When I use a word . . . Medical analogies and analogues

Although “analogy” entered English in the 15th century, via the Latin analogia and the French analogie, with the now obsolete meaning of proportion or mathematical agreement of ratios, most of its other meanings did not emerge until the 16th century or slightly later, including “a thing which (or occasionally person who) corresponds to or resembles another; a parallel, an equivalent ….”1

Here, for example, is the Elizabethan physician Philip Barrough in The Methode of Physicke in 1583: “Therefore by the example and analogie of one difference of tumours, which are caused thorough defluxion (for here we mind to intreate only of them) we will understand also other differences, which do grow through the flowing of humours.”2

The term “analogue” took even longer to emerge, not appearing until the start of the 19th century. The French naturalist Étienne Geoffroy Saint-Hilaire (1794–1866) is remembered for his work on teratology and, through his theory of analogy, his contributions to evolutionary biology.34 He introduced his theory in 1818: “La prévision à laquelle nous porte cette vérité, c’est-à-dire le pressentiment que nous trouverons toujours, dans chaque famille, tous les matériaux organiques que nous aurons aperçus dans une autre, est ce que j’ai embrassé dans le cours de mon ouvrage sous la désignation de Théorie des Analogues.”5

Then in 1837 William Whewell (1772–1844), the inventor of several scientific words, including scientist, physicist, and linguistics, reported Geoffroy Saint-Hilaire’s theory6: “The disciples of the former of the two schools express their tenets by the phrases unity of plan, unity of composition; and the more detailed developement [sic] of these doctrines has been termed the Theory of Analogues, by Geoffroy Saint-Hilaire, who claims this theory as his own creation. According to this theory, the structure and functions of animals are to be studied by the guidance of their analogy only; our attention is to be turned, not to the fitness of the organization for any end of life or action, but to its resemblance to other organizations by which it is gradually derived from the original type.”

The most widely familiar use of the term analogue today is probably in analogue technology, contrasted with digital technology, as applied for example to radio and television. For example, consider the difference between an analogue and a digital timepiece. An analogue watch, as the OED puts it,7 is “a timepiece [that] displays information using moving hands or pointers on a continuous, typically circular scale”; in contrast, a digital one displays the time in numbers.

Visual analogue scales

A different type of scale, the visual analogue scale, gives another insight into the use of analogy. The term “visual analogue scale” is surprisingly not to be found in the OED or in any other standard dictionary that I’ve looked at. However, it is defined in the medical dictionaries. For instance, the Merriam-Webster Medical Dictionary defines it as “a testing technique for measuring subjective or behavioral phenomena (as pain or dietary consumption) in which a subject selects from a gradient of alternatives (as from ‘no pain’ to ‘worst imaginable pain’ or from ‘every day’ to ‘never’) arranged in linear fashion.” And in the 30th edition of Dorland’s Illustrated Medical Dictionary it is defined as “[a scale] that enables a patient to indicate the perceived level of intensity of a symptom (e.g. of pain) by locating its position on a line representing a range from least intensity to greatest intensity.”

In other words, a visual analogue scale converts an idea that cannot be represented in numbers or a picture into an analogous system that depicts it as being quasi-dimensional. The line that forms the scale is used as a numerical analogue of the feeling or opinion being measured, for example, from none at all to the worst possible or the smallest to the greatest.

Visual analogue scales were introduced in the 1920s to represent phenomena that couldn’t be measured directly. The earliest example was used to provide ratings of workers by their superiors.8 Other types of graphic rating scales that were introduced shortly afterwards mimicked ordinal scales,9 of which the Likert scale was an early example.10

A colleague, a statistician, who had asked me about the use of the word analogue in this context responded to my explanation by suggesting that a scale can be “analogue” in the sense I had described while still being “discrete.” “A five-point scale is, one might say, analogue in one sense and digital in another.” I agreed with his assessment. A visual analogue scale can be rather like an electron or a photon of light—both a wave and a particle at the same—in that it can be both analogue and digital at the same time. In other words, it represents what one might whimsically refer to as quantum statistics.

Using analogies

Analogy and precedent are related, and in the context of the law the latter is defined as “a judicial decision which constitutes an authoritative example or rule for subsequent analogous cases.”11 Thus, a barrister, advocate, or attorney, depending on the jurisdiction, might argue that the judgement in a previous case was relevant to the case in hand. The judge or jury would then have to decide to what extent the previous case was sufficiently analogous to allow a similar conclusion to be reached in the current case.

Analogies can be useful in education, when a new concept may be made more familiar by comparing it with a concept that is already understood.

It is also often tempting to use analogies in reaching conclusions about new events. For example, a new medicine with a mechanism of action that is the same as existing medicines may be expected to produce similar outcomes in terms of both benefits and harms. However, that is not always the case. There are, for example, many differences between the pharmacological actions of different β-adrenoceptor antagonists (β-blockers), differences that have to be understood in order to use them properly.12 When cerivastatin was introduced, it was expected that it would have a similar benefit-harm balance to that of other statins; it didn’t—the risk of rhabdomyolysis was about 10 times higher.13

Mathematical models offer analogies of the systems they represent. For example, pharmacokinetic compartmental models are analogies of the way in which the body is constructed, representing different tissues by compartments in the model; the compartments are connected by mathematical constructs that represent the rates at which medicines are transferred from compartment to compartment, analogous to the actual transfer of the medicine from the blood to a tissue or group of tissues and back again, or to metabolism or excretion of the drug from the body. Physiologically based pharmacokinetic (PBPK) models constitute the most detailed type of such models, since each of the mathematical compartments in such a model represents an individual tissue, such as the liver, and the transfers between the blood and the tissues take into account the expected blood flow though that tissue. Modelling of this type is useful in many areas of pharmacology, including drug development,14 methods of drug delivery,15 and drug use in pregnancy.16

More variable in predictive power are the Markov models used in assessing the cost-effectiveness of a therapeutic intervention, typically a medicine. Here the result is susceptible to wide variation, depending on the structural nature of the model and the values of the inputs. For example, the estimated incremental cost-effectiveness ratios (ICERs) submitted by manufacturers to the National Institute for Health and Care Excellence (NICE) in the UK are typically considerably lower than those submitted by independent assessment groups, implying better cost-effectiveness than may actually be the case.17

But no matter how good or bad an analogy or an analogical model is, it can only establish the probability of a conclusion.18