Nov 23

天路客按:“《论语·卫灵公》有言曰:“工欲善其事,必先利其器。居是邦也,事其大夫之贤者,友其士之仁者。”作为中国人相比都耳熟能详,这话的意思是要做好工作,先要使工具锋利。小说里面也有这话,如:“工欲善其事,必先利其器。今既一无所有,纵使大禹重生,亦当束手。”(清·李汝珍《镜花缘》第三十六回)。”(见天路客《现代方法学的进步终会打破中医残梦——兼谈中医缺少证伪能力》XYS20091104),科学有效的方法固然重要,但也不要迷信,惟方法论,任何方法学(Methodology)都是有局限的,都不是绝对的,不能唯方法论,否则科学方法更会使人迷失,一个清醒的人要避免被方法所掌握,即使RCT也远非万能,特转载此文以作说明,有兴趣者可以继续讨论。

 
              文章来源:BMJ  2003;327:1459-1461 (20 December), doi:10.1136/bmj.327.7429.1459

Hazardous journey

Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials

Gordon C S Smith, professor1, Jill P Pell, consultant2

1 Department of Obstetrics and Gynaecology, Cambridge University, Cambridge CB2 2QQ, 2 Department of Public Health, Greater Glasgow NHS Board, Glasgow G3 8YU

Correspondence to: G C S Smith gcss2@cam.ac.uk

Abstract

Objectives To determine whether parachutes are effective in preventing major trauma related to gravitational challenge.

Design Systematic review of randomised controlled trials.

Data sources: Medline, Web of Science, Embase, and the Cochrane Library databases; appropriate internet sites and citation lists.

Study selection: Studies showing the effects of using a parachute during free fall.

Main outcome measure Death or major trauma, defined as an injury severity score > 15.

Results We were unable to identify any randomised controlled trials of parachute intervention.

Conclusions As with many interventions intended to prevent ill health, the effectiveness of parachutes has not been subjected to rigorous evaluation by using randomised controlled trials. Advocates of evidence based medicine have criticised the adoption of interventions evaluated by using only observational data. We think that everyone might benefit if the most radical protagonists of evidence based medicine organised and participated in a double blind, randomised, placebo controlled, crossover trial of the parachute.

Introduction

The parachute is used in recreational, voluntary sector, and military settings to reduce the risk of orthopaedic, head, and soft tissue injury after gravitational challenge, typically in the context of jumping from an aircraft. The perception that parachutes are a successful intervention is based largely on anecdotal evidence. Observational data have shown that their use is associated with morbidity and mortality, due to both failure of the intervention1 2 and iatrogenic complications.3 In addition, “natural history” studies of free fall indicate that failure to take or deploy a parachute does not inevitably result in an adverse outcome.4 We therefore undertook a systematic review of randomised controlled trials of parachutes.

Methods

Literature search
We conducted the review in accordance with the QUOROM (quality of reporting of meta-analyses) guidelines.5 We searched for randomised controlled trials of parachute use on Medline, Web of Science, Embase, the Cochrane Library, appropriate internet sites, and citation lists. Search words employed were “parachute” and “trial.” We imposed no language restriction and included any studies that entailed jumping from a height greater than 100 metres. The accepted intervention was a fabric device, secured by strings to a harness worn by the participant and released (either automatically or manually) during free fall with the purpose of limiting the rate of descent. We excluded studies that had no control group.

Definition of outcomes
The major outcomes studied were death or major trauma, defined as an injury severity score greater than 15.6

Meta-analysis
Our statistical apprach was to assess outcomes in parachute and control groups by odds ratios and quantified the precision of estimates by 95% confidence intervals. We chose the Mantel-Haenszel test to assess heterogeneity, and sensitivity and subgroup analyses and fixed effects weighted regression techniques to explore causes of heterogeneity. We selected a funnel plot to assess publication bias visually and Egger’s and Begg’s tests to test it quantitatively. Stata software, version 7.0, was the tool for all statistical analyses.

Results

Our search strategy did not find any randomised controlled trials of the parachute.

smig95752_f1.gif
Parachutes reduce the risk of injury after gravitational challenge, but their effectiveness has not been proved with randomised controlled trials

Credit: HULTON/GETTY

 

Discussion

Evidence based pride and observational prejudice
It is a truth universally acknowledged that a medical intervention justified by observational data must be in want of verification through a randomised controlled trial. Observational studies have been tainted by accusations of data dredging, confounding, and bias.7 For example, observational studies showed lower rates of ischaemic heart disease among women using hormone replacement therapy, and these data were interpreted as advocating hormone replacement for healthy women, women with established ischaemic heart disease, and women with risk factors for ischaemic heart disease.8 However, randomised controlled trials showed that hormone replacement therapy actually increased the risk of ischaemic heart disease,9 indicating that the apparent protective effects seen in observational studies were due to bias. Cases such as this one show that medical interventions based solely on observational data should be carefully scrutinised, and the parachute is no exception.

Natural history of gravitational challenge
The effectiveness of an intervention has to be judged relative to non-intervention. Understanding the natural history of free fall is therefore imperative. If failure to use a parachute were associated with 100% mortality then any survival associated with its use might be considered evidence of effectiveness. However, an adverse outcome after free fall is by no means inevitable. Survival has been reported after gravitation challenges of more than 10 000 metres (33 000 feet).4 In addition, the use of parachutes is itself associated with morbidity and mortality.1-3 10 This is in part due to failure of the intervention. However, as with all interventions, parachutes are also associated with iatrogenic complications.3 Therefore, studies are required to calculate the balance of risks and benefits of parachute use.

The parachute and the healthy cohort effect
One of the major weaknesses of observational data is the possibility of bias, including selection bias and reporting bias, which can be obviated largely by using randomised controlled trials. The relevance to parachute use is that individuals jumping from aircraft without the help of a parachute are likely to have a high prevalence of pre-existing psychiatric morbidity. Individuals who use parachutes are likely to have less psychiatric morbidity and may also differ in key demographic factors, such as income and cigarette use. It follows, therefore, that the apparent protective effect of parachutes may be merely an example of the “healthy cohort” effect. Observational studies typically use multivariate analytical approaches, using maximum likelihood based modelling methods to try to adjust estimates of relative risk for these biases. Distasteful as these statistical adjustments are for the cognoscenti of evidence based medicine, no such analyses exist for assessing the presumed effects of the parachute.

The medicalisation of free fall
It is often said that doctors are interfering monsters obsessed with disease and power, who will not be satisfied until they control every aspect of our lives (Journal of Social Science, pick a volume). It might be argued that the pressure exerted on individuals to use parachutes is yet another example of a natural, life enhancing experience being turned into a situation of fear and dependency. The widespread use of the parachute may just be another example of doctors’ obsession with disease prevention and their misplaced belief in unproved technology to provide effective protection against occasional adverse events.

 

What is already known about this topic

Parachutes are widely used to prevent death and major injury after gravitational challenge

Parachute use is associated with adverse effects due to failure of the intervention and iatrogenic injury

Studies of free fall do not show 100% mortality

What this study adds

No randomised controlled trials of parachute use have been undertaken

The basis for parachute use is purely observational, and its apparent efficacy could potentially be explained by a “healthy cohort” effect

Individuals who insist that all interventions need to be validated by a randomised controlled trial need to come down to earth with a bump

 

 

Parachutes and the military industrial complex
However sinister doctors may be, there are powers at large that are even more evil. The parachute industry has earned billions of dollars for vast multinational corporations whose profits depend on belief in the efficacy of their product. One would hardly expect these vast commercial concerns to have the bravery to test their product in the setting of a randomised controlled trial. Moreover, industry sponsored trials are more likely to conclude in favour of their commercial product,11 and it is unclear whether the results of such industry sponsored trials are reliable.

A call to (broken) arms
Only two options exist. The first is that we accept that, under exceptional circumstances, common sense might be applied when considering the potential risks and benefits of interventions. The second is that we continue our quest for the holy grail of exclusively evidence based interventions and preclude parachute use outside the context of a properly conducted trial. The dependency we have created in our population may make recruitment of the unenlightened masses to such a trial difficult. If so, we feel assured that those who advocate evidence based medicine and criticise use of interventions that lack an evidence base will not hesitate to demonstrate their commitment by volunteering for a double blind, randomised, placebo controlled, crossover trial.


Contributors: GCSS had the original idea. JPP tried to talk him out of it. JPP did the first literature search but GCSS lost it. GCSS drafted the manuscript but JPP deleted all the best jokes. GCSS is the guarantor, and JPP says it serves him right.

Funding: None.

Competing interests: None declared.

Ethical approval: Not required.

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    Nov 20

    对照研究中一定要注意研究组间基线资料的可比性
    ———评近来关于中国的基础教育好坏的讨论

         作者:天路客

          拜读了网友金海的文章《再看钱学森论教育的问题》,讨论了中国的“人才”问题和教育问题,也拜读了未火的评论文章《说中国的基础教育好有什么根据?》,我比较倾向于未火网友。

          统计学上强调对照的原则,目的无非是为了增加两组之间的可比性,增加研究结果的真实性。两组之间的比较,不仅仅是研究结果的分析比较,更重要的是要注意在研究开始时,两组之间的基本数据是否有可比性(compatibility),也就是说这涉及到分组的方法问题,以及分组后两组的资料数据是否平衡的问题。

          统计学中有个基本概念“基线”(baseline),所谓基线是指研究措施执行前,被研究对象的基本特征。具体到文章中涉及到的问题,因素有很多,未火网友也说到了一部分,不再重复,实际上,同一个研究方法或者同一种处理措施,在不同的研究中,得到不同的结果,往往是由于被研究对象之间的基线资料差别所致。

          在研究中一定要对得到的数据作进一步的分析,确保基线资料的平衡,这样研究结果才有意义,才有说服力。所以,统计学中一般会对资料做可比性分析、变异因素分析等,这里就不展开了,有兴趣但又不太清楚的朋友们可以自己查资料了解一下。

          保证结果可靠的关键是一定要保证基线资料的可比性,具体操作起来也不是一两句话说得清楚,简单说一下,一般会有如下措施(本文不是讨论统计学,都不展开说明):

        确定受试对象的选择条件
        制定统一的检测标准
        必要时分层分组
        防止向均数回归现象的影响
        减少基线资料的变异

          所以,未火的说法还是比较靠谱的,选择适当的数学模型采用适当的统计方法才能得出比较准确的结论,金海的结论的是建立在基线资料可比性不强的基础上泛泛得出来的。

         2009.11.21

    Oct 08

    “伪随机”数不是“假随机”数
                              ———— 评奥卡姆剃刀评《天路客评<通过比喻理解“大样本随机双盲对照临床试验”>》

            作者:天路客

           首先回答关于第二个问题,文章发出后,我觉得有的地方说法欠妥,考虑到文章已发出,就没有及时通知更正作进一步的说明,现在也还没有想好怎么论述这个问题。

           至于第一个问题,“随机”的概念严格说来不是统计学的基本概念,但却是更基本的东西。在医学统计学中,对“随机”概念并不作介绍,基本概念中也不介绍,为避免不太了解的朋友产生误会,我略作一下说明。

          首先说明一点:计算机不会产生绝对随机的随机数,这是确定的。

          那怕计算机技术(这里指据冯诺依曼思想发展起来的电子计算机)发展到什么程度,也只是能够产生一串“伪随机”( pseudo-random)数,不会产生绝对“真”的随机数。其实,设定一个程序后,随机数的计算方法在不同的计算机中是不同的,即使在相同的计算机中安装的不同的操作系统中也是不同的、相同的随机种子在不同计算机或系统中生成的随机数也不同。

           计算机的伪随机数是由随机种子根据一定的计算方法算出来的,而随机种子如果没有特殊规定在默认情况下一般来自系统时钟,所以,只要计算方法一定,随机种子一定,那么产生的随机数就是固定的。

           绝对随机的随机数字只是一种理想状态,举个不太恰当的比喻,随机数字序列就像物理学中讲的“质点”,将物体简化后得到的只有质量而不计大小、形状的一个几何点,这是经典力学中常用的理想化模型,是实际物体在一定条件下的科学抽象。

           所以说,所谓“伪随机数”只是一种相对的随机数,就是说,是有规律的随机数,也就是奥卡姆剃刀朋友所说的“过足够长的一段后,可能出现与前面完全相同的序列。”

          但是,这里所说的“伪”不是“假”,应该理解为“有规律”比较确切,也就是说计算机(例如通过统计软件包SAS等)产生的随机数既是随机的又是有规律的,但说到底还算随机数即使是“伪随机数”,也许将来的计算机会产生基于自然规律的不可重复的“真”随机数,但这里的真还是得加上一个引号。

         在生物、医学或工程的常规科研中,伪随机数已经足够用了,只要不搞成假随机。

         至于在实验中到处可见的闭着眼睛抓白鼠之类的“随机”我实在看不出有多少益处。

       “伪随机”数不是“假随机”数

    Oct 04

           天路客

          bangbu1996在该文中说:“比就比,李婆拿出十个苹果,可她眼花;王婆也拿出十个苹果,可她眼毒着呢。这不用比,王婆的苹果好。慢着,消费者可不是被骗大的,两个婆婆都蒙住眼,随便从箱里摸出十个来比比。最后是李婆的苹果好。嘿嘿,这个就是“随机”的意义。”

          从统计学来说,这个比方并不恰当,这样并不构成随机,而是直接编号,使用随机数字表、计算器或者使用用SAS编制程序来构成随机。

          另,在“买苹果,王婆打开了一箱。“又甜又脆的苹果喽!不甜不脆不要钱”,说的好听!当然了,卖苹果的说的都好听。如果你是个智力正常的消费者,不需要考虑就会挑几个看看。不错,确实没有烂的。但是只看“几个”是不够的,最好把整箱都翻看一遍才放心。只是你没有时间,或者王婆也不让干。那么,你检验的越多,“这一箱苹果都是好的”的可能性就越大。这就是“大样本”的意义。”一段中,“那么,你检验的越多,“这一箱苹果都是好的”的可能性就越大。”

          这样说也不确切,也有可能“这一箱苹果都是好的”的可能性越小,如果烂苹果多些的话。

          与bangbu1996朋友商榷。