The newest objection, which is at the heart of the Simpson defense team's argument to exclude DNA evidence, is the testing lab's error rate. In a field as complicated as forensic science there are many sources of error, most of which will lead to an inconclusive or no result. A false positive or negative error rate is impossible to measure because these are such rare events. These are the types of errors caused by human error or fraud. It should be noted that most of these types of mix-ups or failures in the chain of custody would lead to a false negative result which would be work in the accused's favor. These also are more likely to occur before the evidence is received by the laboratory. There is no rate of these kinds of errors that is acceptable. Fortunately, an error resulting in a miscarriage of justice has yet to be demonstrated in forensic DNA casework, although it is perhaps inevitable that it will occur someday.
    Errors intrinsic to the testing systems, such as the inability to precisely measure DNA restriction fragment lengths, are well compensated for by interpretation guidelines which take these kinds of errors into account. The series of quality control steps built into the process also provide an excellent assurance of the quality of individual and laboratory performance. In most cases these steps should lead to corrective action long before a catastrophic error has occurred.
    Minimizing laboratory errors requires a quality control program such as the ones which already are in place on a voluntary basis in the forensic laboratories. Almost all forensic DNA laboratories participate in programs which include proficiency testing and confirm that a minimum level of performance has been achieved. External proficiency testing also provides an ongoing comparison of inter-laboratory measurement error. These programs, led by the American Society of Crime Laboratory Directors Laboratory Accreditation Board are rapidly gaining momentum.
    In the forensic field, the final arbiters of quality are the courts where experts under examination and cross-examination submit their results to the scrutiny of the opposing experts and the judge and jury. This added level of scrutiny is necessary to ensure quality forensic work and includes review of casework, retesting, and observation of particular tests by opposing experts. Both sides need to have equal access to forensic expertise in the interests of fairness and justice.
    Population Genetics Estimates
    The most contentious debate in forensic DNA involves the use of statistics to estimate the rarity of a given DNA profile. This is to be expected because the extraordinary rarity of these profiles is what gives them their conclusiveness as evidence. The rarities of the genetic profiles depend on the number of genes examined （usually four or five, often more）. The frequencies of the results of each gene are multiplied to reach a combined profile frequency or the final estimate which is presented to the court.
    Critics contend that among certain ethnic sub-groups, there may be arrangements of gene frequencies that differ markedly from those found in the general population. They maintain that the population base used to give frequency statistics must be drawn from the suspect's particular gene pool, i.e., if a suspect is half Vietnamese and half French, the population database used to compute the probability ra
    證據(jù)的本質(zhì)
    證據(jù)是從中可以推理出合理結(jié)論的任何陳述或者具體的物質(zhì)。證據(jù)是一個(gè)很廣泛的類別，可以包括任何可被五官感覺到的東西，具體包括書證、物證、雙方同意的事實(shí)、證人證詞等。刑事審判中的證據(jù)關(guān)系到犯罪的目的、動(dòng)機(jī)、方法和機(jī)會(huì)等。
    一般來講，證據(jù)被劃分為兩種，間接證據(jù)和實(shí)物證據(jù)。間接證據(jù)包括通過證人收集的信息以及指向某一個(gè)人為罪犯的書證。實(shí)物證據(jù)包括與犯罪有關(guān)或者與罪犯相聯(lián)系的真實(shí)的物質(zhì)，如尸體、武器、人體的液體斑點(diǎn)、指紋、毛發(fā)和纖維等。
    法醫(yī)科學(xué)家的工作就是要檢驗(yàn)實(shí)物證據(jù)，使用科學(xué)方法重建構(gòu)成犯罪的事件。然后，控方必須將這些數(shù)據(jù)與證人證言、書證如信件、電話錄音、信用卡收據(jù)等結(jié)合起來，以便能夠在法庭演示整個(gè)案件過程。
    科學(xué)證據(jù)在民事和刑事審判中都占據(jù)著日益重要的地位。法醫(yī)學(xué)是一個(gè)發(fā)展中的產(chǎn)業(yè)。分析實(shí)物證據(jù)的新技術(shù)正在迅速發(fā)展，私人公司正在成為司法系統(tǒng)日益重要的資源。專家證據(jù)是引入科學(xué)證據(jù)的最初方法。因?yàn)檫@些專家要向外行人講授“外行人知識(shí)領(lǐng)域之外”的知識(shí)，他們就必須提出根據(jù)一般證據(jù)規(guī)則所不允許的直接觀察資料、觀點(diǎn)和傳聞之外的知識(shí)。外行證人被限制在僅就他們直接觀察到的事物作證。專家證人則被允許根據(jù)事實(shí)進(jìn)行法官或者陪審團(tuán)不能作出的推理。他們還可以根據(jù)他們的日常工作如討論會(huì)、出版物、記錄或者與其他專家的會(huì)談等作出推理。
    發(fā)現(xiàn)程序
    盡管可以進(jìn)行相反的假定推理（Perry Mason是主要的例子），在實(shí)際的審判中幾乎不會(huì)出現(xiàn)任何驚人的發(fā)現(xiàn)。這是因?yàn)榉Q之為“發(fā)現(xiàn)”的程序，根據(jù)這個(gè)程序，在實(shí)際的審判前，對(duì)方律師被允許獲知另一方掌握的案件事實(shí)和專家意見。除此之外，在審判開始之前，每一方都必須提供給另一方一份證人名單。
    在刑事案件中提供發(fā)現(xiàn)材料是有限的，僅就指控事項(xiàng)提供材料，除了美國(guó)幾個(gè)州和加拿大以外，其他地方一般都是如此。通過發(fā)現(xiàn)程序獲取資料是辯方獲得有關(guān)在法庭上控方將會(huì)呈現(xiàn)對(duì)被告不利的證據(jù)的信息的主要途徑。這一程序保證了辯方對(duì)證據(jù)的再詢問，以及研究對(duì)控方案件的替代性的假定。
    在加利福尼亞州，辯方有權(quán)使用科學(xué)證據(jù)是在格里芬案的判決中詳細(xì)說明的，這個(gè)判決中說，辯方只有在控方完成鑒定后才能使用科學(xué)證據(jù)。同樣，根據(jù)格里芬案以及美國(guó)法院在Arizona v. Youngblood案中的判決，控方可以毀滅證據(jù)，只要他們秉承善意。
    1989年初，就DNA之戰(zhàn)中的發(fā)現(xiàn)程序爆發(fā)了一場(chǎng)激戰(zhàn)。通常，除了實(shí)驗(yàn)室程序手冊(cè)和熟練鑒定結(jié)果之外，辯方一直都能調(diào)查所涉案件的自動(dòng)射線照相、實(shí)驗(yàn)室報(bào)告以及實(shí)驗(yàn)室記錄等來支持他們。辯方要求額外的資料如其他自動(dòng)射線照相、確認(rèn)研究、人口資料數(shù)據(jù)和原始數(shù)據(jù)等則面臨著苛刻的詳細(xì)審查，并經(jīng)常被拒絕。早期的DNA案件在發(fā)現(xiàn)程序上都是耗時(shí)耗力的，并因此而聞名。辯方主張控方和他們雇用的實(shí)驗(yàn)室都妨礙了辯方的發(fā)現(xiàn)請(qǐng)求。實(shí)驗(yàn)室反對(duì)辯方發(fā)現(xiàn)，并堅(jiān)持主張說，辯方請(qǐng)求發(fā)現(xiàn)的資料是受到特權(quán)保護(hù)的，構(gòu)成商業(yè)秘密，在法律上也是不相關(guān)的。
    法醫(yī)實(shí)驗(yàn)室還主張，辯方定期要求他們提供極度繁重的、有意欺騙的大量材料，他們難以負(fù)擔(dān)。如果他們必須要滿足辯方要求的話，實(shí)驗(yàn)室的工作人員就無法從事其他的工作，只能從事鑒定和復(fù)制發(fā)現(xiàn)材料的工作了。DNA發(fā)現(xiàn)程序之戰(zhàn)仍舊繼續(xù)進(jìn)行著。實(shí)際上，O.J.辛普森案件的主要律師，Robert Shapiro就賦予Cellmark以“發(fā)現(xiàn)逃犯”的稱號(hào)。然而，大多數(shù)評(píng)論人士同意圍繞著發(fā)現(xiàn)程序的許多問題已經(jīng)通過訴訟或者其他途徑解決了。
    科學(xué)證據(jù)的采納標(biāo)準(zhǔn)
    在是否采納科學(xué)證據(jù)的問題上的關(guān)鍵因素是科學(xué)證據(jù)是否是值得信賴的。要被認(rèn)為是值得信賴的，它必須具有精確性（有效性）和連貫性（可靠性）。是否采納可以根據(jù)Frye規(guī)則來判定的，這個(gè)規(guī)則強(qiáng)調(diào)“普遍接受”；或者根據(jù)聯(lián)邦證據(jù)規(guī)則（許多州法院采納此規(guī)則），這個(gè)規(guī)則強(qiáng)調(diào)有用性、可靠性和關(guān)聯(lián)性。