PubMed Health⌕ Search

Biomedical subjects

Michael Smithson

Publications and source records attributed to Michael Smithson.

4 recordsLinked to original sources
PubMed2006

A better lemon squeezer? Maximum-likelihood regression with beta-distributed dependent variables.

Uncorrectable skew and heteroscedasticity are among the "lemons" of psychological data, yet many important variables naturally exhibit these properties. For scales with a lower and upper bound, a suitable candidate for models is the beta distribution, which is very flexible and models skew quite well. The authors present maximum-likelihood regression models assuming that the dependent variable is conditionally beta distributed rather than Gaussian. The approach models both means (location) and variances (dispersion) with their own distinct sets of predictors (continuous and/or categorical), thereby modeling heteroscedasticity. The location sub-model link function is the logit and thereby analogous to logistic regression, whereas the dispersion sub-model is log linear. Real examples show that these models handle the independent observations case readily. The article discusses comparisons between beta regression and alternative techniques, model selection and interpretation, practical estimation, and software.

Analysis of Variance↗
PubMed2005

Impact of Federal drug law enforcement on the supply of heroin in Australia.

AIMS: To conduct an empirical investigation of the efficacy of law enforcement in reducing heroin supply in Australia. Specifically, this paper addresses the question of whether heroin purity levels in the Australian Capital Territory (ACT) could be predicted by heroin seizures at the national level by the Australian Federal Police (AFP) in the preceding year. DESIGN: We considered two forms of evidence. First, a Bayesian Markov Chain Monte Carlo (MCMC) change-point model was used to discover (a) if there was a substantial increase in heroin seizures by the AFP, (b) when the increase began and (c) whether it occurred after increased funding to the Australian Federal Police for the purpose of drug law enforcement. Second, standard time-series methods were used to ascertain whether fluctuations in heroin seizure weights or the frequency of large-scale seizures after the aforementioned changes in seizure levels predicted fluctuations in heroin purity levels in the ACT after autocorrelation had been removed from the purity series. FINDINGS: A Bayesian MCMC change-point model supported the hypothesis that heroin seizures rapidly increased about a year before the estimated decline in heroin purity and after the increased funding of AFP. The autoregression models suggested that 10-20% of the variance in the residuals of the heroin purity series was predicted by appropriately lagged residuals of the seizure-number and log-weight series, after autocorrelation had been removed. CONCLUSION: The overall results are consistent with the hypothesis that large-scale heroin seizures by the AFP reduce street-level heroin supply a year or so later, although the short-term dynamics suggest an 'opponent' response to residual fluctuations in seizures. To our knowledge, this is first time a connection has been identified between large-scale heroin seizures and street-level supply.

Australia↗
PubMed2004

The impact of illicit drug supply reduction on health and social outcomes: the heroin shortage in the Australian Capital Territory.

AIMS: We seek to establish whether a substantial decline in the supply of heroin, as measured by indicators such as drug purity, is related to changes in drug-related health indicators such as ambulance callouts to heroin overdoses and numbers participating in methadone treatment programmes, and to changes in levels of property crime. The guiding hypothesis is that reduced supply will result in positive health and social outcomes. DESIGN: Standard time-series methods are employed to analyse official data from local law-enforcement and heroin supply indicators and several health and social outcome indicators within the Australian Capital Territory (ACT), spanning the late 1990s to early 2002. Autoregressive moving average (ARMA) models are estimated to remove autocorrelation from these series. Cross-correlation and autoregression models are then employed to identify the best predictive models. FINDINGS: When autocorrelation has been removed, a reduction in heroin purity predicts a large decline in heroin-related ambulance callouts and an increase in methadone treatment programme enrolments. There is little evidence of an increase in negative outcomes due to heroin users switching to other drugs. A reduction in purity also predicts declines in robbery and burglary but not in theft. CONCLUSIONS: The overall evidence indicates modest links between the declines in heroin supply and increases in positive health outcomes and decreases in crime, as predicted by a simple economic model. Due to the shortness of some of the series and consequent limitations in statistical power, these conclusions should be regarded as tentative.

Australian Capital Territory↗