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Scoring and estimating score precision using multidimensional IRT
Get PDFThe ultimate goal of measurement is to produce a score by which individuals can be assessed and differentiated. Item response theory (IRT) modeling views responses to test items as indicators of a respondent’s standing on some underlying psychological attributes (van der Linden & Hambleton, 1997) – we often call them latent traits – and devises special algorithms for estimating this standing. This chapter gives an overview of methods for estimating person attribute scores using one-dimensional and multi-dimensional IRT models, focusing on those that are particularly useful with patient-reported outcome (PRO) measures.
To be useful in applications, a test score has to approximate the latent trait well, and importantly, the precision level must be known in order to produce information for decision-making purposes. Unlike classical test theory (CTT), which assumes the precision with which a test measures the same for all trait levels, IRT methods assess the precision with which a test measures at different trait levels. In the context of patient-reported outcomes measurement, this enables assessment of the measurement precision for an individual patient. Knowing error bands around the patient’s score is important for informing clinical judgments, such as deciding upon significance of any change, for instance in response to treatment etc. (Reise & Haviland, 2005). At the same time, summary indices are often needed to summarize the overall precision of measurement in a research sample, population group, or in the population as a whole. Much of this chapter is devoted to methods for estimating measurement precision, including the score-dependent standard error of measurement and appropriate sample-level or population-level marginal reliability coefficients.
Patient-reported outcome measures often capture several related constructs, the feature that may make the use of multi-dimensional IRT models appropriate and beneficial (Gibbons, Immekus & Bock, 2007). Several such models are described, including a model with multiple correlated constructs, a model where multiple constructs are underlain by a general common factor (second-order model), and a model where each item is influenced by one general and one group factor (bifactor model). To make the use of these models more easily accessible for applied researchers, we provide specialized formulae for computing test information, standard errors and reliability. We show how to translate a multitude of numbers and graphs conditioned on several dimensions into easy-to-use indices that can be understood by applied researchers and test users alike. All described methods and techniques are illustrated with a single data analysis example involving a popular PRO measure, the 28-item version of the General Health Questionnaire (GHQ28; Goldberg & Williams, 1988), completed in mid-life by a large community sample as a part of a major UK cohort study
Plutonium and uranium isotope ratio measurement by MC-ICP-MS (abstract of paper presented at 12th Annual V.M. Goldschmidt Conference, Davos, Switzerland, August 18-23, 2002)
No full textA new ground-level fallout record of uranium and plutonium isotopes for northern temperate latititudes
No full textPlutonium and uranium isotope ratios can be used to differentiate the sources of nuclear contamination from nuclear weapon establishments (Environ. Sci. Technol. 34 (2000) 4496; Internal Report for AWRE Aldermaston, UK (1961)), weapon fallout (Geochim. Cosmochim. Acta 51 (1987) 2623; Earth Planet. Sci. Lett. 63 (1983) 202; Earth Planet. Sci. Lett. 22 (1974) 111; Geochim. Cosmochim. Acta 64 (2000) 989), reprocessing plants, reactor or satellite accidents (Science 105 (1979) 583; Science 238 (1987) 512) and in addition they provide markers for post-1952 geochronology of environmental systems. A good record of plutonium and uranium isotope ratios of the background resulting from atmospheric nuclear testing is essential for source characterisation studies. Using recently developed mass spectrometric techniques (J. Anal. At. Spectrom. 16 (2001) 279) we present here the first complete records between 1952 and the present day of northern temperate latitude 240Pu/239Pu and 238U/235U atom ratios for atmospheric deposition. Such information was not derived directly during the period of atmospheric testing because suitable mass spectrometric capability was not available. The currently derived records are based on an annual herbage archive and a core from an Alpine glacier. These studies reveal hitherto unseen fluctuations in the 238U/235U atmospheric fallout record, some of which are directly related to nuclear testing. In addition, they also provide the first evidence that plutonium contamination originating from Nevada Desert atmospheric weapon tests in 1952 and 1953 extended eastwards as far as northwestern Europe. The results presented here demonstrate that we now have the capability to detect and precisely identify sources of plutonium in the environment with implications for the development of atmospheric transport models, recent geochronology and environmental studies
Environmental applications of 240Pu/239Pu measurements using the IsoProbe MC-ICP-MS (abstract of paper presented at AGU Fall Meeting, San Francisco, CA, 10-14 Dec 2001)
No full textPre-concentration of naturally occurring radionuclides and the determination of 212Pb from fresh waters
No full textA novel technique has been developed for determining the 212Pb activity of fresh waters. This is of interest to environmental monitoring programmes that utilise gross ?-activity methods to screen for anthropogenic radionuclides. The contribution from 212Pb varies, and is difficult to experimentally measure due to its relatively short half-life (t½ = 10.6 h) and low environmental activity (<0.1 Bq l?1). The use of a three-stage technique that encompasses a unique form of pre-concentration, separation and analysis by liquid scintillation counting allows a lower detection limit of 0.006 Bq l?1 with a chemical yield of 92.5 ± 5.6%. The measurement can be obtained within 7 h of sample collection, and is calculated using the radioactive decay of 212Bi. Other naturally occurring radionuclides may also be extracted using the pre-concentration stage of the technique, with efficiencies above 90% at a range of pH values
Pre-concentration of short-lived radionuclides using manganese dioxide precipitation from surface waters
No full textRapid determination of 222Rn and 220Rn progeny (214Pb, 212Pb, 214Bi, 212Bi) is achievable using manganese dioxide (MnO2) precipitation with analysis by c-spectrometry. This is of interest to environmental monitoring programmes that utilise gross activity methods to screen for anthropogenic radionuclides. The contribution from these naturally occurring radionuclides (NOR) varies, and is difficult to experimentally measure due to short half-lives (t? = 19.9 m–10.64 h) and low environmental activity (\0.1 Bq L-1). The extraction efficiency of the technique is above 90%, and above 80% for other nuclides (232Th, 238U, 235U, 228Ac, 226Ra, 224Ra, 210Pb, 54Mn). Short-lived NOR have been measured at two surface water locations, and indicates elevated 214Bi activity of 4.0 ± 1.1 Bq L-1
Clinician opinions about the appropriateness and severity of general practitioner referrals to specialist mental health services: a cross-sectional survey
No full textA cross-sectional survey of 794 referrals from 41 general practices working in a defined catchment area is reported. The outcomes studied were the proportion of patients (1) declined on the basis of a referral letter, (2) not attending, (3) considered 'appropriate' by clinicians carrying out assessments and (4) judged to meet 'severity' criteria based upon risk or role impairment. Forty-one (5%) referrals were declined on the basis of their referral letter and 221 (27%) did not attend. Of the remainder, 125 (28%) were considered as inappropriate referrals. There was an inconsistent relationship between clinicians' opinions about 'appropriateness' and judgements in the four domains of severity: 66 (52%) of the 'inappropriate' referrals were judged to meet at least one of four severity criteria, whereas, conversely, of the referrals considered 'appropriate', 64 (20%) did not meet any risk or severity criteria. Nearly half (47%) of all referrals to psychiatric services either fail to attend or are considered 'inappropriate' when seen, findings which highlight serious inefficiencies at the interface between primary care and secondary mental health services. The policy of 'prioritisation' on the basis of severity has proved difficult to implement consistently. In order to reduce these inefficiencies, structured referral criteria are needed involving users, general practitioners and mental health service providers
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
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