What coin doubles in value when half is deducted
 UKHL 2
LORD NICHOLLS OF BIRKENHEAD
1. This appeal raises a question which has divided courts and commentators throughout the common law world. The division derives essentially from different perceptions of what constitutes injustice in a common form type of medical negligence case. Some believe a remedy is essential and that a principled ground for providing an appropriate remedy can be found. Others are not persuaded. I am in the former camp.
2. This is the type of case under consideration. A patient is suffering from cancer. His prospects are uncertain. He has a 45% chance of recovery. Unfortunately his doctor negligently misdiagnoses his condition as benign. So the necessary treatment is delayed for months. As a result the patient's prospects of recovery become nil or almost nil. Has the patient a claim for damages against the doctor? No, the House was told. The patient could recover damages if his initial prospects of recovery had been more than 50%. But because they were less than 50% he can recover nothing.
3. This surely cannot be the state of the law today. It would be irrational and indefensible. The loss of a 45% prospect of recovery is just as much a real loss for a patient as the loss of a 55% prospect of recovery. In both cases the doctor was in breach of his duty to his patient. In both cases the patient was worse off. He lost something of importance and value. But, it is said, in one case the patient has a remedy, in the other he does not.
4. This would make no sort of sense. It would mean that in the 45% case the doctor's duty would be hollow. The duty would be empty of content. For the reasons which follow I reject this suggested distinction. The common law does not compel courts to proceed in such an unreal fashion. I would hold that a patient has a right to a remedy as much where his prospects of recovery were less than 50-50 as where they exceeded 50-50. Perforce the reasoning is lengthy, in parts intricate, because this is a difficult area of the law.
The present case
5. First I must mention the salient facts of this appeal. These are not quite so straightforward or extreme as in the example just given. At the risk of over-simplification they can be summarised as follows. The defendant Dr Scott negligently diagnosed as innocuous a lump under the left arm of the claimant Mr Malcolm Gregg when in fact it was cancerous (non-Hodgkin's lymphoma). This led to nine months' delay in Mr Gregg receiving treatment. During this period his condition deteriorated by the disease spreading elsewhere. The deterioration in Mr Gregg's condition reduced his prospects of disease-free survival for ten years from 42%, when he first consulted Dr Scott, to 25% at the date of the trial. The judge found that, if treated promptly, Mr Gregg's initial treatment would probably have achieved remission without an immediate need for high dose chemotherapy. Prompt treatment would, at least initially, have prevented the cancer spreading to the left pectoral region.
6. However, the judge found also that, although Mr Gregg's condition deteriorated and in consequence his prospects were reduced in this way, a better outcome was never a probability. It was not possible to conclude on the balance of probability that, in the absence of the negligence, Mr Gregg's medical condition would have been better or that he would have avoided any particular treatment. Before the negligence Mr Gregg had a less than evens chance (45%) of avoiding the deterioration in his condition which ultimately occurred. The delay did not extinguish this chance but reduced it by roughly half. The judge assessed this reduction at 20%. That was the extent to which the negligence reduced Mr Gregg's prospects of avoiding the deterioration in his condition which ultimately occurred. The facts can be found more fully stated in the judgments of the Court of Appeal  EWCA Civ 1471 and in the speech of my noble and learned friend Lord Phillips of Worth Matravers.
7. On these findings the trial judge, Judge Inglis, dismissed the claim. He considered he was driven to this conclusion by the reasoning of your Lordships' House in Hotson v East Berkshire Area Health Authority  AC 750. The Court of Appeal by a majority (Simon Brown and Mance LJJ, Latham LJ dissenting) dismissed Mr Gregg's appeal.
Past facts and future prospects
8. In order to set the question now before the House in its legal perspective I must next say something about the common law approach to proof of actionable damage, that is, damage which the law regards as founding a claim for compensation. It is trite law that in the ordinary way a claimant must prove the facts giving rise to a cause of action against the defendant. Where the claim is based on negligence the facts to be proved include those constituting actionable damage as well as those giving rise to the existence of a duty of care and its breach.
9. In the normal way proof of the facts constituting actionable damage calls for proof of the claimant's present position and proof of what would have been the claimant's position in the absence of the defendant's wrongful act or omission. As to what constitutes proof, traditionally the common law has drawn a distinction between proof of past facts and proof of future prospects. A happening in the past either occurred or it did not. Whether an event happened in the past is a matter to be established in civil cases on the balance of probability. If an event probably happened no discount is made for the possibility it did not. Proof of future possibilities is approached differently. Whether an event will happen in the future calls for an assessment of the likelihood of that event happening, because no one knows for certain what will happen in the future.
10. This distinction between past and future is applied also when deciding what would have happened in the past or future but for a past happening such as the defendant's negligent act. What would have happened in the past but for something which happened in the past is, at least generally, a question decided by the courts on the all-or-nothing basis of the balance of probability. On this the authorities are not altogether consistent, but this seems to be the generally accepted practice. In contrast, what would have happened in the future but for something which happened in the past calls for an assessment of likelihood.
11. Thus the question whether a claimant's hand was damaged in an accident at work is a matter to be decided on the balance of probability. So also is the hypothetical question whether, if the employer had duly provided the necessary protective equipment, the claimant would have worn it: see, for instance, McWilliams v Sir William Arrol & Co Ltd  1 WLR 295, 306-307, 309, Allied Maples Group Ltd v Simmons & Simmons  1 WLR 1602, 1610G, per Stuart-Smith LJ, and Barnett v Chelsea and Kensington Hospital Management Committee  1 QB 428, 439. By way of contrast, whether the claimant's damaged hand will develop osteoarthritis in later life calls for an estimate of the chances of that happening. Whether, hypothetically, his hand would have been likely to develop osteoarthritis in the future even without the accident also calls for such an estimate.
12. This distinction was summarised in the well known words of Lord Diplock in Mallett v McMonagle  AC 166, 176:
- 'The role of the court in making an assessment of damages which depends upon its view as to what will be and what would have been is to be contrasted with its ordinary function in civil actions of determining what was. In determining what did happen in the past a court decides on the balance of probabilities. Anything that is more probable than not it treats as certain. But in assessing damages which depend upon its view as to what will happen in the future or would have happened in the future if something had not happened in the past, the court must make an estimate as to what are the chances that a particular thing will or would have happened and reflect those chances, whether they are more or less than even, in the amount of damages it awards.'
Lord Reid made similar observations in Davies v Taylor  AC 207, 212-213.
13. This sharp distinction between past events and future possibilities is open to criticism. Whether an event occurred in the past can be every bit as uncertain as whether an event is likely to occur in the future. But by and large this established distinction works well enough. It has a comfortable simplicity which accords with everyday experience of the difference between knowing what happened in the past and forecasting what may happen in the future.
14. In practice the distinction is least satisfactory when applied to hypothetical events (what would have happened had the wrong not been committed). The theory underpinning the all-or-nothing approach to proof of past facts appears to be that a past fact either happened or it did not and the law should proceed on the same footing. But the underlying certainty, that a past fact happened or it did not, is absent from hypothetical facts. By definition hypothetical events did not happen in the past, nor will they happen in the future. They are based on false assumptions. The defendant's wrong precluded them from ever materialising.
Loss of an opportunity or chance as actionable damage
15. It is perhaps not surprising therefore that it is principally in the field of hypothetical past events that difficulties have arisen in practice. Sometimes, whether a claimant has suffered actionable damage cannot fairly be decided on an all-or-nothing basis by reference to what, on balance of probability, would have happened but for the defendant's negligence. Sometimes this would be too crude an approach. What would have happened in the absence of the defendant's negligence is altogether too uncertain for the all-or-nothing approach to be satisfactory. In some cases what the claimant lost by the negligence was the opportunity or chance to achieve a desired result whose achievement was outside his control and inherently uncertain. The defendant's wrong consisted of depriving the claimant of a chance he would otherwise have had to achieve a desired outcome.
16. Then, the greater the uncertainty surrounding the desired future outcome, the less attractive it becomes to define the claimant's loss by whether or not, on balance of probability, he would have achieved the desired outcome but for the defendant's negligence. This definition of the claimant's loss becomes increasingly unattractive because, as the uncertainty of outcome increases, this way of defining the claimant's loss accords ever less closely with what in practice the claimant had and what in practice he lost by the defendant's negligence.
17. In order to achieve a just result in such cases the law defines the claimant's actionable damage more narrowly by reference to the opportunity the claimant lost, rather than by reference to the loss of the desired outcome which was never within his control. In adopting this approach the law does not depart from the principle that the claimant must prove actionable damage on the balance of probability. The law adheres to this principle but defines actionable damage in different, more appropriate terms. The law treats the claimant's loss of his opportunity or chance as itself actionable damage. The claimant must prove this loss on balance of probability. The court will then measure this loss as best it may. The chance is to be ignored if it was merely speculative, but evaluated if it was substantial: see Davies v Taylor  AC 207, 212, per Lord Reid.
18. Some familiar examples will suffice. A woman who was wrongly deprived of the chance of being one of the winners in a beauty competition was awarded damages for loss of a chance. The court did not attempt to decide on balance of probability the hypothetical past event of what would have happened if the claimant had been duly notified of her interview: Chaplin v Hicks  2 KB 786. When a solicitor's failure to issue a writ in time deprived a claimant of the opportunity to pursue court proceedings damages were not assessed on an all-or-nothing basis by reference to what probably would have been the outcome if the proceedings had been commenced in time. The court assessed what would have been the claimant's prospects of success in the proceedings which the solicitor's negligence prevented him from pursuing: Kitchen v Royal Air Force Association  1 WLR 563. When an employer negligently supplied an inaccurate character reference, the
employee did not need to prove that, but for the negligence, he would probably have been given the new job. The employee only had to prove he lost a reasonable chance of employment, which the court would evaluate: Spring v Guardian Assurance Plc  2 AC 296, 327.
19. In Allied Maples Group Ltd v Simmons & Simmons  1 WLR 1602 a solicitor's negligence deprived the claimant of an opportunity to negotiate a better bargain. The Court of Appeal applied the 'loss of chance' approach. Stuart-Smith LJ, at page 1611, regarded the case as one of those where 'the plaintiff's loss depends on the hypothetical action of a third party, either in addition to action by the plaintiff or independently of it.' It is clear that Stuart-Smith LJ did not intend this to be a precise or exhaustive statement of the circumstances where loss of a chance may constitute actionable damage and his observation should not be so understood.
20. Against this background I turn to the primary question raised by this appeal: how should the loss suffered by a patient in Mr Gregg's position be identified? The defendant says 'loss' is confined to an outcome which is shown, on balance of probability, to be worse than it otherwise would have been. Mr Gregg must prove that, on balance of probability, his medical condition after the negligence was worse than it would have been in the absence of the negligence. Mr Gregg says his 'loss' includes proved diminution in the prospects of a favourable outcome. Dr Scott's negligence deprived him of a worthwhile chance that his medical condition would not have deteriorated as it did.
21. Of primary relevance on this important issue is an evaluation of what, in practice. a patient suffering from a progressive illness loses when the treatment he needs is delayed because of a negligent diagnosis. What the patient loses depends, of course, on the circumstances of the individual case. No doubt in some cases medical opinion will be that, given his pre-existing condition, the patient lost nothing by the delay in treatment because he never had any realistic prospect of recovery. The doctor's misdiagnosis made no significant difference to the patient's prospects of recovery. In other cases medical opinion may be that the patient lost everything. Barring unforeseen complications he would have made a complete recovery had his condition been diagnosed properly and had he then received appropriate treatment.
22. These two types of case are, in the present context, straightforward. But there are also many cases of serious illness or injury where a patient's existing chances of recovery fall between these extremes. There are occasions where medical opinion will be that, given prompt and appropriate treatment, the outcome was uncertain but the patient's prospects of recovery were appreciable, sometimes exceeding 50%, sometimes not.
23. This is hardly surprising. Enormous advances have been made in medical knowledge and skills in recent years, in this country and internationally. New and improved drugs and procedures make possible ever more alleviation of illnesses and injuries. But the outcome of medical treatment in any particular case remains beyond anyone's control. It is often a matter of considerable uncertainty, in some types of case more than others. Doctors cannot guarantee outcomes. Every person and his personal circumstances and history are different. The way some drugs work is not understood fully. The response of patients to treatment is not uniform, nor is it always predictable. Faced with a serious illness or injury doctors can often do no more than assess a patient's prospects of recovery. Limitations on human knowledge mean that, to greater or lesser extent, the prognosis for a patient is inherently uncertain. Indeed, sometimes the very diagnosis itself may be problematic.
24. Given this uncertainty of outcome, the appropriate characterisation of a patient's loss in this type of case must surely be that it comprises the loss of the chance of a favourable outcome, rather than the loss of the outcome itself. Justice so requires, because this matches medical reality. This recognises what in practice a patient had before the doctor's negligence occurred. It recognises what in practice the patient lost by reason of that negligence. The doctor's negligence diminished the patient's prospects of recovery. And this analysis of a patient's loss accords with the purpose of the legal duty of which the doctor was in breach. In short, the purpose of the duty is to promote the patient's prospects of recovery by exercising due skill and care in diagnosing and treating the patient's condition.
25. This approach also achieves a basic objective of the law of tort. The common law imposes duties and seeks to provide appropriate remedies in the event of a breach of duty. If negligent diagnosis or treatment diminishes a patient's prospects of recovery, a law which does not recognise this as a wrong calling for redress would be seriously deficient today. In respect of the doctors' breach of duty the law would not have provided an appropriate remedy. Of course, losing a chance of saving a leg is not the same as losing a leg: see Tony Weir, 'Tort Law' (2002), p 76. But that is not a reason for declining to value the chance for whose loss the doctor was directly responsible. The law would rightly be open to reproach were it to provide a remedy if what is lost by a professional adviser's negligence is a financial opportunity or chance but refuse a remedy where what is lost by a doctor's negligence is the chance of health or even life itself. Justice requires that in the latter case as much as the former the loss of a chance should constitute actionable damage.
Loss of prospects and medical statistics
26. Before pursuing the implications of this approach further, I must mention another feature of medical negligence cases. Thus far I have treated diminution of a patient's prospects of recovery as capable of proof like any other fact. But in medical negligence cases there is a complication not always found in negligence claims brought against lawyers or other professionals. By the same token that the outcome of illness or injury in an individual patient may be inherently uncertain, so it may be difficult to prove in an individual case what the patient lost when he lost the 'chance' of a more favourable outcome.
27. Take, for instance, a professional negligence claim brought against a solicitor who negligently permitted court proceedings to become statute-barred by failing to issue a writ in time. The court deciding the negligence claim is able to assess what would have been the claimant's prospects in the time-barred proceedings by having regard to a wide range of known facts peculiar to the particular case: the nature of the issues, the evidence which would have been available, and so forth. In cases of medical negligence assessment of a patient's loss may be hampered, to greater or lesser extent, by one crucial fact being unknown and unknowable: how the particular patient would have responded to proper treatment at the right time. The patient's previous or subsequent history may assist. No doubt other indications may be available. But at times, perhaps often, statistical evidence will be the main evidential aid.
28. Statistical evidence, however, is not strictly a guide to what would have happened in one particular case. Statistics record retrospectively what happened to other patients in more or less comparable situations. They reveal trends of outcome. They are general in nature. The different way other patients responded in a similar position says nothing about how the claimant would have responded. Statistics do not show whether the claimant patient would have conformed to the trend or been an exception from it. They are an imperfect means of assessing outcomes even of groups of patients undergoing treatment, let alone a means of providing an accurate assessment of the position of one individual patient.
29. Take as an example the statistical evidence that 42% of the patients suffering from the same disease as Mr Gregg achieved ten year survival if treated at the stage when, but for the negligence, Mr Gregg would have been treated, this figure dropping to 25% when the treatment was not given until the disease had reached the more advanced stage at which Mr Gregg was actually treated. Who can know whether Mr Gregg was in the 58% non-survivor category or the 42% survivor category? There was no evidence, peculiar to him or his circumstances, enabling anyone to say whether on balance of probability he was in the former group or the latter group. The response Mr Gregg would have made if treated promptly is not known and never can be known.
30. This difficulty was the foundation of a submission based on the proposition that a 'statistical chance' has no value, so its 'loss' cannot attract an award of compensation. To award compensation for such a loss, calculated by reference to percentage 'chances', would not achieve fairness. That would have the effect of under-compensating patients whose outcome was in fact worsened by the delay in treatment, and over-compensating those whose outcome was not in fact worsened.
31. Despite its impeccable logic this argument cannot be accepted. In suitable cases courts are prepared to adapt their process so as to leap an evidentiary gap when overall fairness plainly so requires. Fairchild v Glenhaven Funeral Services Ltd  1 AC 32 is a recent illustration of this in a different context. In the present context use of statistics for the purpose of evaluating a lost chance makes good sense. The practical arguments in favour of use of statistics in this way are compelling. Everyone knows that when speedy medical treatment is essential delay may have adverse effects on a patient. When cancer spreads it may be less susceptible to treatment. Medical experience of other patients has shown this to be so. This experience of the outcome for other patients, recorded in statistics, may afford the only basis on which a court can evaluate the diminution in prospects of recovery of an individual patient which may be expected to follow from delay in giving him treatment.
32. The value of the statistics will of course depend upon their quality: the methodology used in their compilation, how up to date they are, the number of patients involved in the statistics, the closeness of their position to that of the claimant, the clarity of the trend revealed by the figures, and so on. But to reject all statistical evidence out of hand would not be acceptable. This argument, if accepted, would effectually nullify the use of statistics in all cases of delayed treatment save perhaps where the figures approached 0% or 100%. Despite its imperfection, in practice statistical evidence of a diminution in perceived prospects will often be the nearest one can get to evidence of diminution of actual prospects in a particular case. When there is nothing better courts should be able to use these figures and give them such weight as is appropriate in the circumstances. This conclusion is the more compelling when it is recalled that the reason why the actual outcome for the claimant patient if treated promptly is not known is that the defendant by his negligence prevented that outcome becoming known.
33. Use of statistics in this way is not a revolutionary step. Medical statistics are widely used and have been so for many years. Courts habitually use statistics as an aid when compensating claimants for a risk of an outcome which may materialise, whether the risk is more than 50% or less than 50%. If a head injury carries with it a 20% increased risk of epilepsy in the future a court takes this into account when assessing compensation in the round.
Identifying a lost chance in medical negligence cases
34. I come next to a further twist in the story. It concerns an additional complication. It is a difficult part of this appeal. With 'loss of chance' cases such as Chaplin v Hicks  2 KB 786 identifying the 'chance' the claimant lost is straightforward enough. The position of the claimant in the Chaplin case, had there been no wrong, could not be decided satisfactorily because no one could know what would have been the outcome of the beauty contest if the claimant had appeared at the interview. It was this uncertainty which made it appropriate to treat her loss of a chance as itself actionable damage. Otherwise she would have had no remedy. The chance she lost was the opportunity to attend and be considered at the interview. Thus, in this type of case the claimant's actual position at the time of the negligence, proved on balance of probability if disputed, is not determinative of the crucial hypothetical fact: what would have been the claimant's position in the absence of the wrong?Source: www.publications.parliament.uk