggest that the analysis should be based on the differences between measurements on the same subject by the two methods. The mean difference would be the estimated bias, the systematic difference between methods, and the standard deviation of the differences would measure random fluctuations around this mean. We recommended 95% limits of agreement, mean difference plus or minus 2 standard deviations (or, more precisely, 1.96 standard deviations), which would tell us how far apart measurements by the two methods were likely to be for most individuals.

via Comparing methods of measurement: why plotting difference against standard method is misleading.

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Kotaku has details on the ways in which Kinect has been improved since the devices first release in 2010, take a look at the diagrams below. I know some of the details on it are bothering some people (with apologies to gamers [those who are really funding this tool and allowing it to be mass produced] my interests lie beside the intended uses), the possibilities as a scanning device are making me drool a little (ok, a lot). Please, please, Microsoft, help this become a tool that is used in multiple fields… keep it as potentially open as the original Kinect (or more). The results available from the original are pretty great, I have used it, and tested it out a bit, checking resolutions and details; but seeing these specs…

via The Next Xbox Has Mandatory Kinect, Game-Swapping and New Controllers, According To Leaked Info.

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Abstract

We introduce a low-cost and compact spectral imaging camera design based on unmodified consumer cameras and a custom camera objective. The device can be used in a high-resolution configuration that measures the spectrum of a column of an imaged scene with up to 0.8 nm spectral resolution, rivalling commercial non-imaging spectrometers, and a mid-resolution hyperspectral mode that allows the spectral measurement of a whole image, with up to 5 nm spectral resolution and 120×120 spatial resolution. We develop the necessary calibration methods based on halogen/fluorescent lamps and laser pointers to acquire all necessary information about the optical system. We also derive the mathematical methods to interpret and reconstruct spectra directly from the Bayer array images of a standard RGGB camera. This objective design introduces accurate spectral remote sensing to computational photography, with numerous applications in color theory, colorimetry, vision and rendering, making the acquisition of a spectral image as simple as taking a high-dynamic-range image.

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The bodies had been buried with great care in a pit that was partly dug into the ground and covered over with debris from the midden. They had been protected by a roof made of antlers and provided with a number of grave goods including pieces of flint and boar bones, and jewellery made of sea shells drilled and assembled into necklaces, bracelets and ringlets for the legs.[6] The grave assemblage was excavated from the site in one piece and is now on display at the museum of prehistory in Toulouse, where its restoration in 2010 earned a national award.

[edit]

via Téviec – Wikipedia, the free encyclopedia.

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Prepare for a massive series on PIE. Many folks love PIE. Renfrew, Anatolia, Kurgan culture Gimbutas, Mallory… Will try to hit it all (just because this article is first, has no bearing on “ranking” of positions. Just have to start somewhere (yeah, this is a poor place to start, didn’t want to bookmark, or lose the link, so hey!) Actually, I will return and resequence/recontextualize once I decide on more articles to use.

Where Proto-Indo-European came from and who originally spoke it has been a mystery ever since Sir William Jones, a British judge and scholar in India, posited its existence in the late 18th century. As a result, Anthony writes, the question of its origins was “politicized almost from the beginning.” Numerous groups, ranging from the Nazis to adherents of the “goddess movement” (who saw the Indo-Europeans as bellicose invaders who upended a feminine utopia), have made self-interested claims about the Indo-European past.

via The Horse, the Wheel, and Language – David W. Anthony – Book Review – New York Times.

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Acoustic Archaeology

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