Oligomerization and ring closure of immunoglobulin E class antibodies by divalent haptens

Cross-linking of antibodies constitutes a widespread initiation signal for their respective effector functions. Cross-linking IgE-class antibodies provide the triggering signal to mast cells for their degranulation process. To obtain a quantitative insight into these cross-linking processes, the interactions between a DNP-specific monoclonal antibody of the IgE class and a series of divalent DNP haptens with spacers of different length and flexibility have been studied by fluorescence titration experiments. These were analyzed by employing the theoretical model developed by Dembo and Goldstein [Dembo, M., & Goldstein, B. (1978) J. Immunol. 121, 345-353] in a fitting procedure. Equilibrium constants that describe the aggregation and ring-closure processes caused by divalent hapten binding have been used as free parameters. The intrinsic binding constants were determined by fluorescence titrations with corresponding monovalent haptens. The main results are the following: (1) The divalent haptens with a short and flexible spacer [i.e., N alpha, N epsilon-di-(DNP)-L-lysine,meso-bis[(DNP-beta-Ala)amino]succinate, and bis[(DNP-tri-D-Ala)amino]heptane, having a maximal DNP-DNP distance of gamma = 14, 21, and 45 A, respectively] effect aggregation of the antibodies mainly into closed dimers. (2) The divalent hapten family with long and rigid oligoproline spacers di(DNP)-Ahx-Asp-(Pro)n-Lys with n = 24, 27, and 33 (i.e., gamma = 100, 110, and 130 A) causes aggregation of the antibodies predominantly into closed dimers and trimers. The corresponding equilibrium constants of the respective ring-closure processes decrease significantly with longer spacer length. (3) Evidence was found that intramolecularly monomeric ring closure of the IgE antibodies is caused by haptens containing oligoproline spacers with n = 37 or 42 (gamma = 130-150 A). The equilibrium constant of the ring-closure process increases with spacer length. This increase in stability indicates a difference in the imposed strain. Furthermore, the latter results imply that the distance between the two binding sites of the IgE molecule lies in the range dictated by the rigid oligoproline part of the respective hapten's spacer, i.e., 115-130 A. (4) Nearly all oligomeric ring-closure processes proceed relatively slowly with an approximate lower limit of a half-life of 5-10 s. This slowing down of the aggregation and ring-closure processes most probably reflects steric factors.

Does the bifunctional uridylate synthase channel orotidine 5'-phosphate? Kinetics of orotate phosphoribosyltransferase and orotidylate decarboxylase activities fit a noninteracting sites model

Uridylate synthase is a bifunctional protein that first forms orotidine 5'-phosphate (OMP) from orotate via its orotate phosphoribosyltransferase activity (EC 2.4.2.10) and then converts OMP to uridine 5'-phosphate (UMP) via the OMP decarboxylase activity (EC 4.1.1.23). A computer modeling analysis of the experiments that led to the proposal [Traut, T.W., & Jones, M.E. (1977) J. Biol. Chem. 252, 8374-8381] that uridylate synthase channels intermediate OMP suggests that the experimental results do not demonstrate preferential use of OMP generated in the bifunctional complex as against exogenous OMP. This analysis shows that the experimentally observed amounts of [6-14C]UMP from [6-14C]orotate in the presence of various amounts of exogenous [7-14C]OMP agree well with the amounts predicted by the computer simulations. Thus we conclude that uridylate synthase does not channel OMP. Additionally, the subsequent suggestion that channeling of OMP occurs to protect the intermediate from degradation by a nucleotidase [Traut, T.W. (1980) Arch. Biochem. Biophys. 200, 590-594] seems unlikely. The appropriate computer simulation demonstrates that low transient levels of OMP and protection of the intermediate are provided for strictly by the kinetic parameters of orotate phosphoribosyltransferase, OMP decarboxylase, and the nucleotidase. Additionally, calculations show that, in both sets of published experiments, the concentration of transient OMP greatly exceeded the concentration of OMP decarboxylase active sites. Thus, channeling of OMP by the bifunctional complex cannot be invoked to explain the evolution of uridylate synthase, and that event must be the result of some other selective pressure.

Molecular shrinkage of proteoglycans

Viscometry and gel chromatography of mixtures of proteoglycans with other linear flexible polymers suggest that proteoglycans shrink as the concentration of the linear polymer is increased. Similar behavior was observed for binary proteoglycan solutions using a differential viscometry procedure. The shrinkage does not involve specific chemical properties of the linear polymer, but rather is a consequence of purely entropic excluded volume interactions with the proteoglycans. Comparison with a hydrodynamic model supports this conclusion. The polydisperse proteoglycan preparation was subfractionated, and the individual fractions were tested for shrinkage. Fractions of lower molecular weight were found to shrink to a greater extent, suggesting that the molecules are more flexible when they contain fewer glycosaminoglycan chains.

Prediction of resources needed to achieve the national target for treatment of renal failure

A model of the treatment of end stage renal failure has been primed with observed survival statistics and used to predict the steady state that will be achieved when the present annual target of at least 40 new patients per million population is in equilibrium with the death rate. The number of patients expected to receive each type of dialysis or a transplant is given per million of population. The personnel and facilities required to care for these patients and the costs of each programme were derived using analyses of workloads and costs in the North Western Regional Health Authority. The study has documented the considerable need for "back up" beds for patients on dialysis who require temporary care as inpatients. The cost effectiveness of transplantation has been demonstrated; at steady state it is calculated that each successful graft saves the service pounds 30,000. Implementation of the minister's minimum target requires a build up to some three times the resources currently allocated, with parallel increases in numbers of medical, nursing, and other essential staff.

Stopped-flow studies on the mechanism of oxidation of N-methyl-4-phenyltetrahydropyridine by bovine liver monoamine oxidase B

The kinetic mechanism of monoamine oxidase B involves either a binary or a ternary complex, depending on the substrate. In this study, stopped-flow kinetic data provide direct evidence for ternary complexes not only of reduced enzyme, oxygen, and product but also of reduced enzyme, oxygen, and substrate, both for benzylamine and for the tertiary amine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). However, the mechanism for a given substrate is not exclusive but, rather, is determined by competition between the alternate pathways as a result of different rate constants for the oxidation of the reduced enzyme, the reduced enzyme-product complex, and the reduced enzyme-substrate complex, as well as the different dissociation constants for the complexes. Comparison of the rate constants obtained from the stopped-flow studies with steady-state data indicates that the overall rate of reaction for the oxidation of MPTP by monoamine oxidase is dominated by the reductive step, but for benzylamine the steady-state rate is determined by a complex function of the rates of both the reductive and oxidative half-reactions.

Measures of social class based on education for use in health studies in developing countries

In this paper we consider the appropriateness of education, compared to occupation and income, as a measure of social class for use in health-related studies in developing societies in transition. Three evaluation criteria were used, namely, the feasibility of constructing the measure, its sensitivity in reflecting relevant social class life conditions, and its ability to produce a family-level measure of social class. We used two data sets from community health surveys in areas of Amman city, Jordan, and in Beirut city, Lebanon, to define a family-based average educational score. We then proceeded, using the Beirut data, to test the score's ability to discriminate social class effects on family health, compared to a more standard representation based on the educational level of the head of the family. It was found that the performance of the average educational score was often better than, but not consistently superior to, the educational level of the head of the family.

Analysis of total mortality in the Rotterdam sample of the Kaunas-Rotterdam Intervention Study (KRIS)

In 3284 middle-aged Rotterdam men, the dependency of 9-year survival on a number of cardiovascular risk factors is analysed. The purpose of this analysis is twofold. First, there is interest in the relationship between total mortality and the continuous variables blood pressure and plasma cholesterol. In accordance with other studies, the results provide further evidence for a U-shaped relationship. The second purpose is to compare three models theoretically as well as empirically: a piecewise exponential distribution of survival time, Cox's proportional hazards model (both with a long-linear dependency specification of the mortality rate), and a logistic model. The disadvantage of a logistic model is that it is theoretically not appropriate in longitudinal (prospective) epidemiologic studies. The theoretical advantage of the piecewise exponential distribution to Cox's model is that longitudinal (time) effects on the mortality rate can be specified and estimated directly. Empirically, with 342 deaths all three methods yield quite similar estimates and therefore are almost equally capable of detecting relationships between mortality and the risk factors in the data set at hand.

Clinician decisions and computers

The clinician's decisions are subject to numerous distorting influences. Computer decision aids can help avoid these distortions by placing the clinician's limited personal experience into broader perspective through comparison with a larger repository of clinically relevant information; by making explicit the assumptions implied by his or her decisions; and by alerting the clinician whenever the decisions made do not appear consistent with these assumptions, with the available information or with the conventional rules of logic. Practical standards of performance with respect to the development, validation and clinical application of these decision aids are still in evolution, however, and a variety of ethical and legal issues have yet to be addressed. Despite the promise of computer decision aids, it remains to be seen whether their diffusion into medical practice will improve the quality and cost of health care.

Aggregate health care expenditures and national income. Is health care a luxury good?

It is well known that a strong relationship exists between national expenditures on health care and national income. This has been used to suggest that health care is a luxury good, and that factors such as the type of health care delivery system in a country are of little importance in determining expenditure levels. This paper argues that these implications rely upon the application of microeconomic analysis to macroeconomic data, and that this is not appropriate. As well as raising questions about the inferences drawn from previous studies, new empirical evidence is presented which casts some doubts on previous findings. International comparisons are based on Purchasing Power Parity rather than exchange rate conversions, underlining the importance of prices as well as quantities in the relationship, and leading to the conclusion that the aggregate data show health care to be, if anything, a necessity rather than a luxury good.

A demographic-economic explanation of political stability: Mauritius as a microcosm

"This paper examines current models of economic and political development--social modernization theory, political and economic characteristics of stable regimes, and cross country analysis of political stability--and tests them on the Indian Ocean Island of Mauritius. The analysis continues with a causal explanation for political stability in Mauritius' recent history, derived from an examination of economic policies and demographic patterns. Political change in Mauritius over the past sixty years seems to be explained best by a model for political stability which integrates specific economic and demographic factors. The model, applicable to development in other third world nations, revises Malthus' conclusion that population and economic conditions move in an oscillatory relationship and replaces it with a more comprehensive theory, suggesting that political stability is a function of both economic development and a repeating cyclical relationship between economics and population."

Strategic planning for hospitals--a health-needs approach

This paper presents a strategic planning approach which provides methods for analysing hospitals in relation to their environment and for assessing their community dependency. Hospitals must not only focus on their clienteles but must also plan their future in relation to the needs of their constituency while taking into account the other elements of the health services network. The proposed approach also takes into account the need for all the actors concerned with the future of the hospital to become involved through an active-reactive-adaptive process.

Computation of the luminance and pattern components of the bar pattern electroretinogram

Pattern onset electroretinograms (PERGs) were recorded from four normal subjects. Square-wave gratings of 75% contrast were presented in three approximately contiguous, concentric zones of outer angular radius, 5.1 degrees, 12.6 degrees, and 23.6 degrees. The zones were calculated to give equal numbers of ganglion cell receptive fields. The recorded PERGs were considered to include luminance and pattern components which have low and bandpass spatial tuning functions respectively. These components combine in the PERG to produce a broad spatial tuning characteristic. The amplitude of PERGs in response to low spatial frequency stimuli is widely reported to be linearly related to contrast. The retinal illuminance response at every spatial frequency was computed from the eye's modulation transfer function. This function characterizes the reduction in contrast that occurs because of optical degradation. The computed retinal illuminance response was subtracted from the PERG waveform and a pattern-specific response was revealed. The latter had a highly tuned bandpass function which peaked at higher spatial frequencies than the PERG at corresponding peripheral angles.

The effects of image degradation on retinal illuminance and pattern responses to checkerboard stimuli

The contrast of a retinal image is less than that of the external stimulus owing to a process of optical degradation. Theoretical studies have shows that this affects the pattern and illuminance detectors of the retina differently and provides a new insight into the nature of contrast stimulation and the mechanisms responsible for the pattern electroretinogram. Consensus data on the optical transfer function of the eye are applied to the Fourier transform of the pattern stimulus and the retinal illuminance distribution is determined. The checkerboard image is shown to undergo substantial degradation for those check sizes used in experimental and clinical observations. Current concepts of contrast and modes of stimulation are examined and methods are described for quantifying the effects on stimulation of illuminance and pattern detectors. The findings are applied to experimental data on spatial tuning functions of electroretinograms elicited by checkerboard pattern stimulation. It is concluded that the signal predominantly originates from local illuminance, but that this can account for only part of the response with small check sizes. The remainder must be a highly selective response to spatial frequency. If the predicted degradation is accepted, a similar conclusion must be reached for many previously reported tuning functions of the pattern electroretinogram as well as to the present experiment on eight normal eyes.

Spreadsheets simplify sensitivity analysis for capital decisions

The availability and ease of use of electronic spreadsheets removes the previously cumbersome number crunching burden of sensitivity analysis. In this article, the advantages of sensitivity analysis will be reintroduced and a model will be presented using a pro forma statement for a freestanding magnetic resonance imaging center.

Calculation of inhibitor Ki and inhibitor type from the concentration of inhibitor for 50% inhibition for Michaelis-Menten enzymes

The use of I50 (concentration of inhibitor required for 50% inhibition) for enzyme or drug studies has the disadvantage of not allowing easy comparison among data from different laboratories or under different substrate conditions. Modifications of the Michaelis-Menten equation for treatment of inhibitors can allow both the determination of the type of inhibition (competitive, noncompetitive, and uncompetitive) and the Ki for the inhibitor. For competitive and uncompetitive inhibitors when the assay conditions are [S] = Km, then Ki = I50/2. For different conditions of [S] there is a divergence between competitive and uncompetitive inhibitors that may be used to identify the type of inhibitor. The equation for Ki also differs. For noncompetitive inhibitors the Ki = I50 and this relationship is valid with changing [S]. The equations developed require a single substrate, reversible-type inhibitors, and kinetics of the Michaelis-Menten type. Examples of the use of the equations are illustrated with experimental data from scientific publications.

Bronchial mechanical properties and maximal expiratory flows

Flow limitation in a collapsible elastic tube is dependent on the area (A) vs. pressure (P) relationship (the "tube law") for the tube. In this paper, a tube law in which A varies as (1-P)-n1 at negative pressures is assumed. It is shown that wave-speed limitation is possible at negative pressures only if n1 is greater than 0.5. Dissipative limitation is also investigated. Viscous limitation can occur if n1 is greater than 0.5, and turbulent limitation can occur if n1 is not less than 0.4. For values of n1 less than 0.4, flow cannot be limited at negative pressures. Model simulations are used to show that a combination of a value of n1 less than 0.3 together with an area minimum in the bronchial tree produce a minimum (a "hook") in the flow-volume curve. In the vicinity of such hooks, density dependence exceeds the usually accepted theoretical maximum value. Simulations also show that, when n1 is sufficiently small, apparently supramaximal flows appear to be possible.

The specification of a hospital cost function. A comment on the recent literature

In the empirical estimation of hospital cost functions, two radically different types of specifications have been chosen to date, ad-hoc forms and flexible functional forms based on neoclassical production theory. This paper discusses the respective strengths and weaknesses of both approaches and emphasizes the apparently unreconcilable conflict between the goals of maintaining functional flexibility and keeping the number of variables manageable if at the same time patient heterogeneity is to be adequately reflected in the case mix variables. A new specification is proposed which strikes a compromise between these goals, and the underlying assumptions are discussed critically.

Health insurance and the demand for medical care: evidence from a randomized experiment

We estimate how cost sharing, the portion of the bill the patient pays, affects the demand for medical services. The data come from a randomized experiment. A catastrophic insurance plan reduces expenditures 31 percent relative to zero out-of-pocket price. The price elasticity is approximately -0.2. We reject the hypothesis that less favorable coverage of outpatient services increases total expenditure (for example, by deterring preventive care or inducing hospitalization).

Multiregional two-sex population model

"The theory of stable population has been developed quite thoroughly in the demographic literature, but it deals with single-region populations that are assumed to be undisturbed by migration. Rogers...proposed the stable population model for the case of a multiregional population that experiences internal migration for single-sex. In this paper, considering a birth-function dependent on both sexes, the multiregional stable population model is extended for two-sex population."

Developing new services for hospitals: a suggested model

Hospitals tend to use an incomplete means of developing new services. The result is a lack of attention to the needs of the intended target markets. A model for developing new hospital services is suggested, one that allows greater input from the service recipients. An illustrative case is presented.

Three components in the establishment of a certification testing program

Certification in the health professions is growing both in number of programs and in quality. Three important components in the development of any sound and defensible certification testing program are presented and discussed. The three are interrelated and sequential, and each has a growing technology. Guidelines are presented and discussed that should assist those involved in establishing new certification program testing that is more adequately prepared for the difficult challenges typically faced.

Effectiveness of a salt transport cascade in the renal medulla: computer simulations

Based on morphological observations, it has been concluded that the upper parts of the long descending limbs of Henle's loops should be able to secrete salts into the tubular fluid (Kriz, W. Federation Proc. 42:2379-2385, 1983). In the same article, a hypothesis is given depicting how this active transport in concert with certain characteristics of the medullary architecture might produce a transport cascade toward the papillary tip, thus supporting the accumulation of salts in this region. The effectiveness of the proposed mechanism can be judged by a mathematical model only. The computer simulations of the present study demonstrate that this mechanism indeed leads to an increase of the concentrating capability of the renal countercurrent system.

The volume-outcome relationship: practice-makes-perfect or selective-referral patterns?

Various studies have demonstrated that hospitals with larger numbers of patients with a specific diagnosis or procedure have lower mortality rates. In some instances, these results have been interpreted to mean that physicians and hospital personnel with more of these patients develop greater skills and that this results in better outcomes--the "practice-makes-perfect" hypothesis. An alternative explanation is that physicians and hospitals with better outcomes attract more patients--the "selective-referral pattern" hypothesis. Using data for 17 categories of patients from a sample of over 900 hospitals, we examine the patterns of selected variables with respect to hospital volume. To explore the plausibility of each hypothesis, a simultaneous-equation model is also used to test the relative importance of the two explanations for each diagnosis or procedure. The results suggest that both explanations are valid, and that the relative importance of the practice or referral explanation varies by diagnosis or procedure, in ways consistent with clinical aspects of the various patient categories.

Horse serum butyrylcholinesterase kinetics: a molecular mechanism based on inhibition studies with dansylaminoethyltrimethylammonium

The kinetics of the hydrolysis of butyrylthiocholine by horse serum butyrylcholinesterase (acylcholine acylhydrolase; BuChE; EC 3.1.1.8) exhibit an activation phenomenon at high substrate concentrations. At least two mechanistic models can account for the enzyme kinetics: one assumes the binding of an additional substrate molecule on the acyl-enzyme intermediate, and the other hypothesizes the existence of a peripheral regulatory site for the substrate. (1-Dimethylaminonaphthalene-5-sulfonamidoethyl)-trimethylammonium perchlorate, a potent reversible inhibitor, appears to affect BuChE activity by binding to a peripheral site. The inhibition is of the mixed type at low substrate concentrations and of the competitive type at high substrate concentrations. This is consistent with a peripheral site for the binding of the substrate responsible for the activation phenomenon.

An equation for the systemic availability of drugs undergoing simultaneous enterohepatic cycling, first-pass metabolism, and intestinal elimination

A relationship between systemic availability and its determinants has been derived for a physiologically realistic model of drug disposition that includes enterohepatic cycling (EHC), gallbladder emptying (with an arbitrary time course), first-pass metabolism to noncycling metabolites, and fecal excretion. Systemic availability (F) has been shown to be determined by the fraction of the dose initially absorbed (f*a), the fraction of the drug excreted into the GI tract that is reabsorbed with each cycle (fa), the hepatic extraction ratio (E), and the fraction of extracted drug that is transported to the gallbladder for EHC (fg) according to the relationship F = f*a(1 - E)/(1 - fafgE) The implications of the above relationship are that (1) systemic availability is dependent on EHC, (2) values of F calculated to be greater than unity cannot be explained simply by the presence of EHC, (3) calculations of E based on the usual expression F = f*a(1 - E) are erroneous for drugs subject to EHC, and (4) a compound that has a high systemic availability and is subject to EHC is not necessarily inefficiently metabolized. The quantitative interrelationship of systemic availability and its determinants is illustrated using a contour plot. Slices through the surface are used to demonstrate that the presence of EHC changes the sensitivity of F to changes in E.

A statistical model examining repetitive criminal behavior in acts of violence

A simple statistical model for examining repetitive criminal behavior in acts of violence is described. The units, called "parameters," are nonquestionable data concerning environment of the crime, personal properties, and postmortem findings of the victim, obtained by double-blind investigation performed by two forensic pathologists. Parameters shared by two or more criminal acts allegedly committed by the same assailant were compared with the same parameters recorded from 50 or 100 other mutually independent criminal acts committed by other known assailants. This allowed an evaluation of the probability (p) of a crime pattern expressed as a parameter score to recur in mutually independent cases. The distribution of the score, when plotted on a logarithmic scale in all examples, showed an approximately normal distribution. The relation between probability (p), the estimated mean (means), and standard deviation (SD) yielded a normal curve. Different patterns of action by different perpetrators and patterns indicating repetitive behavior could be obtained. The method is applicable during investigation of crimes in which the perpetrator acts in a repetitive manner, as in serial murders.

A model for diabetes health care delivery in developing countries

Developing countries can make considerable progress in providing diabetes health care if they adopt ideas and technologies already being applied in diabetes health care in other parts of the world, but they must adapt them to their own particular circumstances. Too often efforts in this direction fail because it is not possible to transfer programs from developed nations intact and expect them to function in the circumstances of less developed nations. Success on a national level requires the application of a systematic health care model tailored to the needs of the developing country. As discussed in this paper, the model's elements include organization of the available national health care delivery system; education of health care personnel, patients, their families, and communities; diabetes-related epidemiological research; education program planning; and continuous program evaluation and adjustment.

[A further test of a multivariate forecasting model for fertility]

A model developed by J. Nelissen and A. Vossen, for making short-term population projections, is first described. In this model, the fertility rate is related to the unemployment rate and the fraction of the labor force employed in agriculture. The model is then applied to Dutch data for the period 1980-1985. "The main conclusions are that the results depend heavily on the choice of the forecasting period and that the ex-post forecasts of Nelissen and Vossen's model are out-performed by univariate ARIMA-projections." (SUMMARY IN ENG)

Rival Conceptions in Doctoral Education in Nursing and Their Outcomes: An Update

The need for two distinct types of doctoral programs in nursing was historically well conceived and well articulated. The growing literature on the doctoral education in nursing, however, shows that the implementation of the concept has not produced the intended results, that is the coexistence of academic and professional doctoral programs in nursing has not resulted in the differentiated career paths for the graduates. Instead, the competing aims of doctoral education in nursing, paved the way to varying doctoral titles, and weakened the claims of professional doctoral programs as preparing for advanced clinical practice. More significantly, however, this development is threatening the future existence of professional doctoral programs in nursing.

Secular changes in early neonatal mortality in Norway, 1967-1981

Two five-year cohorts, 1967-1971 and 1977-1981, of the Medical Birth Registry of Norway, were utilized to analyze secular trends in early neonatal mortality rates, controlling simultaneously for birth weight, parity, maternal age, and sex. Improvement in the crude early neonatal mortality rate, from 6.5 to 2.9 per 1,000, was partitioned into one portion (18.4%) attributable to changes in the distribution of the birth weight or the other independent variables and another portion (81.6%) attributable to improved survival within each specific subgroup in the multivariate table. The early neonatal mortality rate was found to decrease in all subgroups, the low birth weight groups showing a significantly greater decline than the normal birth weight groups. Women with diabetes, epilepsy, or blood group antibodies were studied in separate analyses. For women with diabetes, the odds ratio of the early neonatal mortality rate between the first and the second periods was found to be three times the odds ratio for women without diabetes, and 2.9% of the total improvement of the crude early neonatal mortality rate was attributable to an improved survival in these infants.

Perceived health damages during a physicians' strike in Israel

This study analyzed the relationship between perceived need for health services and perceived damages to the health of the public due to curtailment of medical services during a physicians' strike. A model describing this relationship was hypothesized and tested in one city in Israel during the course of the strike. Results indicated that significant damage was perceived by the public as a result of the strike, a perception that persisted despite utilization of alternative care centers set up by the strikers. Furthermore, it was shown that financially weaker segments of the population were more likely to perceive damages to their health during a strike than were the more affluent segments of society.

Hidden bias in the use of archival data

Nonresponses in archival data may violate the missing-at-random assumption in ways difficult to detect. Standard methods of comparing sociodemographics of respondents and nonrespondents are inappropriate when the units of analysis are not also the individuals who maintain the archival record. Under these circumstances, the distribution of missing data may be correlated with the dependent variable and traits of the record keepers. This will distort relationships, especially when listwise deletion of missing values is used in multivariate analysis. Data are used from a large clinical chart study of mentally ill patients to demonstrate the process of identifying hidden bias and the implications of such bias.

Hospital challenge: using change theory and processes to adopt and implement the marketing concept

Getting the marketing concept understood and accepted is one of the biggest challenges faced by a hospital. The authors discuss a variety of issues related to the process of changing a hospital's internal cultural values and managing those changes needed to embrace the marketing concept as part of the overall hospital culture.

Models for temporal variation in cancer rates. II: Age-period-cohort models

Our first paper reviewed methods for modelling variation in cancer incidence and mortality rates in terms of either period effects or cohort effects in the general multiplicative risk model. There we drew attention to the difficulty of attributing regular trends to either period or cohort influences. In this paper we turn to the more realistic problem in which neither period nor cohort effects alone lead to an adequate description of the data. We describe the age-period-cohort model and show how its ambiguities surrounding regular trends 'intensify'. We recommend methods for presenting the results of analyses based upon this model which minimize the serious risk of misleading implications and critically review previous suggestions. The discussion is illustrated by an analysis of breast cancer mortality in Japan with special reference to the phenomenon of 'Clemmesen's hook'.

Innovative retention strategies for nursing staff

The creation of innovative retention strategies will be a major focus for nursing administration as a shortage of nurses recurs and turnover of staff becomes a problem. A recent study provides information on which to formulate retention strategies. The findings suggest that retention strategies, to be effective, need to be targeted specifically to particular conditions of the nursing staff, e.g., educational preparation and the clinical service on which staff are functioning. The authors outline their research findings and the innovative strategies that have been constructed.

[Description of the magnetic resonance relaxation behavior of tissues using a suitable theoretical model]

An introduction into the physicochemical background of MR imaging is given by illustrating the MR relaxation processes that are relevant for body tissues. In particular the importance of molecular mobility (correlation time tau c) and resonance frequency omega o to NMR relaxation times T1 and T2 is explained.