Data generation for the Cox proportional hazards model with time-dependent covariates: A method for medical researchers

The proliferation of longitudinal studies has increased the importance of statistical methods for time-to-event data that can incorporate time-dependent covariates. The Cox proportional hazards model is one such method that is widely used. As more extensions of the Cox model with time-dependent covariates are developed, simulations studies will grow in importance as well. An essential starting point for simulation studies of time-to-event models is the ability to produce simulated survival times from a known data generating process. This paper develops a method for the generation of survival times that follow a Cox proportional hazards model with time-dependent covariates. The method presented relies on a simple transformation of random variables generated according to a truncated piecewise exponential distribution, and allows practitioners great flexibility and control over both the number of time-dependent covariates and the number of time periods in the duration of follow-up measurement. Within this framework, an additional argument is suggested that allows researchers to generate time-to-event data in which covariates change at integer-valued steps of the time scale. The purpose of this approach is to produce data for simulation experiments that mimic the types of data structures applied researchers encounter when using longitudinal biomedical data. Validity is assessed in a set of simulation experiments and results indicate that the proposed procedure performs well in producing data that conform to the assumptions of the Cox proportional hazards model

Network Structure, Interracial Contacts, and the Evolution of Social Norms

In this paper I explore the underlying mechanisms of the changes in public discourse with respect to the issue of racial equality that have been observed in the United States over the course of its history, with a particular focus on the changes that occurred in the latter half of the twentieth century. Specifically, I provide a formal model of social interactions in which agents are assigned to non-homophilic networks, are heterogeneous with respect to preferences for equality between the races, and have preferences both to express their true preferences and to not appear deviant from the group. In a series of numerical experiments, results indicate that the probability of a transition in norms from an equilibrium around inequality to an equilibrium around equality is increasing in the size of the minority population and decreasing in the size of groups to which individuals are assigned

Informant single screening questions for delirium and dementia in acute care – a cross-sectional test accuracy pilot study

Background Cognitive impairment often goes undetected in older people in hospital. Efficient screening tools are required to improve detection.<p></p> To determine diagnostic properties of two separate informant-based single screening questions for cognitive impairment (dementia and delirium) in hospitalised older people.<p></p> Methods Patients over 65 years non-electively admitted to medical or geriatric wards within a teaching hospital. Our index tests were single screening questions (SSQ), one for dementia (“How has your relative/friend’s memory changed over the past 5 years (up to just before their current illness)?”) and one for delirium (“How has your relative/friend’s memory changed with his/her current illness?”), which were assessed with informant response given on a five point Likert scale.<p></p> Any deterioration on our index tests of SSQ-dementia and SSQ-delirium was accepted as a positive screen for cognitive impairment. Scores were compared to the Informant Questionnaire for Cognitive Decline in the Elderly (IQCODE) >3.38 accepted as dementia, and Confusion Assessment Method (CAM) diagnosis of delirium. We also collected direct cognitive screening data using Mini Mental Status Examination (MMSE).<p></p> Results Informant responses were obtained in 70/161 (43.5%) patients, median age 80.8 (range:67–97) years; mean MMSE score 18.5 (SD: 8.1). The SSQ-dementia when compared to the IQCODE had a sensitivity of 83.3% and specificity of 93.1%. The SSQ-delirium when compared to CAM diagnosis had sensitivity of 76.9% and a specificity of 56.1%.<p></p> Conclusions These findings show promise for use of an informant single screening question tool as the first step in detection of dementia in older people in acute hospital care, although this approach appears to be less accurate in screening for delirium.<p></p&gt

Reassessing Schoenfeld residual tests of proportional hazards in political science event history analyses

An underlying assumption of proportional hazards models is that the effect of a change in a covariate on the hazard rate of event occurrence is constant over time. For scholars using the Cox model, a Schoenfeld residual-based test has become the disciplinary standard for detecting violations of this assumption. However, using this test requires researchers to make a choice about a transformation of the time scale. In practice, this choice has largely consisted of arbitrary decisions made without justification. Using replications and simulations, we demonstrate that the decision about time transformations can have profound implications for the conclusions reached. In particular, we show that researchers can make far more informed decisions by paying closer attention to the presence of outlier survival times and levels of censoring in their data. We suggest a new standard for best practices in Cox diagnostics that buttresses the current standard with in-depth exploratory data analysis

Evaluation of delirium screening tools in geriatric medical inpatients: a diagnostic test accuracy study

Introduction: screening all unscheduled older adults for delirium is recommended in national guidelines, but there is no consensus on how to perform initial assessment. Aim: to evaluate the test accuracy of five brief cognitive assessment tools for delirium diagnosis in routine clinical practice. Methods: a consecutive cohort of non-elective, elderly care (older than 65 years) hospital inpatients admitted to a geriatric medical assessment unit of an urban teaching hospital. Reference assessments were clinical diagnosis of delirium performed by elderly care physicians. Routine screening tests were: Abbreviated Mental Test (AMT-10, AMT-4), 4 A's Test (4AT), brief Confusion Assessment Method (bCAM), months of the year backwards (MOTYB) and informant Single Question in Delirium (SQiD). Results: we assessed 500 patients, mean age 83 years (range = 66−101). Clinical diagnoses were: 93 of 500 (18.6%) definite delirium, 104 of 500 (20.8%) possible delirium and 277 of 500 (55.4%) no delirium; 266 of 500 (53.2%) were identified as definite or possible dementia. For diagnosis of definite delirium, AMT-4 (cut-point < 3/4) had a sensitivity of 92.7% (95% confidence interval (CI): 84.8–97.3), with a specificity of 53.7% (95% CI: 48.1–59.2); AMT-10 (<4/10), MOTYB (<4/12) and SQiD showed similar performance. bCAM had a sensitivity of 70.3% (95% CI: 58.5–80.3) with a specificity of 91.4% (95% CI: 87.7–94.3). 4AT (>4/12) had a sensitivity of 86.7% (95% CI: 77.5–93.2) and specificity of 69.5% (95% CI: 64.4–74.3). Conclusions: short screening tools such as AMT-4 or MOTYB have good sensitivity for definite delirium, but poor specificity; these tools may be reasonable as a first stage in assessment for delirium. The 4AT is feasible and appears to perform well with good sensitivity and reasonable specificity

Ballot secrecy concerns and voter mobilization: new experimental evidence about message source, context, and the duration of mobilization effects

Recent research finds that doubts about the integrity of the secret ballot as an institution persist among the American public. We build on this finding by providing novel field experimental evidence about how information about ballot secrecy protections can increase turnout among registered voters who had not previously voted. First, we show that a private group’s mailing designed to address secrecy concerns modestly increased turnout in the highly contested 2012 Wisconsin gubernatorial recall election. Second, we exploit this and an earlier field experiment conducted in Connecticut during the 2010 congressional midterm election season to identify the persistent effects of such messages from both governmental and non-governmental sources. Together, these results provide new evidence about how message source and campaign context affect efforts to mobilize previous non-voters by addressing secrecy concerns, as well as show that attempting to address these beliefs increases long term participation

Self-interest, beliefs, and policy opinions: understanding how economic beliefs affect immigration policy preferences

Research on how economic factors affect attitudes toward immigration often focuses on labor market effects, concluding that, because workers’ skill levels do not predict opposition to low- versus highly skilled immigration, economic self-interest does not shape policy attitudes. We conduct a new survey to measure beliefs about a range of economic, political, and cultural consequences of immigration. When economic self-interest is broadened to include concerns about the fiscal burdens created by immigration, beliefs about these economic effects strongly correlate with immigration attitudes and explain a significant share of the difference in support for highly versus low-skilled immigration. Although cultural factors are important, our results suggest that previous work underestimates the importance of economic self-interest as a source of immigration policy preferences and attitudes more generally

Subtle linguistic cues may not affect voter behavior: new evidence

One of the most important recent developments in social psychology is the discovery of minor interventions that have large and enduring effects on behavior. A leading example of this class of results is Bryan et al. (2011), which shows that administering a set of survey items worded so that subjects think of themselves as voters (noun treatment) rather than as voting (verb treatment) substantially increases political participation (voter turnout) among subjects. We revisit these experiments by replicating and extending their research design in a large-scale field experiment. In contrast to the 11 to 14 percentage point greater turnout among those exposed to the noun rather than verb treatment reported in Bryan et al. (2011), we find no statistically significant difference in turnout between the noun and verb treatments (the point estimate of the difference is approximately zero). Further, when we benchmark these treatments against a standard get-outthe- vote message, we find that both are less effective at increasing turnout than a much shorter basic mobilization message. In sum, in our experiments, we find no evidence that describing a subject as a voter rather than as voting has a positive relative or absolute effect on subject behavior. In our conclusion, we detail how our study differs from Bryan et al. (2011) and discuss how our results might be interpreted

AD-8 for diagnosis of dementia across a variety of healthcare settings

This is a protocol for a Cochrane Review (Diagnostic test accuracy). The objectives are as follows: To determine the diagnostic accuracy of the informant‐based questionnaire AD‐8, in detection of all‐cause (undifferentiated) dementia in adults. We will present data for each healthcare setting where AD‐8 may be employed (community; primary care; secondary care)