Where does attenuation collide?

The gradual loss of a signal's intensity as it travels through a medium, usually reported in decibels per unit length. As light, sound, or radio energy passes through glass, tissue, air, or cable, part of it is absorbed or scattered, so the power arriving at the far end is smaller than the power sent in. The rate of this loss depends on the medium and on the wavelength, and engineers work hard to keep it low over long spans. Keck, Maurer, and Schultz's 1973 On the ultimate lower limit of attenuation in glass optical waveguides asked how low the loss in a glass fibre could be pushed, work that helped make long-distance optical communication practical. The referent is the physical weakening of a wave over distance, measured on a logarithmic scale, and it is not a weakened microbe, a halted transcript, or a shrunken correlation.[1]

Unit

The fraction of signal power lost over distance, in decibels per kilometre

Recovery means

Not applicable; the lost energy is absorbed or scattered away and is not returned, though the signal can be amplified again

Measured by

Measured by comparing input and output power and expressing the ratio in decibels, often per unit length of the medium

A live microbe whose disease-causing power has been deliberately weakened so that it can prompt immunity without causing illness. Attenuation is often achieved by growing the pathogen for many generations in an unusual host or cell culture, a process called passage, which selects variants that reproduce poorly in humans while still provoking a protective immune response. Vaccines against measles, mumps, rubella, the oral form of polio, and yellow fever use such live-attenuated strains. Minor's 2015 Live attenuated vaccines: historical successes and current challenges reviewed how these vaccines are made, why they work, and the safety questions they raise. Here the referent is a living organism made less virulent, not a fading wave, a stopped transcript, or a biased statistic.[2]

Unit

A live pathogen strain of reduced virulence, judged by how mild it is and how well it immunises

Recovery means

A live-attenuated strain can in rare cases revert toward virulence, which is a central safety concern for these vaccines

Measured by

Assessed by tests of virulence in animals or cell systems and by the strength of the immune response the strain produces

A mechanism that regulates a bacterial gene by causing RNA polymerase to stop transcription prematurely, before it reaches the genes it would otherwise copy. In the tryptophan (trp) operon of Escherichia coli, a short leader region called the attenuator can fold into alternative RNA structures: when the amino acid tryptophan is plentiful, a translating ribosome lets a terminator hairpin form and transcription is cut short, whereas when tryptophan is scarce, an alternative fold lets transcription continue. Yanofsky's 1981 Attenuation in the control of expression of bacterial operons set out how this coupling of transcription and translation tunes gene expression to need. The referent is premature termination of transcription as a form of genetic control, and it shares nothing but the word with signal loss, a weakened microbe, or a shrunken correlation.[3]

Unit

The fraction of transcripts terminated early at the attenuator, as a share of those begun

Recovery means

Not applicable; each round of transcription is decided anew, and low tryptophan allows read-through on the next attempt

Measured by

Measured by the ratio of short terminated transcripts to full-length transcripts, and by expression of the downstream genes

The systematic shrinking of an observed correlation below the true relationship between two variables, caused by imperfect measurement. Because any real measurement carries random error, the scores we record are noisier than the underlying quantities, and that noise dilutes the association, so the correlation we compute comes out smaller than the one that links the true values. Spearman's 1904 The proof and measurement of association between two things described this effect and gave a correction that estimates the true correlation from the reliabilities of the two measures. Range restriction, where the sample covers only part of the possible spread, attenuates a correlation in a related way. The referent is a downward bias in a statistical estimate, not a fading signal, a weakened germ, or a stopped transcript.[4]

Unit

The gap between an observed correlation and the true one, in correlation units

Recovery means

Not applicable as a physical process; the true value can be estimated by correcting for the reliability of each measure

Measured by

Gauged by comparing the observed correlation with a disattenuated estimate computed from the reliabilities of the measures