INTRODUCTION. S INCE it was shown in a previous paper (16) that the phenomena commonly called thermotropic in roots were due largely to hydrotropism, it seemed advisable to investigate this tropism more in detail, and to determine to some extent the laws which govern it, and the limits within which it acts. The subject is of more particular interest because the question of the limitation of hydrotropic sensitivity to the root-tip has never been definitely settled since Darwin first suggested it.

Estudios sobre el valor fisiol6gico relativo de las luces espectrales.11. La sensibilidad del Volvox a longitudes de onda de igual contenido de energia.La sensibilidad del Volvox a la radiacidn de diferentes longitudes de onda e igual energia (sensibilidad a la radiaci6n de la misma energia) ha sido investigada por 10s autores siguiendo dos mktodos: a) la duraci6n relativa del tiempo de presentacibn, y b) la velocidad relativa de la locomoci6n (y precisi6n de la orientaci6n). Una longitud de onda de 494p tiene el mayor valor estimulante, como demuestran ambos mktodos. La eficacia de las otras longitudes de onda presenta una disminuci6n gradual cuando se emplean longitudes de onda mas o menos largas. Los autores insisten sobre la necesidad de usar un espectro de igual energia en tales investigaciones. Tambikn llaman la atenci6n acerca de la presente incapacidad para hacer comparaciones con la visibn humana (normal o dalthica), es decir, con las curvas de luminosidad fot6pica y scot6pica (0 acromAtica) por las siguientes razones: a) a causa de nuestra ignorancia sobre las reacciones fotoquimicas y la naturaleza de las substancias fotosensitivas, y b) porque las investigaciones efectuadas recientemente sobre la visibilidad de la radiaci6n por el ojo normal y dalt6nico demuestran que el efecto maxim0 no depende de Ias longitudes de onda mencionadas en investigaciones mas antiguas.

This study examined the effectiveness of the threeparameter IRT model in vertically equating five overlapping levels of a mathematics computation test. One to four test levels were administered within intact classrooms to randomly equivalent groups of third through eighth grade students. Test characteristic curves were derived for each grade/test level combination. It was generally found that an examinee would receive a higher ability estimate if the test level administered had been calibrated on less able examinees.Practical implications for "out-of-level" and adaptive testing are discussed.It is often considered desirable to test a student in a given subject matter area periodically throughout his/her formal schooling, and to compare the scores obtained across the various testings. Because knowledge in many subject areas is closely linked to school curricula, standardized achievement tests are usually developed in levels that attempt to rnirr®r '6tYp~c~l&dquo; curriculum placement of different aspects of a subject area. This usually results in a standardized test battery with levels corresponding, at least roughly, to grades in school.In order to compare test scores across these levels, a scale must be developed that allows comparisons of raw scores obtained on tests differing in content and difficulty. This is the problem that vertical equating attempts to solve-how to develop a score scale across test levels which (1) differ in difficulty and (2) are designed for groups of examinees who differ in average ability level.This study was designed to examine the effectiveness of the three-parameter item response theory (IRT) model in vertically equating the mathematics computation test of the Iowa Tests of Basic Skills (Hieronymus, Lindquist, & Hoover, 1977).IRT methods are frequently suggested as the prefeffed vertical equating approach for two reasons:(1) It is recognized that problems exist with the classical test theory methods (see, e.g., Lord, 197?; Lord & Wingersky, 1984), and (2) IRT methods are usually conceived of as having &dquo;person-free&dquo; 9 calibration and &dquo;item-free&dquo; measurement. These properties imply that the item parameters which are estimated are invariant for all subgroups of examinees, and that, once the items are calibrated, the same 0 estimate would be obtained (except for errors of measurement) for an individual regardless of the subset of items he/she was administered. These properties, if they held, would essentially solve the problem of vertical equating.The two IRT models that have been most prominent in the vertical equating literature are the oneparameter (Rasch) model and the three-parameter model (see, e.g., Hambleton & Swaminathan, 1984). Although the Rasch model possesses certain desirable properties, such as simplicity and a monotonic relationship between raw score and estimated examinee ability, there are indications that the model does not perform well in practice in vertical equatat Kungl Tekniska Hogskolan / Royal Institute of Technology on August 24, 20...

W. GOODSPEED 219 MSS. intended for publication and books, etc., intended Tor review should be sent to Professor J. McKeen Cattell, Garrisonon-Hudson. N. Y.

No abstract