Carbohydrate-induced stimulation of saccharin intake: Yoked controls

Dilute, intragastric infusions of carbohydrate stimulate saccharin intake. To determine whether this effect is due to an associative process, a yoked control was employed. One group of rats was trained in the conventional fashion: they received carbohydrate (maltodextrin) infusions whenever they drank saccharin. Whenever these rats were infused with carbohydrate, yoked rats were infused with the same amount of carbohydrate. In an experiment in which saccharin was available continuously, carbohydrate infusions stimulated saccharin intake in conventionally trained rats but not in yoked rats. In a subsequent experiment, in which rats were offered saccharin for only 2.5 h/day, carbohydrate infusions stimulated intake in conventionally trained and in yoked rats. However, when these rats were switched to continuous saccharin availability, saccharin intake declined in yoked rats but remained elevated in conventionally trained rats. Thus, carbohydrate infusions stimulate saccharin intake via an associative process.     

The scattering of X-rays

Present Status of the Problem.—The scattering of X-rays is one of the outstanding problems of electromagnetic radiation which has not been solved satisfactorily. All theories (based on classical electrodynamics) presented thus far do not explain either the diminution in the scattering coefficient, or the observed asymmetry in the scattering, or both. Among such theories we may mention:J. J. Thompson's Theory.—Assuming that the scattering is done by a point electron, and making use of certain additional hypotheses, Thomson showed that the scattering coefficient of any substance is given by

$\text{σ=}\text{8πNpe}{}^4\text{3m}{}^2\text{c}{}^4$

and that the intensity of the scattered radiation is given by

$\text{I}{}_{\mathrm{\theta }}\text{=I}\text{e}{}^4\text{(I+cos}{}^2\text{θ)}\text{2r}{}^2\text{m}{}^2\text{c}{}^4$

where N is the number of atoms per c.c., p the number of electrons per atom, e the electronic charge, m the electronic mass, c the velocity of light, I_θ the intensity of the scattered radiation at an angle θ between the incident beam and the radius vector joining the centre of the electron and the point P distant r from the electron, and I is the intensity of the incident beam. This theory explains neither the asymmetry nor the decrease in the coefficient of scattering.Schott's Theory.—Among other things, the assumption is here made that the atom consists of coaxal rings of electron. The electrons in each ring are spaced at equal intervals and revolve with a uniform angular velocity, which, however, may be different for different rings. This theory fails to explain the observed diminution in the scattering coefficient.Debye's Theory.—In its essentials, Debye's theory has the same merits and demerits as that of Schott. Debye assumes that all the electrons in an atom are arranged in a single ring, and that they are spaced at equal intervals. This theory (and also Schott's theory) explains the asymmetry and the “excess scattering,” but is altogether unable to explain the diminution in the scattering coefficient.Modification of the Classical Theory.—The present paper presents a discussion of the possibility of modifying the classical theory (that of J. J. Thomson) so as to account for the decrease in the scattering coefficient as well as the dissymmetry. By assuming that the electron is made up of a number of parts—for simplicity, of two parts—it has been found possible to account for the diminution in the scattering coefficient without, at the same time, explaining the observed asymmetry. To accomplish both objects is what was aimed at in the combination of the present work with that of Debye. In this research the goal has not been perfection between predicted and observed results, but rather to discuss some possible modifications of the classical theory and their consequences.     

Developing a geographic search filter to identify randomised controlled trials in Africa: finding the optimal balance between sensitivity and precision

Background: Research on identifying trials using geographic filters is limited. Objectives:  To test the sensitivity and precision of a filter to identify African randomised controlled trials (RCTs). Methods: We searched medline and embase for RCTs published in 2004 using a Cochrane filter for RCTs. The search was limited to HIV/AIDS but irrespective of location. Two investigators independently identified African RCTs from the retrieved records forming a reference set. We then repeated the search using an African geographic filter comprising country and regional terms forming the filter set. We compared the sensitivity and precision of the sets. Results: The medline reference set comprised 1799 records with 23 African RCTs; for embase , the reference set comprised 763 records with 37 African RCTs. The medline filter set comprised 180 records with 17 African RCTs; the embase filter set comprised 98 records with 27 African RCTs. Sensitivity of the filter was 74% (medline ) and 73% (embase ). Addition of the filter improved precision from 1.3% to 9.4% (medline ) and from 5% to 28% (embase ). Conclusion: The African filter improved precision with some loss in sensitivity. Incomplete reporting of trial location in electronic bibliographic records restricts efficiency of geographic filters. Prospective trial registration should alleviate this.     

Class size and achievement in introductory economics: Evidence from the TUCE III data

Does class size affect achievement in introductory economics? This paper addresses this question using a national economic education data base (TUCE III), consolidating these data so that each observation represents a single class rather than a single student, thereby alleviating several estimation problems that plague empirical work in this area. Our results indicate that class size does not affect student achievement, and further, that class characteristics over which instructors or department chairs have control also do not influence achievement. These results are remarkably robust to different specifications, so long as students' Scholastic Aptitude Test (SAT) scores are used to control for ability and account is taken of whether the TUCE test result counted toward the students' final grade.