Many trades and professions have long required prospective members to serve a probationary period to gain entry to their ranks. Increasingly the building professions have moved toward academic qualification, to the point that it is now rare for new members of the older professions to start with less than a bachelor’s degree. As soon, however, as newly qualified professionals begin to practise, it is their actions – not qualifications – which become the gauge of their competence.
In common law, professional competence is assessed in comparative rather than absolute terms. For a professional to be in error is not of itself grounds for a claim for professional negligence. Unless the express terms of the contract state to the contrary, or in cases where there is an implied or statutory duty to achieve ‘fitness for purpose’, it is not sufficient merely to show that a defect has arisen from someone’s errors or omissions.
For example, many engineers have, in the recent past, wrongly specified high-alumina cement. However, they did so at a time when a large part of the profession believed this to be sound. The mere fact that they were wrong and that damage has resulted from this error does not make them liable for the consequential losses. Each engineer, in common with other design professionals, is judged by comparison with their peers. Thus, for an engineer to be liable for the cost of a defect, they would have to have performed their duties to a lower standard than could be expected of a representative body of ordinarily competent engineers.
The standard of skill and care in all cases is similar. The duty owed to a client is to act with the skill and care of the ordinary prudent person holding themselves out as possessing the particular skill offered. It is not simply a question of giving correct or incorrect advice, but of demonstrating a standard of skill commonly found in persons of like discipline.
Legal opinion on this can be found in various decided court cases.
To quote from the privy council in Chin Keow v Government of Malaysia; ‘ … where you get a situation which involves the use of some special skill or competence, then the test whether there has been negligence or not is not the test of the man on the top of the Clapham Omnibus, because he has not got this special skill. The test is the standard of the ordinary skilled man exercising and professing to have that special skill ... it is well established law that it is sufficient if he exercises the ordinary skill of an ordinary competent man exercising that particular art.’
The applicable common law is well set out in the following:
‘The law requires of a professional man that he live up in practice to the standard of the ordinary skilled man exercising and professing to have his special professional skills. He need not possess the highest expert skill; it is enough if he exercises the ordinary skill of an ordinary competent man exercising his particular art 
… No matter what profession it may be, the common law does not impose on those who practise it any liability for damage resulting from what in the result turn out to have been errors of judgement, unless the error was such as no reasonable well-informed and competent member of that profession could have made’.
It can be drawn from the decided cases on negligence that a professional should:
· command the corpus of knowledge which forms part of the professional equipment of the ordinary member of their profession;
· not lag behind other ordinarily assiduous and intelligent members of a profession in knowledge of new advances and developments in their field;
· have such awareness as an ordinarily competent practitioner would have of the deficiencies in their knowledge and the limitations on their skill;
· be alert to the hazards and risks inherent in any professional task they undertake to the extent that other ordinarily competent members of their profession would be alert;
· bring to any professional task they undertake no less expertise, skill and care than any other ordinarily competent members of that profession would bring, but no more.
The standard is that of the reasonable average. The law does not require of a professional to be exceptional (unless, perhaps, if in plying for work they hold themselves out to be exceptional).
In deciding whether someone has fallen short of the standards observed by ordinarily skilled and competent members of their profession, it is the standards prevailing at the time of acts or omissions that provide the relevant yardstick.
‘He is not … to be judged by the wisdom of hindsight. This of course means that knowledge of an event which happened later should not be applied when judging acts and/or omissions which took place before that event …; … it is necessary, if the Defendant’s conduct is to be fairly judged, that the making of [any] retrospective assessment should not of itself have the effect of magnifying the significance of the … risk as it appeared or should reasonably have appeared to [an] ordinarily competent practical man with a job to do at the time’. 
In applying a test of skill and competence, circumstances also need to be taken into account. Negligent advice which is not relied upon or which self-evidently ought not to be relied upon is not a good basis for a claim in negligence. The same negligent advice given in circumstances where it is relied upon may form the basis for a sustainable claim.
For example, in relation to a proposed building project, Archie, the Project Architect, asked Edward, the Project Engineer, to carry out a soils investigation. The investigation was limited to shallow test holes. Edward advised Archie accordingly, noting that deeper test holes were required before the capacity of the ground to support the proposed building could be established with confidence. This action was not negligent because the limits of the reliance that could be placed on the test results were stated.
Later, Owen, the Employer, requested a copy of the report. Edward wrote to Owen supplying the requested information, but omitted to warn of the need for deeper trial holes. In writing to Owen, Edward gave his opinion of the load-bearing capacity of the soil. In subsequent legal action it was held that Edward was in breach of his duty of care for not advising Owen that it would be prudent to take further tests before drawing firm conclusions on the bearing capacity of the ground.
 For example: Marshall v Lindsey County Council (1935) 1 KB 516, 540 (CA), Greaves and Co (Contractors) Ltd v Banham Mikle and Partners (1975) 3 ALL ER 104 and McNare J in Bolam v Friern Hospital Management Committee  2 All ER 118.
 Bolam v Friern Hospital Management Committee  1 WLR 582.
 Saif Ali v Sydney Mitchell & Co.  AC 198 at 220 D per Lord Diplock.
 Bingham LJ dissenting opinion in the court of Appeal in the Abbeystead case (Eckersley & Others v Binnie & Partners, Edmund Nuttall Ltd. & North West Water Authority  18 ConLR 1).