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Basic statistics

Basic statistics. www.bradfordvts.co.uk. EBM SKILLS - STATISTICS. CHANCE - p = 1 in 20 (0.05). > 1 in 20 (0.051) = not significant < 1 in 20 (0.049) = statistically significant CONFIDENCE INTERVALS

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Basic statistics

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  1. Basic statistics www.bradfordvts.co.uk

  2. EBM SKILLS - STATISTICS • CHANCE - p = 1 in 20 (0.05). • > 1 in 20 (0.051) = not significant • < 1 in 20 (0.049) = statistically significant • CONFIDENCE INTERVALS • what is the range of values between which we could be 95% certain that this result would lie if this intervention was applied to the general population

  3. EBM SKILLS - A BASIC INTRODUCTION CHANCE, BIAS, CONFOUNDING VARIABLES

  4. TYPES OF STUDY - HYPOTHESIS FORMING • CASE REPORTS / CASE SERIES • CROSS SECTIONAL / PREVALENCE STUDIES measure personal factors & disease states - hypothesis FORMING - cannot indicate cause & effect • CORRELATIONAL / ECOLOGICAL / GEOGRAPHIC STUDIES. prevalence &/or incidence measurement in one population c/w another pop.

  5. TYPES OF STUDY - HYPOTHESIS TESTING CASE CONTROL STUDIES

  6. CASE CONTROL EXAMPLE -SMOKING & LUNG CANCER DISEASE Cases Controls EXPOSURE Yes a b EXPOSURE No c d Odds Ratio = ad/bc(1 = no association, > 1 = possible association, < 1 = protective effect) DISEASE Cases Controls (lung cancer) EXPOSURE Yes 56 230 (smoking) No 7 246 The odds ratio would therefore be 56 x 246 = 13776 = 8.6. 7 x 230 1610

  7. TYPES OF STUDY - HYPOTHESIS TESTING • COHORT STUDIES

  8. COHORT STUDIES OUTCOME Yes No Exposed a b Not exposed c d Attributable risk (absolute risk or risk difference) "What is the incidence of disease attributable to exposure" Answer = a - c. Relative risk "How many times are exposed persons more likely to develop the disease, relative to non-exposed persons?" i.e. the incidence in the exposed divided by the incidence in the non-exposed. This is expressed as a divided by c . a+b c+d

  9. COHORT STUDY EXAMPLE Deep vein thromboses (DVT) in oral contraceptive users. (Hypothetical results). OUTCOME (DVT) Yes No Exposed ( on oral contraceptive ) 41 9996 Not exposed (not on o.c.)7 10009 These results would give an attributable risk of 34 and a relative risk of 6 - significantly large enough numbers to indicate the possibility of a real association between exposure and outcome. However, the possibility of biases very often arises.

  10. RANDOMISED CONTROLLED TRIALS

  11. RANDOMISED CONTROLLED TRIALS OUTCOME Yes No Comparison intervention a b Experimental intervention c d Relative risk reduction: “ How many fewer patients will get the outcome measured if they get active treatment versus comparison intervention” a /a+b - c/c+d a/a+b Absolute risk reduction: “What is the size of this effect in the population” a/a+b - c/c+d

  12. RCT EXAMPLE - 4S STUDY • STABLE ANGINA OR MYOCARDIAL INFARCTION MORE THAN 6 MONTHS PREVIOUSLY • SERUM CHOLESTEROL > 6.2mmol/l • EXCLUDED PATIENTS WITH ARYHTHMIAS AND HEART FAILURE • ALL PATIENTS GIVEN 8 WEEKS OF DIETARY THERAPY • IF CHOLESTEROL STILL RAISED (>5.5) RANDOMISED TO RECEIVE SIMVASTATIN (20mg > 40mg) OR PLACEBO • OUTCOME DEATH OR MYOCARDIAL INFARCTION (LENGTH OF TREATMENT 5.4 YEARS ) WERE THE OUTCOMES

  13. RCT EXAMPLE - 4S STUDY OUTCOME (death) Yes No Comparison intervention (placebo) 256 1967 2223 Experimental intervention (simvastatin) 182 2039 2221 The ARR is (256/2223) - (182/2221) = 0.115 - 0.082 = 0.033. The RRR is 0.033/0.115 = 0.29 or expressed as a percentage 29%. 1/ARR = NUMBER NEEDED TO TREAT. 1/0.033 = 30. i.e. if we treat 30 patients with IHD with simvastatin as per 4S study, in 5.4 years we will have prevented 1 death.

  14. NNT EXAMPLES Intervention Outcome NNT

  15. Why are RCTs the “gold standard”Breast cancer mortality in studies of screening with mammography; women aged 50 and over (55 in Malmo study, 45 in UK)

  16. SCREENING - WILSON & JUNGEN (WHO, 1968) • IS THE DISORDER COMMON / IMPORTANT • ARE THERE TREATMENTS FOR THE DISORDER • IS THERE A KNOWN NATURAL HISTORY & “WINDOW OF OPPORTUNITY” WHERE SCREENING CAN DETECT DISEASE EARLY WITH IMPROVED CHANCE OF CURE • IS THE TEST ACCEPTABLE TO PATIENTS • SENSITIVE AND SPECIFIC • GENERALISABLE • CHEAP / COST EFFECTIVE • APPLY TO GROUP AT HIGH RISK

  17. SCREENING DISEASE PRESENT ABSENT TEST POSITIVE A B NEGATIVE C D Sensitivity = a/a+c; Specificity = d/b+d; positive predicitive value = a/a+b; negative predicitve value = d/c+d.

  18. Value of exercise ECG in coronary artery stenosis DISEASE PRESENT ABSENT TEST POSITIVE 137 11 NEGATIVE 90 112 Sensitivity = a/a+c = 60%; Specificity = d/b+d = 91%; positive predicitive value = a/a+b = 93%; negative predicitve value = d/c+d = 55%.

  19. Sensitivities and Specificities for different testsAlcohol dependency or abuse(as defined by extensive investigations in medical and orthopaedic in patients) SENS SPEC GGT 54% 76% MCV 63% 64% LFTs 37% 81% “Yes” to 1 or > of CAGE ?s 85% 81% “Yes” to 3 or > of CAGE ?s 51% 100%

  20. MAKING SENSE OF THE EVIDENCE - ARE THESE RESULTS VALID -i.e. should I believe them? • Randomised (where appropriate)? • Drop outs and withdrawals? • Followup complete? • Analysed in the groups to which randomised?- “Intention to treat”.

  21. MAKING SENSE OF THE EVIDENCE- ARE THESE RESULTS USEFUL?-i.e. should I be impressed by them, are they relevant to my patients (GENERALISABLE) • How large was the treatment effect? • How precise was the estimate of treatment effect • Were all important clinical outcomes considered? • Do benefits outweigh risks?

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