73 research outputs found
Conservative Safety Monitors of Stochastic Dynamical Systems
Generating accurate runtime safety estimates for autonomous systems is vital
to ensuring their continued proliferation. However, exhaustive reasoning about
future behaviors is generally too complex to do at runtime. To provide scalable
and formal safety estimates, we propose a method for leveraging design-time
model checking results at runtime. Specifically, we model the system as a
probabilistic automaton (PA) and compute bounded-time reachability
probabilities over the states of the PA at design time. At runtime, we combine
distributions of state estimates with the model checking results to produce a
bounded time safety estimate. We argue that our approach produces
well-calibrated safety probabilities, assuming the estimated state
distributions are well-calibrated. We evaluate our approach on simulated water
tanks
Safe Planning in Dynamic Environments using Conformal Prediction
We propose a framework for planning in unknown dynamic environments with
probabilistic safety guarantees using conformal prediction. Particularly, we
design a model predictive controller (MPC) that uses i) trajectory predictions
of the dynamic environment, and ii) prediction regions quantifying the
uncertainty of the predictions. To obtain prediction regions, we use conformal
prediction, a statistical tool for uncertainty quantification, that requires
availability of offline trajectory data - a reasonable assumption in many
applications such as autonomous driving. The prediction regions are valid,
i.e., they hold with a user-defined probability, so that the MPC is provably
safe. We illustrate the results in the self-driving car simulator CARLA at a
pedestrian-filled intersection. The strength of our approach is compatibility
with state of the art trajectory predictors, e.g., RNNs and LSTMs, while making
no assumptions on the underlying trajectory-generating distribution. To the
best of our knowledge, these are the first results that provide valid safety
guarantees in such a setting
Conformal Prediction Regions for Time Series using Linear Complementarity Programming
Conformal prediction is a statistical tool for producing prediction regions
of machine learning models that are valid with high probability. However,
applying conformal prediction to time series data leads to conservative
prediction regions. In fact, to obtain prediction regions over time steps
with confidence , {previous works require that each individual
prediction region is valid} with confidence . We propose an
optimization-based method for reducing this conservatism to enable long horizon
planning and verification when using learning-enabled time series predictors.
Instead of considering prediction errors individually at each time step, we
consider a parameterized prediction error over multiple time steps. By
optimizing the parameters over an additional dataset, we find prediction
regions that are not conservative. We show that this problem can be cast as a
mixed integer linear complementarity program (MILCP), which we then relax into
a linear complementarity program (LCP). Additionally, we prove that the relaxed
LP has the same optimal cost as the original MILCP. Finally, we demonstrate the
efficacy of our method on a case study using pedestrian trajectory predictors
Causal Repair of Learning-enabled Cyber-physical Systems
Models of actual causality leverage domain knowledge to generate convincing
diagnoses of events that caused an outcome. It is promising to apply these
models to diagnose and repair run-time property violations in cyber-physical
systems (CPS) with learning-enabled components (LEC). However, given the high
diversity and complexity of LECs, it is challenging to encode domain knowledge
(e.g., the CPS dynamics) in a scalable actual causality model that could
generate useful repair suggestions. In this paper, we focus causal diagnosis on
the input/output behaviors of LECs. Specifically, we aim to identify which
subset of I/O behaviors of the LEC is an actual cause for a property violation.
An important by-product is a counterfactual version of the LEC that repairs the
run-time property by fixing the identified problematic behaviors. Based on this
insights, we design a two-step diagnostic pipeline: (1) construct and
Halpern-Pearl causality model that reflects the dependency of property outcome
on the component's I/O behaviors, and (2) perform a search for an actual cause
and corresponding repair on the model. We prove that our pipeline has the
following guarantee: if an actual cause is found, the system is guaranteed to
be repaired; otherwise, we have high probabilistic confidence that the LEC
under analysis did not cause the property violation. We demonstrate that our
approach successfully repairs learned controllers on a standard OpenAI Gym
benchmark
Megadrought and Megadeath in 16th Century Mexico
The native population collapse in 16th century Mexico was a demographic catastrophe with one of the highest death rates in history. Recently developed tree-ring evidence has allowed the levels of precipitation to be reconstructed for north central Mexico, adding to the growing body of epidemiologic evidence and indicating that the 1545 and 1576 epidemics of cocoliztli (Nahuatl for "pest”) were indigenous hemorrhagic fevers transmitted by rodent hosts and aggravated by extreme drought conditions
Distributionally Robust Statistical Verification with Imprecise Neural Networks
A particularly challenging problem in AI safety is providing guarantees on
the behavior of high-dimensional autonomous systems. Verification approaches
centered around reachability analysis fail to scale, and purely statistical
approaches are constrained by the distributional assumptions about the sampling
process. Instead, we pose a distributionally robust version of the statistical
verification problem for black-box systems, where our performance guarantees
hold over a large family of distributions. This paper proposes a novel approach
based on a combination of active learning, uncertainty quantification, and
neural network verification. A central piece of our approach is an ensemble
technique called Imprecise Neural Networks, which provides the uncertainty to
guide active learning. The active learning uses an exhaustive neural-network
verification tool Sherlock to collect samples. An evaluation on multiple
physical simulators in the openAI gym Mujoco environments with
reinforcement-learned controllers demonstrates that our approach can provide
useful and scalable guarantees for high-dimensional systems
Incidence of human brucellosis in the Kilimanjaro Region of Tanzania in the periods 2007-2008 and 2012-2014
Background:
Brucellosis causes substantial morbidity among humans and their livestock. There are few robust estimates of the incidence of brucellosis in sub-Saharan Africa. Using cases identified through sentinel hospital surveillance and health care utilization data, we estimated the incidence of brucellosis in Moshi Urban and Moshi Rural Districts, Kilimanjaro Region, Tanzania, for the periods 2007–2008 and 2012–2014.
Methods:
Cases were identified among febrile patients at two sentinel hospitals and were defined as having either a 4-fold increase in Brucella microscopic agglutination test titres between acute and convalescent serum or a blood culture positive for Brucella spp. Findings from a health care utilization survey were used to estimate multipliers to account for cases not seen at sentinel hospitals.
Results:
Of 585 patients enrolled in the period 2007–2008, 13 (2.2%) had brucellosis. Among 1095 patients enrolled in the period 2012–2014, 32 (2.9%) had brucellosis. We estimated an incidence (range based on sensitivity analysis) of brucellosis of 35 (range 32–93) cases per 100 000 persons annually in the period 2007–2008 and 33 (range 30–89) cases per 100 000 persons annually in the period 2012–2014.
Conclusions:
We found a moderate incidence of brucellosis in northern Tanzania, suggesting that the disease is endemic and an important human health problem in this area
Comparison of the estimated incidence of acute leptospirosis in the Kilimanjaro Region of Tanzania between 2007-08 and 2012-14
Background:
The sole report of annual leptospirosis incidence in continental Africa of 75–102 cases per 100,000 population is from a study performed in August 2007 through September 2008 in the Kilimanjaro Region of Tanzania. To evaluate the stability of this estimate over time, we estimated the incidence of acute leptospirosis in Kilimanjaro Region, northern Tanzania for the time period 2012–2014.
Methodology and Principal Findings:
Leptospirosis cases were identified among febrile patients at two sentinel hospitals in the Kilimanjaro Region. Leptospirosis was diagnosed by serum microscopic agglutination testing using a panel of 20 Leptospira serovars belonging to 17 separate serogroups. Serum was taken at enrolment and patients were asked to return 4–6 weeks later to provide convalescent serum. Confirmed cases required a 4-fold rise in titre and probable cases required a single titre of ≥800. Findings from a healthcare utilisation survey were used to estimate multipliers to adjust for cases not seen at sentinel hospitals. We identified 19 (1.7%) confirmed or probable cases among 1,115 patients who presented with a febrile illness. Of cases, the predominant reactive serogroups were Australis 8 (42.1%), Sejroe 3 (15.8%), Grippotyphosa 2 (10.5%), Icterohaemorrhagiae 2 (10.5%), Pyrogenes 2 (10.5%), Djasiman 1 (5.3%), Tarassovi 1 (5.3%). We estimated that the annual incidence of leptospirosis was 11–18 cases per 100,000 population. This was a significantly lower incidence than 2007–08 (p<0.001).
Conclusions:
We estimated a much lower incidence of acute leptospirosis than previously, with a notable absence of cases due to the previously predominant serogroup Mini. Our findings indicate a dynamic epidemiology of leptospirosis in this area and highlight the value of multi-year surveillance to understand leptospirosis epidemiology
Risk factors for human brucellosis in northern Tanzania
Little is known about the epidemiology of human brucellosis in sub-Saharan Africa. This hampers prevention and control efforts at the individual and population levels. To evaluate risk factors for brucellosis in northern Tanzania, we conducted a study of patients presenting with fever to two hospitals in Moshi, Tanzania. Serum taken at enrollment and at 4–6 week follow-up was tested by Brucella microagglutination test. Among participants with a clinically compatible illness, confirmed brucellosis cases were defined as having a ≥ 4-fold rise in agglutination titer between paired sera or a blood culture positive for Brucella spp., and probable brucellosis cases were defined as having a single reciprocal titer ≥ 160. Controls had reciprocal titers < 20 in paired sera. We collected demographic and clinical information and administered a risk factor questionnaire. Of 562 participants in the analysis, 50 (8.9%) had confirmed or probable brucellosis. Multivariable analysis showed that risk factors for brucellosis included assisting goat or sheep births (Odds ratio [OR] 5.9, 95% confidence interval [CI] 1.4, 24.6) and having contact with cattle (OR 1.2, 95% CI 1.0, 1.4). Consuming boiled or pasteurized dairy products was protective against brucellosis (OR 0.12, 95% CI 0.02, 0.93). No participants received a clinical diagnosis of brucellosis from their healthcare providers. The under-recognition of brucellosis by healthcare workers could be addressed with clinician education and better access to brucellosis diagnostic tests. Interventions focused on protecting livestock keepers, especially those who assist goat or sheep births, are needed
Towards elimination of dog-mediated human rabies: experiences from implementing a large-scale demonstration project in Southern Tanzania
No abstract available
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