Basic Statistics for Clinicians_3. Assessing Effects of Treatment. Measures of Association.pdf

# Basic Statistics for Clinicians_3. Assessing Effects of Treatment. Measures of Association.pdf

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[------- statistics * statistique I BASIC STATISTICS FOR CLINICLkNS: 3. ASSESSING THE EFFECTS OF TREATMENT: MEASURES OF ASSOCIATION Roman Jaeschke,*t MD; Gordon Guyatt,*t MD; Harry Shannon,* PhD; Stephen Walter,* PhD; Deborah Cook,*t MD; Nancy Heddle,t MSc sS \$G S§, In the third of a series of four articles the authors show the calcuLlation of measures of association and discuss their use- fUlness in clinical decision making. From the rates of death or other "events' in experimental and control groups in a clinical trial, we can calculate the relative risk (RR) of the event after the experimental treatment, expressed as a percentage of the risk without such treatment. The absolute risk reduction (ARR) is the difference in the risk of an event between the groups. The relative risk reduction is the percentage of the baseline risk (the risk of an event in the control patients) re- moved as a result of therapy. The odds ratio (OR), which is the measure of choice in case-control studies, gives the ratio of the odds of an event in the experimental group to those in the control group. The OR and the RR provide limited infor- mation in reportinig the results of prospective trials because they do not reflect changes in the baseline risk. The ARR and the number needed to treat, which tells the clinician how many patients need to be treated to prevent one event, re- flect both the baseline risk and the relative risk reduction. If the timing of events is important - to determine whether treatment extends life, for example - survival CLrves are Lised to show when events occuLr over time. DLans le troisi&me article d'une s6rie de quatre, les auteurs il- lustrent le calCulI de mesures d'association et discutent de leuLr The reader familiar with the first two articles in this se- ties will, when presented with the results of a clinical trial, know how to discover the range within which the treatment effect likely lies. This treatment effect is worth considering if it comes from a study that is valid.' In this ar- ticle, we explore the ways investigators and representatives utilite dans la prise de decisions cliniques. A partir des taux de dc&s OU dfautres ,&6nements,, dans des groupes de sujets experimentaux et des grouLpes de sujcts temoins au coLurs dLun essai clinique, nous pouvons calculer le risque relatif (RR) de I&v6nement apres oll traitement experimental, exprim& en pourcentage du risquLe sanis le recours aLl traitement en qLes- tion. l a reduction du risque absolu (RRA) est la diff6rence entre les groopes quant auL risque qu/un &6nement se pro- duise. La r6dUction du risque relatif est le pourcentage du risque de base (le risque quoUn 6v6nement se prodotise clhez les patilents do grouLpC trmoin 6liminM a l suite do traite- ment. Le ratio des probabilit&s (RP), qLui represente la mesuLre privik1gice dans les etudes cas-t6moins, repn5sente Ic ratio entre les probabilites quUn evenement se produise dans Ie groupe de sujets exp6rimentaux et celles qU'il se produise dans le groupe de sujets t&moins.

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