Statistical process control of mortality series in the Australian and New Zealand Intensive Care Society (ANZICS) adult patient database: implications of the data generating process

for the ANZICS Centre for Outcome and Resource Evaluation (CORE) of the Australian and New Zealand Intensive Care Society (ANZICS)BACKGROUND Statistical process control (SPC), an industrial sphere initiative, has recently been applied in health care and public health surveillance. SPC methods assume independent observations and process autocorrelation has been associated with increase in false alarm frequency. METHODS Monthly mean raw mortality (at hospital discharge) time series, 1995–2009, at the individual Intensive Care unit (ICU) level, were generated from the Australia and New Zealand Intensive Care Society adult patient database. Evidence for series (i) autocorrelation and seasonality was demonstrated using (partial)-autocorrelation ((P)ACF) function displays and classical series decomposition and (ii) “in-control” status was sought using risk-adjusted (RA) exponentially weighted moving average (EWMA) control limits (3 sigma). Risk adjustment was achieved using a random coefficient (intercept as ICU site and slope as APACHE III score) logistic regression model, generating an expected mortality series. Application of time-series to an exemplar complete ICU series (1995-(end)2009) was via Box-Jenkins methodology: autoregressive moving average (ARMA) and (G)ARCH ((Generalised) Autoregressive Conditional Heteroscedasticity) models, the latter addressing volatility of the series variance. RESULTS The overall data set, 1995-2009, consisted of 491324 records from 137 ICU sites; average raw mortality was 14.07%; average(SD) raw and expected mortalities ranged from 0.012(0.113) and 0.013(0.045) to 0.296(0.457) and 0.278(0.247) respectively. For the raw mortality series: 71 sites had continuous data for assessment up to or beyond lag ₄₀ and 35% had autocorrelation through to lag ₄₀; and of 36 sites with continuous data for ≥ 72 months, all demonstrated marked seasonality. Similar numbers and percentages were seen with the expected series. Out-of-control signalling was evident for the raw mortality series with respect to RA-EWMA control limits; a seasonal ARMA model, with GARCH effects, displayed white-noise residuals which were in-control with respect to EWMA control limits and one-step prediction error limits (3SE). The expected series was modelled with a multiplicative seasonal autoregressive model. CONCLUSIONS The data generating process of monthly raw mortality series at the ICU level displayed autocorrelation, seasonality and volatility. False-positive signalling of the raw mortality series was evident with respect to RA-EWMA control limits. A time series approach using residual control charts resolved these issues.John L Moran, Patricia J Solomo

Longevity of a Controlled Burn’s Impacts on Species Composition and Biomass in Northern California Annual Rangeland During Drought

Controlled burning timed in early summer can dramatically change the species composition of annual rangeland the following season. Although this has been well documented, the longevity of these shifts has not. Presented is a case study of a single 200-ha burn to begin to understand how long plant communities and biomass production remain diverged between burned and unburned annual rangeland. Species composition and biomass production were monitored before and for 3 yr after burning. Burning drastically reduced medusahead (Taeniatherum caputmedusae; P < 0.01) the following year from 69% in the control to 4% cover in the area burned. In the same year, filaree (Erodium spp.; P < 0.01) filled in the area left vacant, subsequently lessening production (P < 0.01) in the burn area by over half that of the control. No difference existed in the occurrence of native wildflower species due to fire. Three consecutive drought years following the burn shifted the control from medusahead dominance to filaree in a linear fashion. At the same time, in the burned area medusahead cover increased fourfold between 1 and 3 yr after the burn. By 3 yr post burning, the area had 4% more medusahead cover than the control and was equal in filaree, rose clover (Trifolium hirtum), and soft brome (Bromus hordeaceus) cover. Our results suggest that a controlled burn followed by drought can cause the divergence in species composition and production to become void in as little as 3 yr after a well-timed burn in a low-elevation annual rangeland system. © Published by Elsevier Inc. on behalf of The Society for Range Management.The Rangeland Ecology & Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact [email protected] for further information