However, writing such a report can cause problems, because in simplifying the case for the jury to understand, the expert may omit a vital piece of evidence. Throughout the course of the investigation, the forensic expert may not know the details of the case, and so there is a possibility that he does not know which piece of information discovered is crucial to the case. Due to his inadequate knowledge of the case at hand, important information might be omitted from the report for the sake of brevity and clarity.

Forensic scientists are often criticized for this habit. However, it is a necessary part of their job. If they knew better the importance of their work within the individual case, they may be able to reduce the size of their report with a degree of discretion, while being of no hindrance to the case, or the jury, and at the same time not hiding any crucial information to the case.

If something of importance has been left out of the report by the forensic expert, it can be extracted from the expert by careful questioning from the prosecution, or even the defence, whoever values the evidence the most. However, if the right question is not asked, the information may never be found out, until an appeal is launched. An example of this happening is Preece v HM Advocate, where Dr Clift, a prosecution expert, omitted some vital information from his report in the court, although it had been included in his written report for the defence. His report stated that the blood samples found at the scene of the crime were of group A (secretor) and that the accused was also of this blood group, meaning that the probability of the accused being guilty was relatively high. However, he omitted in his oral report the fact that the victim�s blood was also of group A (secretor). This was before the development of DNA profiling, and so it could not be proven who the blood belonged to. However, given the information that both the accused and the victim were of the same blood type, it greatly reduced the probability that the blood at the scene of the crime was that of the accused. This evidence is obviously of importance to the case, but Dr Clift�s training caused him to omit it. He was not asked the relevant question, and so this information was not extracted from him. As a result of this, the conviction was quashed on appeal, and Dr Clift was severely criticized for his oversight.

It is also often difficult to express to the jury that forensic science is not infallible, contrary to what they have been led to believe. It is not objective and completely factual, as there is interpretation of the facts involved. In the future forensic science evidence used in a case could be proved to have been wrong for four main reasons. Firstly it is possible that scientists will disagree, secondly contamination may have occurred. The other two reasons are a lack of time and a lack of finance. If a certain test is expensive to conduct, it may be deemed to be unnecessary, and omitted. In the same way, if forensic science evidence was only asked for a few months before the trial is to take place, there may not be sufficient time to carry out all the necessary tests, so the least important ones may be omitted. Both these constraints add to the chance that the evidence may not be as exact as it otherwise could be, and it can be difficult to make a jury understand this.

A problem that is often found in the courtroom concerning forensic evidence is the experts themselves. An expert is employed by either the prosecution or the defence. These experts tend to represent the same side every time they appear in court. This may introduce bias into the case, and so only certain types of evidence beneficial to their side will be investigated. This is a point of controversy and there is much discussion among forensic scientists whether they should be objective and impartial and serve only the truth or whether they should "deliver the goods" that they have been paid for.

This can be a difficult thing to do for forensic scientists, not because they are fickle people who aim to please only for financial gains, but because during the investigation they have been asked to do tests on a particular specimen from a certain viewpoint. It is often the case that a test cannot take place if the scientist does not know about a certain aspect of the case � information usually supplied by the side they represent. For this reason, it is not often that a forensic scientist can conform to the desirable measures of objective scientific truth. Some of the information they uncover may only be a scientific proof providing that another factor is true. It is this other factor that is determined by the scientist�s previous knowledge of the case.

Another reason why the information that scientists give to the court may be so beneficial to the prosecution is that the police, when collecting evidence, often have a suspect in mind before they send any evidence off to the FSS, and so the forensic scientist is only asked to confirm the suspicions of the police. This is where it is often said that the forensic scientist is biased towards the prosecution. The other problem with this method is that the police may tend to collect evidence that will uphold their theory that their suspect is the correct person.

The problem that the forensic scientist may be biased is difficult to solve because they will have a certain amount of background information for a variety of reasons. They may have been at the scene of the crime, which automatically gives them background information that they might not have otherwise obtained. If they want to draw inferences from the evidence they have been presented with, it is important that the scientist knows some background information. Unfortunately for the scientist, this makes him "inappropriately influenced", which is a problem in the courtroom. It is however impossible for the scientist to work without such knowledge in some branches of forensic science.

Fact and opinion are usually obviously two different things. However, in forensic science, they can intertwine, because fact, in this sphere, is created by the scientists themselves, and if there are disagreements between scientists, or there is an uncertainty in what is being tested, fact and opinion will be blurred. The general rule in evidence law is that a witness may only testify as to the facts that he actually knows. Luckily, this does not apply to expert witnesses. Opinion evidence is acceptable from an expert, as long as he has adequate expertise to testify about the certain subject, and he must only confine his opinion to that which he has expertise in. So if a forensic scientist gives his opinion on a matter, even if there are others that do not agree with him, it may be admissible. However this raises the problem that the prosecution is able to "shop around" for their experts. If one expert has an opinion which is at odds to the opinion they wanted, it may be possible for them to find an expert with a more suitable opinion. This is less likely to occur with the defence, as it is more difficult for them to find an expert. It is also less likely to happen in some areas of forensic science than others.

Forensic scientists are predominantly scientists. Sometimes their job finds them in the courtroom giving evidence. As they are scientists, this adversarial atmosphere may not suit them. They do not get any formal training in the ways of the courtroom, and especially for the expert who has not appeared in court very often, it can be a daunting experience. The barristers who question the scientists are well versed in the ways of the courtroom, and so immediately have the upper hand. They are skilled in getting information from the expert, and if the expert is not careful, they can quickly get him confused. To alleviate this problem, scientists should speak with the prosecution or the defence before the trial to establish what sort of things are going to be asked of him. However this contact may also give rise to a degree of bias in the scientist�s reports. It has often been criticised that the scientist has been "told" what to say beforehand by the prosecution and is effectively their pawn and nothing else. To stop this from taking place, these meetings do not take place as much as they should, under the principle that the less contact the witness has with the prosecution, the less biased their evidence will be. This creates a vicious circle in effect, as then without such meetings, the expert witness may well not know what sort of thing he is up against in the court.

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