Multilevel modelling of cost data: an application to thrombolysis and primary angioplasty in the UK NHS

Cost data are frequently collected from several locations and tend to be non negative and skewed. Generalised linear multilevel models provide a means of dealing with each of these issues. This paper compares several statistical models within this class using data drawn from an observational study of 3,000 patients treated for heart attack in 15 UK NHS hospitals. A number of alternative link functions and covariates were considered. We demonstrate that whilst it is important to take account of clustering in the data, the precise manner in which this is done is equally important. Models which allow for correlation between the random effects components and heteroskedasticity across all hospitals performed best in terms of model fit and made substantial di¤erences to cost estimates

Vástagos cementados y no cementados en artroplastias totales de cadera por coxopatías mecánicas

En la controversia sobre la cementación o no del componente femoral se presenta el análisis clínico, radiológico y de supervivencia de los vástagos de una serie de 121 artoplastias totales de cadera (61 cementadas y 60 sin cementar). El estudio no evidencia diferencias significativas a los 5 años de seguimiento, aunque es destacable una mayor incidencia de dolor en el muslo entre los vástagos no cementados (35%) (p < 0,1) y una tasa de reabsorción ósea proximal más elevada (70%) que en los cementados (23%). Estos fenómenos son diseño dependientes. Un mayor número de cirugías de revisión por aflojamiento aséptico entre las prótesis cementadas nos hacen preferir la fijación sin cemento del componente femoral.We discuss the cemented or non-cement fixation of the femoral components in total hip arthroplasty on a serie of 121 femoral stem (61 cemented stem and 60 non-cemented stem). Survival analysis, clinical and radiographic data showed no significant differences between the two groups at 5 years follow-up. However the incidence of pain thigh phenomenon was higher for the non-cemented stem (35%) (p < 0.1). Stress schielding was evidently higher between the non-cemented implants (70%) too, as compared with cemented stem (23%). These findings are design-dependent. The cemented arthroplasties had a higher rate of revision procedures because of aseptic loosening (13%). In this way we prefer the non-cement fixation of the femoral implant

Common scale valuations across different preference-based measures: estimation using rank data

Background: Different preference-based measures (PBMs) used to estimate Quality Adjusted Life Years (QALYs) provide di¤erent utility values for the same patient. Differences are expected since values have been obtained using different samples, valuation techniques and descriptive systems. Previous studies have estimated the relationship between pairs of PBMs using patient self-reported data. However, there is a need for an approach capable of generating values directly on a common scale for a range of PBMs using the same sample of general population respondents and valuation technique but keeping the advantages of the different descriptive systems. Methods: General public survey data (n=501) where respondents ranked health states described using subsets of six PBMs were analysed. We develop a new model based on the mixed logit to overcome two key limitations of the standard rank ordered logit model, namely, the unrealistic choice pattern (Independence of Irrelevant Alternatives) and the independence of repeated observations. Results: There are substantial differences in the estimated parameters between the two models (mean di¤erence 0.07) leading to di¤erent orderings across the measures. Estimated values for the best states described by di¤erent PBMs are substantially and significantly di¤erent using the standard model, unlike our approach which yields more consistent results. Limitations: Data come from a exploratory study that is relatively small both in sample size and coverage of health states. Conclusions: This study develops a new, �exible econometric model specifically designed to reflect appropriately the features of rank data. Results support the view that the standard model is not appropriate in this setting and will yield very different and apparently inconsistent results. PBMs can be compared using a common scale by implementation of this new approach

Avalanche dynamics in fluid imbibition near the depinning transition

We study avalanche dynamics and local activity of forced-flow imbibition fronts in disordered media. We focus on the front dynamics as the mean velocity $\bar{v}$ of the interface is decreased and the pinning state is approached. Scaling arguments allow us to obtain the statistics of avalanche sizes and durations, which become power-law distributed due to the existence of a critical point at $\bar{v}= 0$. Results are compared with phase-field numerical simulations