Where does linkage collide?

A connected set of rigid bars joined by pivots so that driving one part produces a determined motion in the others, turning one kind of movement into another. The English word linkage for such assemblies was introduced in the 1870s and given a full public treatment in Kempe's 1877 How to Draw a Straight Line: A Lecture on Linkages, which analysed bar-and-joint mechanisms such as the Peaucellier cell that traces a true straight line; the mechanisms themselves, including Watt's parallel motion of 1784 and the Peaucellier design of 1864, predate the naming. Here the word names hardware, a physical assembly of bars and joints described by its geometry and its motion, not the co-inheritance of genes, the matching of data records, or the pull one industry exerts on another.[1]

Unit

One mechanism of rigid bars and pivot joints, described by link lengths and joint positions

Recovery means

Not applicable

Measured by

Analysed by the geometry of its links and joints and by the path and range of motion it produces

The tendency of genes that sit close together on the same chromosome to be inherited together rather than assorting independently, because they seldom separate when chromosomes are shuffled. Morgan's 1911 Random segregation versus coupling in Mendelian inheritance named this linkage and tied it to position on the chromosome, and Sturtevant's 1913 The linear arrangement of six sex-linked factors in Drosophila turned the strength of the association into the first genetic map, spacing genes by how often they are inherited apart. Here linkage is a statistical relationship between locations on a strand of DNA, measured in map distance, not a jointed machine, a match between data records, or a tie between industries.[2][3]

Unit

The co-inheritance of two loci, scaled as recombination frequency or map distance in centimorgans

Recovery means

Recombination during meiosis breaks the association, so distant loci behave as if unlinked

Measured by

Estimated from the frequency of recombinant offspring in controlled crosses or from marker co-segregation in families

The task of deciding which records, drawn from one file or several, refer to the same person or thing, so that scattered information can be joined without a shared identifier. Dunn's 1946 Record linkage coined the phrase for assembling the pages of an individual's life into a single book of record, and Fellegi and Sunter's 1969 A theory for record linkage gave it a formal decision rule that weighs how well fields agree to classify a pair as a match, a non-match, or a case for human review. Here linkage is an inference about identity across datasets, measured by match accuracy, and it has nothing to do with genes on a chromosome, bars in a mechanism, or flows between industries.[4][5]

Unit

A decision that a pair of records does or does not refer to the same entity

Recovery means

Not applicable

Measured by

Evaluated by agreement weights across fields and by match precision and recall against known truth

The connections through which investment in one industry stimulates activity in others, running backward toward its suppliers or forward toward the users of its output. Hirschman's 1958 The Strategy of Economic Development introduced backward and forward linkages as a way to choose where scarce investment would set off the most further activity, favouring industries whose demands pull others into being. Here linkage is a relationship between sectors of an economy, gauged by the strength of inter-industry demand, not a co-inheritance of genes, a machine of jointed bars, or a match between data records.[6]

Unit

The strength of demand that one industry places on its suppliers or buyers

Recovery means

Not applicable

Measured by

Estimated from input-output tables as the share and spread of inter-industry purchases and sales