Multistate Model Approach Research Articles

Multiscale patterns of movement in fragmented landscapes and consequences on demography of the snail kite in Florida

1. Habitat loss and fragmentation are major factors affecting vertebrate populations. A major effect of these habitat alterations is that they reduce movement of organisms. Despite the accepted importance of movement in driving the dynamics of many natural populations, movement of vertebrates in fragmented landscapes have seldom been estimated with robust statistical methods. 2. We estimated movement probabilities of snail kites Rosthramus sociabilis within the remaining wetlands in Florida. Using both radio-telemetry and banding information, we used a multistate modelling approach to estimate transition probabilities at two temporal scales (month; year) and multiple spatial scales. We examined kite movement among wetlands altered by three different levels of fragmentation: among wetlands separated by small physical barriers (e.g. road); among wetlands separated by moderate amount of matrix (< 5 km); and among wetlands separated by extensive matrix areas (> 15 km). 3. Kites moved extensively among contiguous wetlands (movement probability 0.29 per month), but significantly less among isolated wetlands (movement probability 0.10 per month). 4. Kites showed high levels of annual site fidelity to most isolated wetlands (probability ranged from 0.72 to 0.95 per year). 5. We tested the effects of patch size and interpatch distance on movement. Our modelling indicated an effect of both distance and patch size on juveniles' movement (but not adult) when examining movements among fragments. 6. Only a small proportion of kites escaped a regional drought by moving to refugia (wetlands less affected by drought). Many individuals died after the drought. During drought adult survival dropped by 16% while juvenile survival dropped by 86% (possibly because juveniles were less likely to reach refugia). 7. We hypothesize that fragmentation may decrease kite's resistance to drought by restricting exploratory behaviour.

Analyzing the duration of recurrent events in clinical trials: A comparison of approaches using data from the UK700 trial of psychiatric case management

In studies of chronic disease the outcome measure may be based upon the duration of recurring illness. Our example is the UK700 trial of psychiatric case management, where the total number of days spent in hospital over a 2-year follow-up was the primary outcome. Investigations of treatment effect modifiers were undertaken using an analysis of that primary outcome, a comparison of length of hospitalizations, and also a multi-state modeling approach. The days in hospital outcome was relatively straightforward to analyze, and allowed the complete randomized treatment groups to be compared. In contrast, the comparison of length of hospitalizations included only hospitalized patients, with censored observations not being well accommodated. The multi-state model provided separate treatment effect estimates for admission and discharge, this being more informative about how any reduction in days spent in hospital is achieved. Estimation of the treatment effects through the use of proportional hazards regression allowed appropriate incorporation of censored observations. However, with the multi-state model approach treatment effect estimates are not based upon comparisons of complete randomized treatment groups, as individuals are removed from the risk set for admission whilst at risk for discharge, and vice versa. We conclude that total duration is an appropriate primary outcome for clinical trials, but that multi-state models deserve greater use as an informative secondary analysis.

The effect of ursodeoxycholic acid therapy on the natural course of primary biliary cirrhosis

We used a multistate modeling approach to assess the effect of ursodeoxycholic acid (UDCA) therapy on the natural course of primary biliary cirrhosis (PBC), which remains controversial. Our population included 262 patients with PBC who had received 13-15 mg/kg UDCA daily for a mean of 8 years (range, 1-22 years). Data were analyzed using a multistate Markov model, with histologic stage progression, death, and orthotopic liver transplantation (OLT) as main end points. Survival without OLT was compared with that predicted by the updated Mayo model and with the expected survival in the control population. Forty-five patients developed cirrhosis, 20 underwent OLT, and 16 died by the censor date. Ten deaths were due to liver disease. The overall survival rates were 92% at 10 years and 82% at 20 years. Survival rates without OLT were 84% and 66% at 10 and 20 years, respectively, which were slightly lower than the survival rate of an age- and sex-matched control population (relative risk [RR], 1.4; P = .1) but better than the spontaneous survival rate as predicted by the updated Mayo model (RR, .5; P < .01). The survival rate of patients in stage 1 and 2 was similar to that in the control population (RR, .8; P = .5), whereas the probability of death or OLT remained significantly increased in treated patients in late histologic stages (RR, 2.2; P < .05). Treatment with UDCA alone normalizes the survival rate of patients with PBC when given at early stages. However, there is a continued need for new therapeutic options in patients with advanced disease.

Dispersal and survival rates of adult and juvenile Red-tailed tropicbirds (Phaethon rubricauda) exposed to potential contaminants

Annual survival and dispersal rates of adult and juvenile red-tailed tropicbirds were examined in connection with exposure to heavy metals. From 1990-2000 the incineration of a U.S. stockpile of chemical weapons stored at Johnston Atoll exposed nesting tropicbirds to increased levels of human disturbance, smoke stack emissions and potential leaks. Using a multi-state mark-recapture modeling approach, birds nesting in this site (downwind of the plant) were compared to those nesting in a reference site (upwind of the plant) with less human disturbance, no exposure to smoke stack emissions or other potential incineration emissions. We did not find any difference in survival of adults or juveniles when comparing the two sites. Adult breeding dispersal rates did not differ between the sites but we did find differences in the age-specific natal dispersal rates. Birds fledged from downwind areas were less likely to return to their natal area to nest and more likely to immigrate to the upwind area than vice-versa. This asymmetry in emigration rates is believed to be due to differing vegetation densities and has implications for vegetation management in relation to tropicbird nest success and population size.

Multi-state models for bone marrow transplantation studies.

High-dose chemotherapy followed by stem cell recovery, more commonly called a bone marrow transplant, is a common treatment for a number of diseases. This article examines four problems commonly encountered when dealing with bone marrow transplant studies. First, we look at the problem of competing causes of failure and at methods based on a multi-state model to estimate meaningful probabilities for these risks. Second, we examine methods for estimating the prevalence of an intermediate condition, here the prevalence of chronic GVHD. Third, we look at the problem of modeling the post transplant recovery process and we provide two examples of how these estimates can be used to assess dynamically a patient's prognosis or how these probabilities can be used to design trials of new therapy. Finally, we present an estimate of a new measure of treatment efficiency, the current leukemia free survival function, which is derived from a multi-state model approach.

Summary curves for patients transplanted for chronic myeloid leukaemia salvaged by a donor lymphocyte infusion: the current leukaemia-free survival curve.

A significant number of patients who relapse after allogeneic stem cell transplantation (SCT) for chronic myeloid leukaemia (CML) will achieve sustained molecular remissions after treatment with donor lymphocyte infusions (DLI) from the original stem cell donor. Leukaemia-free survival, defined as survival without evidence of relapse at any time after transplant does not account for patients who are successfully treated with DLI. To summarize adequately the response to treatment, a new summary probability, called the current leukaemia-free survival (CLFS), is proposed. This quantity is defined as the probability that a patient is alive and in remission at a given time after transplant. We discuss two statistical methods for estimating CLFS. The first is based on a multistate modelling approach. The second is based on an estimate constructed by looking at appropriate differences between Kaplan-Meier estimates. We compare these estimates using data on 189 consecutive patients who underwent SCT over a 7-year period.

The use of multi-state capture-recapture models to address questions in evolutionary ecology

Multi-state capture-recapture models can be used to estimate survival rates in populations that are stratified by location or by state variables associated with individual animals. In populations stratified by location, movement probabilities can be estimated and used to test hypotheses relevant to population genetics and evolutionary ecology. When the interest is in state variables, these models permit estimation and testing of hypotheses about state-specific survival probabilities. If the state variable of interest is reproductive activity or success, then the multi-state modeling approach can be used to test hypotheses about life history trade-offs and a possible cost of reproduction.