4,801 research outputs found
Forecasting in the Presence of Structural Breaks and Policy Regime Shifts
The value of selecting the best forecasting model as the basis for empirical economic policy analysis is questioned. When no model coincides with the data generation process, non-causal statistical devices may provide the best available forecasts: examples from recent work include intercept corrections and differenced-data VARs. However, the resulting models need have no policy implications. A 'paradox' may result if their forecasts induce policy changes which can be used to improve the statistical forecast. This suggests correcting statistical forecasts by using the econometric model's estimate of the 'scenario' change, and doing so yields reduced biases.
Reformulating empirical macro-econometric modelling
The policy implications of estimated macro-econometric systems depend on the formulations of their equations, the methodology of empirical model selection and evaluation, the techniques of policy analysis, and their forecast performance. Drawing on recent results in the theory of forecasting, we question the role of `rational expectations'; criticize a common approach to testing economic theories; show that impulse-response methods of evaluating policy are seriously flawed; and question the mechanistic derivation of forecasts from econometric systems. In their place, we propose that expectations should be treated as instrumental to agents' decisions; discuss a powerful new approach to the empirical modelling of econometric relationships; offer viable alternatives to studying policy implications; and note modifications to forecasting devices that can enhance their robustness to unanticipated structural breaks. Keywords; economic policy analysis, macro-econometric systems, empirical model selection and evaluation, forecasting, rational expectations, impulse-response analysis, structural breaks
Intensity dependences of the nonlinear optical excitation of plasmons in graphene
Recently, we demonstrated an all-optical coupling
scheme for plasmons, which takes advantage of the
intrinsic nonlinear optical response of graphene.
Frequency mixing using free-space, visible light
pulses generates surface plasmons in a planar
graphene sample, where the phase matching
condition can define both the wavevector and energy
of surface waves and intraband transitions. Here,
we also show that the plasmon generation process is
strongly intensity-dependent, with resonance features
washed out for absorbed pulse fluences greater than
0.1 J m−2. This implies a subtle interplay between the
nonlinear generation process and sample heating. We
discuss these effects in terms of a non-equilibrium
charge distribution using a two-temperature model.Peer ReviewedPostprint (author's final draft
Model Identification and Non-unique Structure
Identification is an essential attribute of any model's parameters, so we consider its three aspects of 'uniqueness', 'correspondence to reality' and 'interpretability'. Observationally-equivalent over-identified models can co-exist, and are mutually encompassing in the population; correctly-identified models need not correspond to the underlying structure; and may be wrongly interpreted. That a given model is over-identified with all over-identifying restrictions valid (even asymptotically) is insufficient to demonstrate that it is a unique representation. Moreover, structre (as invariance under extended information) need not be identifiable. We consider the role of structural breaks to discriminate between such representations.
The Corralitos Observatory program for the detection of lunar transient phenomena
This is a final report on the establishment, observing procedures, and observational results of a survey program for the detection of lunar transient phenomena (LTP's) by electro-optical image conversion means. For survey, a unique detection system with an image orthicon was used as the primary element in conjunction with a 24-in. f/20 Cassegrainian telescope. Observations in three spectral ranges, with 6,466 man-hours of observing, were actually performed during the period from October 27, 1965, to April 26, 1972. Within this entire period, no color or feature change within the detection capabilities of the instrumentation was observed, either independently or in follow up of amateur LTP reports, with the exception of one general bluing and several localized bluings (probably ascribable to the effects of the terrestrial atmosphere) that were observed solely by the Corralitos system. A table is presented indicating amateur and professional reports of LTP's and the results of efforts to confirm these reports through the Corralitos system
Hot phonon decay in supported and suspended exfoliated graphene
Near infrared pump-probe spectroscopy has been used to measure the ultrafast
dynamics of photoexcited charge carriers in monolayer and multilayer graphene.
We observe two decay processes occurring on 100 fs and 2 ps timescales. The
first is attributed to the rapid electron-phonon thermalisation in the system.
The second timescale is found to be due to the slow decay of hot phonons. Using
a simple theoretical model we calculate the hot phonon decay rate and show that
it is significantly faster in monolayer flakes than in multilayer ones. In
contrast to recent claims, we show that this enhanced decay rate is not due to
the coupling to substrate phonons, since we have also seen the same effect in
suspended flakes. Possible intrinsic decay mechanisms that could cause such an
effect are discussed.Comment: 4 pages, 3 figure
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