Why to know statistics
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Why to know statistics

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every research should know basic statistics : this is very important to perform sound methodology

every research should know basic statistics : this is very important to perform sound methodology

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    Why to know statistics Why to know statistics Presentation Transcript

    • Why to know statisticsWhy to know statistics
    •  To understand dataTo understand data
    • ExampleExample  One of your colleague is an oncologyOne of your colleague is an oncology surgeonsurgeon  60% of his cases died60% of his cases died  Does this mean that he is a looser!!Does this mean that he is a looser!!
    •  We should ask what are the resultsWe should ask what are the results of his colleagues inof his colleagues in similarsimilar patientspatients  How many patients he operatedHow many patients he operated upon e.g 2/3!!!!upon e.g 2/3!!!!
    •  To summarize dataTo summarize data
    • ExampleExample  Diastolic Blood pressureDiastolic Blood pressure  80,70,65,90,74,80,60,90,60,75,80,980,70,65,90,74,80,60,90,60,75,80,9 0,100,100,100,950,100,100,100,95  AgeAge  24,30,26,40,28,21,26,31,32,36,27,424,30,26,40,28,21,26,31,32,36,27,4 5,62,58,52,50,605,62,58,52,50,60
    • Vital for researchVital for research  Without the use of statistics it wouldWithout the use of statistics it would be very difficult to make decisionsbe very difficult to make decisions based on the data collected from abased on the data collected from a research projectresearch project
    • Statistical steps in researchStatistical steps in research  Collect dataCollect data  Organise dataOrganise data  Analyse dataAnalyse data  Interpret the dataInterpret the data  Present the dataPresent the data
    • How to read the resultsHow to read the results  An understanding of basic statisticsAn understanding of basic statistics will provide you with thewill provide you with the fundamental skills necessary to readfundamental skills necessary to read and evaluate results section inand evaluate results section in published paperspublished papers
    • Are groups comparableAre groups comparable!!!!!!  the baseline characteristics of thethe baseline characteristics of the groups being studied should begroups being studied should be comparablecomparable  If not, they should be adjusted forIf not, they should be adjusted for differencesdifferences
    • statistical testsstatistical tests  Are they frequently used tests!!Are they frequently used tests!!  If not, why!!If not, why!!
    •  Are the data analysed accordingAre the data analysed according to the original protocol?to the original protocol?
    •  Was follow- up complete?Was follow- up complete? Patients lost to follow-upPatients lost to follow-up………………loss ofloss of subjects biassubjects bias > 10% - 15 %> 10% - 15 % ……………………………………..invalid..invalid resultsresults
    • P valueP value  A P value of <0.05 means that thisA P value of <0.05 means that this result would have arisen by chanceresult would have arisen by chance on less than one occasion in 20on less than one occasion in 20
    • Standardization of measures of outcomeStandardization of measures of outcome:: Odds and odds ratioOdds and odds ratio The odds is the number of patients who fulfilThe odds is the number of patients who fulfil the criteria for a given endpoint divided bythe criteria for a given endpoint divided by the number of patients who do not.the number of patients who do not.
    • For exampleFor example the odds of diarrhoea during treatment with anthe odds of diarrhoea during treatment with an antibiotic in a group of 10 patients may be 4antibiotic in a group of 10 patients may be 4 to 6 (4 with diarrhoea divided by 6 without,to 6 (4 with diarrhoea divided by 6 without, 0.66);0.66); in a control group the odds may be 1 to 9in a control group the odds may be 1 to 9 (0.11). The odds ratio of treatment to control(0.11). The odds ratio of treatment to control group would be 6 (0.66÷0.11).group would be 6 (0.66÷0.11).
    • Risk and relative riskRisk and relative risk The risk is the number of patients whoThe risk is the number of patients who fulfil the criteria for a given end pointfulfil the criteria for a given end point divided by the total number of patients.divided by the total number of patients.
    • For exampleFor example,, the risk of diarrhoea during treatmentthe risk of diarrhoea during treatment with an antibiotic in a group of 10with an antibiotic in a group of 10 patients may be 4 to 10; in the controlpatients may be 4 to 10; in the control group the risks may be 1 to 10. Thegroup the risks may be 1 to 10. The relative risk of treatment to controlrelative risk of treatment to control group would be 4 (0.4÷0.1).group would be 4 (0.4÷0.1).
    • C.IC.I  The confidence interval around aThe confidence interval around a result in a clinical trial indicates theresult in a clinical trial indicates the limits within which the "real"limits within which the "real" difference between the treatments isdifference between the treatments is likely to lie,likely to lie,  hence the strength of the inferencehence the strength of the inference that can be drawn from the resultthat can be drawn from the result
    • Example:Example: 95 %95 % CI for RRR 25 % :CI for RRR 25 % :  sample size 100sample size 100………………………….= -- 38 % to 59.= -- 38 % to 59 %%  Sample size 1000Sample size 1000…………………….= 9 % to 41 %.= 9 % to 41 % The larger the sample size , the narrower andThe larger the sample size , the narrower and more precise the CI , and the greater ourmore precise the CI , and the greater our confidence that the true RRR is closer toconfidence that the true RRR is closer to what we have observedwhat we have observed..
    • OR = 0.34, 95% CI 0.23 - 0.52 • Odds Ratio < 1  decreased risk • Confidence Interval does not cross 1  statistically significant
    •  A statistically significant result mayA statistically significant result may not be clinically significant.not be clinically significant.
    •  The results of intervention trialsThe results of intervention trials should be expressed in terms of theshould be expressed in terms of the likely benefit an individual couldlikely benefit an individual could expect (for example, the absoluteexpect (for example, the absolute risk reduction)risk reduction)
    • How large was the treatmentHow large was the treatment effecteffect?? Treatment effectTreatment effect………………………….. Adverse.. Adverse outcomeoutcome e.g.;e.g.; Risk of outcome without therapyRisk of outcome without therapy ( baseline risk )( baseline risk ) XX ( = 20 % or 0.20 )( = 20 % or 0.20 ) Risk of outcome with therapyRisk of outcome with therapy YY ( =( = 15% 0r 0.15)15% 0r 0.15)
    •  Absolute risk reduction= X -- YAbsolute risk reduction= X -- Y 0.20-0.15= 0.050.20-0.15= 0.05  Relative risk ( RR )= Y / X = 0.15 /0.20 =Relative risk ( RR )= Y / X = 0.15 /0.20 = 0.750.75  Relative risk reduction ( RRR ) =Relative risk reduction ( RRR ) = { X -- Y/ X } x 100 % = 0.05 / 0.2 x{ X -- Y/ X } x 100 % = 0.05 / 0.2 x 100%= 25 % i.e :100%= 25 % i.e : therapy reduced the risk of the outcome by 25therapy reduced the risk of the outcome by 25 % relative to that occurring among the% relative to that occurring among the controlscontrols
    • the greater the RRR, the morethe greater the RRR, the more effective the therapy.effective the therapy.
    • Example: M.IExample: M.I  patients receiving medical treatmentpatients receiving medical treatment have a chance of 404/1324=0.305 orhave a chance of 404/1324=0.305 or 30.5% of being dead at 10 years.30.5% of being dead at 10 years.  Let us call this riskLet us call this risk xx. Patients. Patients randomised to coronary arteryrandomised to coronary artery bypass grafting have a chance ofbypass grafting have a chance of 350/1325=0.264 or 26.4% of being350/1325=0.264 or 26.4% of being dead at 10 years. Let us call this riskdead at 10 years. Let us call this risk yy..
    • RRRR  The relative risk of deathThe relative risk of death——  that is, the risk in surgically treatedthat is, the risk in surgically treated patients compared with medicallypatients compared with medically treated controlstreated controls——isis  y/xy/x or 0.264/0.305=0.87 (87%).or 0.264/0.305=0.87 (87%).
    • RRRRRR  The relative risk reductionThe relative risk reduction——that is,that is, the amount by which the risk ofthe amount by which the risk of death is reduced by the surgerydeath is reduced by the surgery——isis 100%-87% (1-100%-87% (1-yy//xx)=13%.)=13%.
    • ARRARR  The absolute risk reduction (or riskThe absolute risk reduction (or risk difference)difference)——that is, the absolutethat is, the absolute amount by which surgical treatmentamount by which surgical treatment reduces the risk of death at 10 yearsreduces the risk of death at 10 years ——is 30.5%-26.4%=4.1% (0.041).is 30.5%-26.4%=4.1% (0.041).
    • NNTNNT  The number needed to treatThe number needed to treat——howhow many patients need coronary arterymany patients need coronary artery bypass grafting in order to prevent,bypass grafting in order to prevent, on average, one death after 10 yearson average, one death after 10 years ——is the reciprocal of the absoluteis the reciprocal of the absolute risk reduction: 1/ARR=1/0.041=24.risk reduction: 1/ARR=1/0.041=24.
    • ConclusionConclusion  to be able to effectively conductto be able to effectively conduct researchresearch  to be able to read and evaluateto be able to read and evaluate journal articlesjournal articles  to further develop critical thinkingto further develop critical thinking and analytic skillsand analytic skills  to know when you need to hireto know when you need to hire outside statistical helpoutside statistical help