What is anomaly?

Thomas Kuhn, answered from the texts and cited to the page.

Thomas Kuhn answers

An anomaly is a discrepancy between what a paradigm leads scientists to expect and what observation actually delivers — but the definition, stated that baldly, already obscures the most important thing about it, which is how the community responds. Normal science is organized around problems that past success has shown to be tractable. Scientists restrict their attention to puzzles defined by the conceptual and instrumental techniques already at hand, and that restriction is what makes short-term success so reliable.¹

But the same restriction guarantees that the data will never entirely or precisely fit theory-induced expectations. Some of those failures to fit become current research problems; others are pushed to the periphery of consciousness, and some are suppressed entirely.² The suppression is not dishonesty — it is, most of the time, justified. More often than not, minor instrumental adjustments or small articulations of existing theory ultimately reduce the apparent anomaly to law, and pausing over anomalies when they are first confronted is to invite continual distraction.³

So the first thing to say about anomaly is that it is the normal condition of science, not a special event. There are always some discrepancies. Even the most stubborn ones usually respond at last to normal practice, and scientists are often quite reasonably willing to wait.⁴ The moon's perigee remained only half of its predicted value for sixty years after Newton's original computation, and no one took the occasional proposals for modifying the inverse square law very seriously — patience that proved justified when Clairaut showed in 1750 that only the mathematics of the application had been wrong.⁵

Neither the long-recognized discrepancy in the speed of sound nor the anomalous motion of Mercury seriously threatened Newtonian theory, and both were eventually resolved — the first by experiments undertaken for a wholly different purpose, the second by general relativity after a crisis it had had no role in creating.⁶ Apparently neither had seemed sufficiently fundamental to evoke the malaise that goes with crisis.

The philosophically interesting question, then, is not what an anomaly is but what makes one press. Not all anomalies are created equal. Some cannot be indefinitely ignored — either because they are particularly striking or because they are reproduced repeatedly across many different laboratories — and these impinge with gradually increasing force upon the consciousness of the scientific community.⁷

As that pressure mounts, the pattern of research begins to change. Reports of unassimilated observations appear more frequently in notebooks and as asides in published reports; then more and more research turns to the anomaly itself; those attempting to make it lawlike begin increasingly to quarrel over the meaning of concepts and theories they had long held in common.⁸

When an anomaly comes to seem more than just another puzzle of normal science, the transition to crisis has begun.⁹ More and more attention is devoted to it by more and more of the field's most eminent practitioners. If it continues to resist — as it usually does not — many of them may come to view its resolution as the subject matter of their discipline.

Through the proliferation of divergent articulations, increasingly described as ad hoc adjustments, the rules of normal science become increasingly blurred; though a paradigm still exists, few practitioners prove entirely agreed about what it is, and even formerly standard solutions of solved problems are called in question.¹⁰ At the extreme of crisis, the scientist starts to search at random, trying anything at all that might illuminate the nature of the difficulty.

If that difficulty endures long enough, he and his colleagues may even begin to wonder whether their entire approach to the now-problematic range of phenomena is somehow askew.¹¹ Crisis can be resolved in a few ways: sometimes more refined experimental techniques or finer scrutiny of theoretical approximations eliminate the discrepancy entirely; sometimes a discrepancy that has repeatedly defied analysis is simply left as a known anomaly, encysted within the body of more successful applications of the theory.¹²

Anomaly, in short, is the hinge between normal and revolutionary science — but it functions as that hinge only when the community comes to recognize it as such, and that recognition is itself a social and psychological process, not a logical one.

Sources

professionals restrict their attention to problems defined by the conceptual and instrumental techniques already at hand.
The Essential Tension, p. 282
Even the data that this restricted pattern of research presents to the scientist never entirely or precisely fit his theory-induced expectations. Some of those failures to fit provide his current research problems; but others are pushed to the periphery of consciousness and some are suppressed entirely.
The Essential Tension, p. 282
Usually that inability to recognize and confront anomaly is justified in the event. More often than not minor instrumental adjustments or small articulations of existing theory ultimately reduce the apparent anomaly to law. Pausing over anomalies when they are first confronted is to invite continual distraction.
The Essential Tension, p. 282
There are always some discrepancies. Even the most stubborn ones usually respond at last to normal practice. Very often scientists are willing to wait, particularly if there are many problems available in other parts of the field.
The Structure of Scientific Revolutions, p. 71
during the sixty years after Newton's original computation, the predicted motion of the moon's perigee remained only half of that observed... Clairaut in 1750 was able to show that only the mathematics of the application had been wrong and that Newtonian theory could stand as before.
The Structure of Scientific Revolutions, p. 71
No one seriously questioned Newtonian theory because of the long-recognized discrepancies between predictions from that theory and both the speed of sound and the motion of Mercury. The first discrepancy was ultimately and quite unexpectedly resolved by experiments on heat undertaken for a very different purpose; the second vanished with the general theory of relativity after a crisis that it had had no role in creating.
The Structure of Scientific Revolutions, p. 71
Among those that do not are some which, either because they are particularly striking or because they are educed repeatedly in many different laboratories, cannot be indefinitely ignored. Though they remain unassimilated, they impinge with gradually increasing force upon the consciousness of the scientific community.
The Essential Tension, p. 282
At first, reports of unassimilated observations appear more and more frequently in the pages of laboratory notebooks or as asides in published reports. Then more and more research is turned to the anomaly itself. Those who are attempting to make it lawlike will increasingly quarrel over the meaning of the concepts and theories which they have long held in
The Essential Tension, p. 282
When, for these reasons or others like them, an anomaly comes to seem more than just another puzzle of normal science, the transition to crisis and to extraordinary science has begun.
The Structure of Scientific Revolutions, p. 72
Through this proliferation of divergent articulations (more and more frequently they will come to be described as ad hoc adjustments), the rules of normal science become increasingly blurred. Though there still is a paradigm, few practitioners prove to be entirely agreed about what it is. Even formerly standard solutions of solved problems are called in question.
The Structure of Scientific Revolutions, p. 72
the scientist will start to search at random, trying anything at all which he thinks may conceivably illuminate the nature of his difficulty. If that difficulty endures long enough, he and his colleagues may even begin to wonder whether their entire approach to the now problematic range of natural phenomena is not somehow askew.
The Essential Tension, pp. 222–223
Sometimes, as has often happened in chemistry and astronomy, more refined experimental techniques or a finer scrutiny of the theoretical approximations will eliminate the discrepancy entirely. On other occasions, though I think not often, a discrepancy that has repeatedly defied analysis is simply left as a known anomaly, encysted within the body of more successful applications of the theory.
The Essential Tension, pp. 222–223