Basics-of-Study-Design[研究设计的基础知识]课件.ppt
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- 研究设计的基础知识 Basics of Study Design 研究 设计 基础知识 课件
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1、Basics of Study DesignJanice Weinberg ScDAssistant Professor of BiostatisticsBoston University School of Public HealthBasics of Study Design Bias and variability Randomization:why and how?Blinding:why and how?General study designsBias and Variability The clinical trial is considered to be the“gold s
2、tandard”in clinical research Clinical trials provide the ability to reduce bias and variability that can obscure the true effects of treatment Bias affects accuracy Variability affects precision Bias:any influence which acts to make the observed results non-representative of the true effect of thera
3、py Examples:healthier patients given treatment A,sicker patients given treatment B treatment A is“new and exciting”so both the physician and the patient expect better results on A Many potential sources of bias Variability:high variability makes it more difficult to discern treatment differences Som
4、e sources of variability Measurement instrument observer Biologic within individuals between individuals Can not always control for all sources(and may not want to)Fundamental principle in comparing treatment groups:Groups must be alike in all important aspects and only differ in the treatment each
5、group receives In practical terms,“comparable treatment groups”means“alike on the average”Why is this important?If there is a group imbalance for an important factor then an observed treatment difference may be due to the imbalance rather than the effect of treatment Example:Drug X versus placebo fo
6、r osteoporosis Age is a risk factor for osteoporosis Older subjects are enrolled in Drug X group Treatment group comparison will be biased due to imbalance on ageHow can we ensure comparability of treatment groups?We can not ensure comparability but randomization helps to balance all factors between
7、 treatment groups If randomization“works”then groups will be similar in all aspects except for the treatment receivedRandomization Allocation of treatments to participants is carried out using a chance mechanism so that neither the patient nor the physician know in advance which therapy will be assi
8、gned Simplest Case:each patient has the same chance of receiving any of the treatments under studySimple Randomization Think of tossing a coin each time a subject is eligible to be randomized HEADS:Treatment A TAILS:Treatment B Approximately will be assigned to treatments A and B Randomization usual
9、ly done using a randomization schedule or a computerized random number generatorProblem with Simple Randomization:May result in substantial imbalance in either an important baseline factor and/or the number of subjects assigned to each group Solution:Use blocking and/or stratified randomizationBlock
10、ing Example:If we have two treatment groups (A and B)equal allocation,and a block size of 4,random assignments would be chosen from the blocks 1)AABB 4)BABA 2)ABAB 5)BAAB 3)ABBA 6)BABA Blocking ensures balance after every 4th assignmentStratification Example To ensure balance on an important baselin
11、e factor,create strata and set up separate randomization schedules within each stratum Example:if we want prevent an imbalance on age in an osteoporosis study,first create the strata“75 years”and“75 years”then randomize within each stratum separately Blocking should be also be used within each strat
12、um Alternatives to Randomization Randomization is not always possible due to ethical or practical considerations Some alternatives:Historical controls Non-randomized concurrent controls Different treatment per physician Systematic alternation of treatments Sources of bias for these alternatives need
13、 to be consideredBlinding Masking the identity of the assigned interventions Main goal:avoid potential bias caused by conscious or subconscious factors Single blind:patient is blinded Double blind:patient and assessing investigator are blinded Triple blind:committee monitoring response variables(e.g
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