Permutation Inference for Canonical Correlation Analysis

Canonical correlation analysis (CCA) has become a key tool for population neuroimaging, allowing investigation of associations between many imaging and non-imaging measurements. As other variables are often a source of variability not of direct interest, previous work has used CCA on residuals from a model that removes these effects, then proceeded directly to permutation inference. We show that such a simple permutation test leads to inflated error rates. The reason is that residualisation introduces dependencies among the observations that violate the exchangeability assumption. Even in the absence of nuisance variables, however, a simple permutation test for CCA also leads to excess error rates for all canonical correlations other than the first. The reason is that a simple permutation scheme does not ignore the variability already explained by previous canonical variables. Here we propose solutions for both problems: in the case of nuisance variables, we show that transforming the residuals to a lower dimensional basis where exchangeability holds results in a valid permutation test; for more general cases, with or without nuisance variables, we propose estimating the canonical correlations in a stepwise manner, removing at each iteration the variance already explained, while dealing with different number of variables in both sides. We also discuss how to address the multiplicity of tests, proposing an admissible test that is not conservative, and provide a complete algorithm for permutation inference for CCA.Comment: 49 pages, 2 figures, 10 tables, 3 algorithms, 119 reference

Marketing Ecosystem Services from Agricultural Land: Stated Preferences over Payment Mechanisms and Actual Sales of Farm-Wildlife Contracts

Agriculture conventionally supplies food, fiber and fuel that consumers can purchase through the market. With the right incentives, farmers can also provide ecosystem services such as wildlife habitat, climate regulation, surface water flows and waste absorption and breakdown. Such incentives have so far come almost entirely from government-sponsored programs that rely on financial assistance to farmers to encourage them to alter agricultural practices or input mix to enhance ecosystem services. Programs recently implemented in Costa Rica and Columbia rely on payments by the beneficiaries of the ecosystem services, such as municipal water companies and water users (Pagiola et al. 2002). Few of these programs, however, have attempted to establish a market for ecosystem services in which the beneficiaries of such services pay the suppliers their personal values of ecosystem services in an actual market. Markets for ecosystem services must overcome two major challenges. In order to set prices for ecosystem services at the right level, it is imperative to understand consumers preferences. Farmland, however, has multiple attributes such as wildlife habitat services and landscape view; the marginal rate of substitution among those attributes must be understood to design marketable products for ecosystem services. Moreover, many ecosystem services are public goods for which traditional markets are ill-suited, because many individuals can receive benefits simultaneously regardless of whether they have paid part of the cost of provision. Therefore, consumers have an incentive to free-ride on others. Evidence from previous research on public goods clearly suggests that under-contribution is typical (e.g., Ledyard 1995). The overall goal of this study is to explore the potential to establish an actual market in which the public can purchase ecosystem services generated by agricultural land. More specifically, this paper evaluates the performance of alternative elicitation methods that are intended to reduce individuals incentives to free-ride on others payments for ecosystem services. Using a choice experiment involving a large-scale mail survey, we first estimate the marginal rate of substitution consumers place on various attributes of farmland including the ecosystem services such land can provide. We further compare the results across different payment mechanisms and examine which ones are capable of revealing demand that is closer to consumers true value. Second, we attempt to establish an actual market in which individuals are asked to purchase a share of a farm contract to provide ecosystem service with real money under different payment mechanisms. We compare the market outcomes with the choice experiments. To the best of our knowledge, this is the first study to examine the performance of different payment mechanisms for provision of ecosystem services using field experiments both within a hypothetical setting and by developing an actual market. The ecosystem service in question in this study is habitat for a grassland-nesting bird called the Bobolink (Dolichonyx oryivorus). Yellow and black Bobolinks establish ground nests in hay fields from mid-May into early June. Their visibility and entertaining character, combined with evidence that many birds, including bobolinks, are experiencing population declines (Sauer et al. 2004), make the bird a leading candidate to attract public interest in efforts to manage farmland for vulnerable wildlife. Previous studies have established that hay harvesting conducted during the birds five to six week nesting period is devastating to fledgling success (e.g., Mitchell et al. 2000). A fairly moderate shift in the harvest schedule could provide significant refuge for nesting birds while causing some losses of the quantity and quality of the hay harvested. If a market developed that paid farmers acceptable compensation to protect grassland birds, then farmers would have an incentive to add an ecosystem service to their revenue base while enhancing environmental quality for wildlife. In this study, we compare four payment mechanisms applied in field experiments: 1) voluntary contribution mechanism, 2) provision point with a money-back guarantee and proportional rebate of excess contributions (PPMBG-PR), 3) uniform-price, multi-buyer auction and 4) pivotal mechanism. Voluntary contribution mechanism has no provision point but has a money-back guarantee if enough money is not raised. Under PPMBG-PR, the public good is supplied only if a pre-specified amount of money (the provision point) is raised, and contributors receive their money back if the market fails to raise that amount. Under a multi-buyer auction, everyone who is willing to pay above a certain price will pay a price such that the total sum will be enough to cover the cost for a farmer to change harvest practices. Under a pivotal mechanism only those consumers whose payments make a difference in the provision of the good would pay. The pivotal mechanism is incentive compatible and is used as the baseline. We test the following hypotheses on the WTP, market participation rate and total revenue collection: (1) WTP: Voluntary contribution < Multi-buyer Auction < PPMBG-PR < Pivotal (2) Participation rate: Voluntary contribution < PPMBG-PR < Multi-buyer auction < Pivotal (3) Revenue collection: Pivotal < Voluntary < Multi-buyer auction < PPMBG-PR We perform a large-scale cross-mechanism comparison using two types of field experiments, a survey involving hypothetical questions and a functioning market for an ecosystem service. In the survey, the subjects are randomly assigned to one of the elicitation methods. We posit a hypothetical situation involving a market for contracts with farmers, describe the assigned elicitation method and ask respondents to compare several sets of two alternative farm wildlife contracts, each with five attributes presented in a stated-choice format. One of the attributes is to change the timing of hay harvesting to enhance the success of a specified number of bobolink fledglings on a given size of farm. The survey data were collected in fall of 2006. Survey questionnaires were sent to all of the 2,987 households in Jamestown, Rhode Island. The response rate was about 37% after accounting for undelivered surveys. Although still preliminary, a key result from the choice experiment is that consumers are least willing to pay for farm wildlife contracts under the voluntary contribution mechanism. The payment method that generated the greatest total purchases of farm wildlife contracts was the multi-buyer auction, followed by the pivotal mechanism and PPMBG-PR, respectively. This result suggests that some features of the payment mechanisms (such as setting of a provision point, fairness and a money-back guarantee) encourage consumers to reveal demand that is closer to each consumer true value of the good than the demand revealed by a simple voluntary contribution mechanism in a field experiment using a hypothetical situation. We are currently in the process of launching an ecosystem-service market in Jamestown by establishing actual farm wildlife contracts with farmers and selling shares of those contracts to consumers. The market will be open to the public for two months during March and April, 2007. Each farm wildlife contract will be tied to one or more of the payment mechanisms and whether each contract will remain effective during the breeding season will depend on the market outcome under each mechanism. Consumers will be randomly assigned to one of the elicitation methods. Using market data, we will compare the actual consumer behavior across alternative methods and also to their willingness to pay as estimated in the survey choice experiment.Land Economics/Use,

Do Stated Preference Values Predict Revealed Behavior in “New” Markets for Ecosystem Services? A Comparison of Experiments Addressing Establishing A Market for Farmland Ecosystem Services

Resource /Energy Economics and Policy,

Ecosystem Services Beyond Valuation, Regulation and Philanthropy: Integrating Consumer Values into the Economy

Environmental Markets, Ecosystem Service Markets, Payment For Ecosystem Services, Incentives, Nature's Services, Resource /Energy Economics and Policy, Q20, Q57, C93, H41,

Preferences for Residential Development Attributes and Support for the Policy Process: Implications for Management and Conservation of Rural Landscapes

The rural public may not only be concerned with the consequences of land management; residents may also have systematic preferences for policy instruments applied to management goals. Preferences for outcomes do not necessarily imply matching support for the underlying policy process. This study assesses relationships among support for elements of the policy process and preferences for management outcomes. Preferences are examined within the context of alternative proposals to manage growth and conserve landscape attributes in southern New England. Results are based on (a) stated preferences estimated from a multi-attribute contingent choice survey of rural residents, and (b) Likert-scale assessment of strength of support for land use policy tools. Findings indicate general but not universal correlation among policy support indicators and preferences for associated land use outcomes, but also confirm the suspicion that policy support and land use preference may not always coincide.Agricultural and Food Policy,

A Compact Microchip-Based Atomic Clock Based on Ultracold Trapped Rb Atoms

We propose a compact atomic clock based on ultracold Rb atoms that are magnetically trapped near the surface of an atom microchip. An interrogation scheme that combines electromagnetically-induced transparency (EIT) with Ramsey's method of separated oscillatory fields can achieve atomic shot-noise level performance of 10^{-13}/sqrt(tau) for 10^6 atoms. The EIT signal can be detected with a heterodyne technique that provides noiseless gain; with this technique the optical phase shift of a 100 pW probe beam can be detected at the photon shot-noise level. Numerical calculations of the density matrix equations are used to identify realistic operating parameters at which AC Stark shifts are eliminated. By considering fluctuations in these parameters, we estimate that AC Stark shifts can be canceled to a level better than 2*10^{-14}. An overview of the apparatus is presented with estimates of duty cycle and power consumption.Comment: 15 pages, 11 figures, 5 table

Disparity between General Symptom Relief and Remission Criteria in the Positive and Negative Syndrome Scale (PANSS): A Post-treatment Bifactor Item Response Theory Model.

Objective: Total scale scores derived by summing ratings from the 30-item PANSS are commonly used in clinical trial research to measure overall symptom severity, and percentage reductions in the total scores are sometimes used to document the efficacy of treatment. Acknowledging that some patients may have substantial changes in PANSS total scores but still be sufficiently symptomatic to warrant diagnosis, ratings on a subset of 8 items, referred to here as the "Remission set," are sometimes used to determine if patients' symptoms no longer satisfy diagnostic criteria. An unanswered question remains: is the goal of treatment better conceptualized as reduction in overall symptom severity, or reduction in symptoms below the threshold for diagnosis? We evaluated the psychometric properties of PANSS total scores, to assess whether having low symptom severity post-treatment is equivalent to attaining Remission. Design: We applied a bifactor item response theory (IRT) model to post-treatment PANSS ratings of 3,647 subjects diagnosed with schizophrenia assessed at the termination of 11 clinical trials. The bifactor model specified one general dimension to reflect overall symptom severity, and five domain-specific dimensions. We assessed how PANSS item discrimination and information parameters varied across the range of overall symptom severity (θ), with a special focus on low levels of symptoms (i.e., θ&lt;-1), which we refer to as "Relief" from symptoms. A score of θ=-1 corresponds to an expected PANSS item score of 1.83, a rating between "Absent" and "Minimal" for a PANSS symptom. Results: The application of the bifactor IRT model revealed: (1) 88% of total score variation was attributable to variation in general symptom severity, and only 8% reflected secondary domain factors. This implies that a general factor may provide a good indicator of symptom severity, and that interpretation is not overly complicated by multidimensionality; (2) Post-treatment, 534 individuals (about 15% of the whole sample) scored in the "Relief" range of general symptom severity, but more than twice that number (n = 1351) satisfied Remission criteria (37%). 2 in 3 Remitted patients had scores that were not in a low symptom range (corresponding to Absent or Minimal item scores); (3) PANSS items vary greatly in their ability to measure the general symptom severity dimension; while many items are highly discriminating and relatively "pure" indicators of general symptom severity (delusions, conceptual disorganization), others are better indicators of specific dimensions (blunted affect, depression). The utility of a given PANSS item for assessing a patient depended on the illness level of the patient. Conclusion: Satisfying conventional Remission criteria was not strongly associated with low levels of symptoms. The items providing the most information for patients in the symptom Relief range were Delusions, Preoccupation, Suspiciousness Persecution, Unusual Thought Content, Conceptual Disorganization, Stereotyped Thinking, Active Social Avoidance, and Lack of Judgment and Insight. Lower scores on these items (item scores ≤2) were strongly associated with having a low latent trait θ or experiencing overall symptom relief. The inter-rater agreement between Remission and Relief subjects suggested that these criteria identified different subsets of patients. Alternative subsets of items may offer better indicators of general symptom severity and provide better discrimination (and lower standard errors) for scaling individuals and judging symptom relief, where the "best" subset of items ultimately depends on the illness range and treatment phase being evaluated

Diffusion of Computer Applications Among Physicians: A Quasi-Experimental Study

An experimental program involving the use of a hospital information system was implemented and evaluated on four services at Methodist Hospital of Indiana, a 1120-bed, private teaching hospital. Ten other hospital services were assigned to a control group. The program utilized educationally influential physicians to disseminate information concerning the advantages of using predesigned computer-stored personal order sets for the entry of medical orders into a hospital information system. Data from the hospital information system\u27s tapes were collected at three times in order to evaluate the effectiveness of the intervention. A multivariate analysis of variance indicated that the program resulted in a significant increase in personal order set use by physicians, physician assistants, and unit secretaries on the experimental services. The results of the study suggest that the identification and use of educationally influential physicians is an effective means of introducing medical innovations into clinical settings

Conserved currents of massless fields of spin s>0

A complete and explicit classification of all locally constructed conserved currents and underlying conserved tensors is obtained for massless linear symmetric spinor fields of any spin s>0 in four dimensional flat spacetime. These results generalize the recent classification in the spin s=1 case of all conserved currents locally constructed from the electromagnetic spinor field. The present classification yields spin s>0 analogs of the well-known electromagnetic stress-energy tensor and Lipkin's zilch tensor, as well as a spin s>0 analog of a novel chiral tensor found in the spin s=1 case. The chiral tensor possesses odd parity under a duality symmetry (i.e., a phase rotation) on the spin s field, in contrast to the even parity of the stress-energy and zilch tensors. As a main result, it is shown that every locally constructed conserved current for each s>0 is equivalent to a sum of elementary linear conserved currents, quadratic conserved currents associated to the stress-energy, zilch, and chiral tensors, and higher derivative extensions of these currents in which the spin s field is replaced by its repeated conformally-weighted Lie derivatives with respect to conformal Killing vectors of flat spacetime. Moreover, all of the currents have a direct, unified characterization in terms of Killing spinors. The cases s=2, s=1/2 and s=3/2 provide a complete set of conserved quantities for propagation of gravitons (i.e., linearized gravity waves), neutrinos and gravitinos, respectively, on flat spacetime. The physical meaning of the zilch and chiral quantities is discussed.Comment: 26 pages; final version with minor changes, accepted in Proc. Roy. Soc. A (London