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Positive control
A positive control is an experimental condition with a known, expected response, used to verify that an assay can detect that response; a failing control suggests a flaw.
Positive definite matrix
A positive definite matrix is a Hermitian or real symmetric matrix for which the quadratic form is positive for every nonzero vector, equivalently all its eigenvalues are positive.
Positive energy theorem
The positive energy theorem, also called the positive mass theorem, states that an isolated system in general relativity has nonnegative total mass-energy, with equality only for flat Minkowski spacetime.
Positive feedback
Positive feedback is a loop in which a change in a system is amplified, moving it away from equilibrium, as in blood clotting and climate warming.
Positive matrix factorization
Positive matrix factorization (PMF) is a non-negative matrix factorization method, introduced by Paatero and Tapper in 1994, that decomposes environmental measurements into source profiles and contributions, weighting each data point by its uncertainty.
Positive operator-valued measure
A positive operator-valued measure, or POVM, is the most general mathematical description of a quantum measurement, a set of positive semidefinite operators summing to the identity.
Positive position feedback
Positive position feedback (PPF) is a resonant control technique that actively damps vibration in flexible structures, introduced by Goh and Caughey in 1985 for large space structures.
Positive-definite kernel
A positive-definite kernel is a symmetric function whose Gram matrix on any finite set of points is positive semidefinite, used in Gaussian processes and machine learning.
Positivist calendar
The Positivist calendar (Calendrier positiviste) is a calendar reform proposed by Auguste Comte in 1849, with 13 months of 28 days named for figures like Moses and Shakespeare.
Positron
The positron, also called the antielectron, is the electron's antiparticle, with equal mass and a positive charge. Carl D. Anderson discovered it in cosmic rays in 1932.
Positron annihilation lifetime spectroscopy
Positron annihilation lifetime spectroscopy (PALS), a branch of positron annihilation spectroscopy, measures positron lifetimes in materials to characterize vacancy-type defects, free volume, and porosity at the sub-nanometer scale.
Positron emission
Positron emission, also called beta-plus (β+) decay, is a mode of radioactive decay in which a proton in the nucleus becomes a neutron, emitting a positron and an electron neutrino.
Positron fraction excess
The positron fraction excess is the unexpected rise in the cosmic-ray positron fraction above roughly 10 GeV, observed by PAMELA and confirmed by AMS-02 in 2013.
Positronium
Positronium (Ps) is an exotic hydrogen-like atom in which an electron is bound to a positron, its antiparticle, making it the simplest bound state in physics.
Possible world
A possible world, also called a possible universe, is a complete way things could have been, used in modal logic to define possibility and necessity.
Post hoc ergo propter hoc
Post hoc ergo propter hoc, or the post hoc fallacy, is an informal fallacy that treats one event following another as proof the first caused the second.
Post-AGB star
A post-AGB star is a low- or intermediate-mass star that has left the asymptotic giant branch after losing its outer envelope, evolving toward the planetary-nebula and white-dwarf stages.
Post-combustion carbon capture retrofit
A post-combustion carbon capture retrofit adds CO2 capture equipment to an existing power plant, most often amine solvents; the first was Boundary Dam in Canada in 2014.
Post-glacial rebound
Post-glacial rebound, also called glacial isostatic adjustment (GIA), is the slow rise of land depressed by ice sheets, continuing thousands of years after the ice melts.
Post-hoc analysis
A post-hoc analysis is a comparison of treatment means chosen after the data are collected, usually to locate which group differences drive a significant ANOVA result.
Post-Newtonian metric expansion
The post-Newtonian metric expansion, or PN expansion, approximates the spacetime metric in general relativity as a series in the inverse speed of light for weak, slow systems.
Post-Newtonian N-body problem
The post-Newtonian N-body problem is the formulation and numerical integration of equations of motion for many-body gravitational systems with corrections beyond Newtonian gravity, ordered by powers of v/c.
Post-perovskite
Post-perovskite is a high-pressure form of magnesium silicate, formed when bridgmanite transforms near Earth's core–mantle boundary; discovered in 2004, it explains the D″ seismic discontinuity.
Post-selection inference
Post-selection inference, also called selective inference, is a collection of statistical methods giving valid confidence intervals and tests for parameters chosen after a data-driven selection step.
Post-stratification
Post-stratification is a survey-estimation technique in which a sample is classified into strata after data collection and combined using population stratum weights taken from a census or other external source.
Post-transition metal
A post-transition metal, also called a poor metal, is a soft, low-melting p-block element such as gallium, indium, tin, lead, and bismuth, with fixed oxidation states unlike transition metals.
PostBQP
PostBQP is a quantum complexity class, defined by Scott Aaronson in 2004, of decision problems solvable by polynomial-time quantum circuits with postselection; it equals the classical class PP.
Posterior consistency in Bayesian nonparametrics
Posterior consistency in Bayesian nonparametrics is the property that, as observations grow, the posterior concentrates on the true infinite-dimensional parameter rather than staying spread over the prior.
Posterior contraction rates and Bernstein–von Mises phenomena in nonparametric Bayes
Posterior contraction rates and Bernstein–von Mises phenomena in nonparametric Bayes measure how fast posteriors concentrate around the truth and whether they become Gaussian, which depends on the norm used.
Posterior predictive distribution
The posterior predictive distribution is the probability distribution of new observations given observed data, formed in Bayesian inference by averaging the sampling distribution over the posterior parameter distribution.