13,852 research outputs found
Bayesian uncertainty quantification in linear models for diffusion MRI
Diffusion MRI (dMRI) is a valuable tool in the assessment of tissue
microstructure. By fitting a model to the dMRI signal it is possible to derive
various quantitative features. Several of the most popular dMRI signal models
are expansions in an appropriately chosen basis, where the coefficients are
determined using some variation of least-squares. However, such approaches lack
any notion of uncertainty, which could be valuable in e.g. group analyses. In
this work, we use a probabilistic interpretation of linear least-squares
methods to recast popular dMRI models as Bayesian ones. This makes it possible
to quantify the uncertainty of any derived quantity. In particular, for
quantities that are affine functions of the coefficients, the posterior
distribution can be expressed in closed-form. We simulated measurements from
single- and double-tensor models where the correct values of several quantities
are known, to validate that the theoretically derived quantiles agree with
those observed empirically. We included results from residual bootstrap for
comparison and found good agreement. The validation employed several different
models: Diffusion Tensor Imaging (DTI), Mean Apparent Propagator MRI (MAP-MRI)
and Constrained Spherical Deconvolution (CSD). We also used in vivo data to
visualize maps of quantitative features and corresponding uncertainties, and to
show how our approach can be used in a group analysis to downweight subjects
with high uncertainty. In summary, we convert successful linear models for dMRI
signal estimation to probabilistic models, capable of accurate uncertainty
quantification.Comment: Added results from a group analysis and a comparison with residual
bootstra
Peculiar Velocities and the Mean Density Parameter
We study the peculiar velocity field inferred from the Mark III spirals using
a new method of analysis. We estimate optimal values of Tully-Fisher scatter
and zero-point offset, and we derive the 3-dimensional rms peculiar velocity
() of the galaxies in the samples analysed. We check our statistical
analysis using mock catalogs derived from numerical simulations of CDM models
considering measurement uncertainties and sampling variations. Our best
determination for the observations is . We use the
linear theory relation between , the density parameter , and
the galaxy correlation function to infer the quantity where is the linear bias parameter
of optical galaxies and the uncertainties correspond to bootstrap resampling
and an estimated cosmic variance added in quadrature. Our findings are
consistent with the results of cluster abundances and redshift space distortion
of the two-point correlation function. These statistical measurements suggest a
low value of the density parameter if optical galaxies are
not strongly biased tracers of mass.Comment: Accepted for publication in MNRAS. 8 pages latex (mn.sty), including
7 figure
Bootstrap methods for the empirical study of decision-making and information flows in social systems
Abstract: We characterize the statistical bootstrap for the estimation of information theoretic quantities from data, with particular reference to its use in the study of large-scale social phenomena. Our methods allow one to preserve, approximately, the underlying axiomatic relationships of information theory—in particular, consistency under arbitrary coarse-graining—that motivate use of these quantities in the first place, while providing reliability comparable to the state of the art for Bayesian estimators. We show how information-theoretic quantities allow for rigorous empirical study of the decision-making capacities of rational agents, and the time-asymmetric flows of information in distributed systems. We provide illustrative examples by reference to ongoing collaborative work on the semantic structure of the British Criminal Court system and the conflict dynamics of the contemporary Afghanistan insurgency
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