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CONFIDENCE INTERVALS. DR.S.SHAFFI AHAMED ASST. PROFESSOR DEPT. OF F & CM. BACKGROUND AND NEED OF CI. ---Statistical analysis of medical studies is based on the key idea that, we make observations on a sample of subjects from which the sample is drawn.

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  2. BACKGROUND AND NEED OF CI ---Statistical analysis of medical studies is based on the key idea that, we make observations on a sample of subjects from which the sample is drawn. ---If the sample is not representative of the population we may well be misled and statistical procedures cannot help. .

  3. ---Even a well designed study can give only an idea of the answer sought , because of random variation in the sample. ---And the results from a single sample are subject to statistical uncertainty, which is strongly related to the size of the sample.

  4. -- The quantities( single mean, proportion, difference in means, proportions, OR, RR, Correlation,-----------) will be imprecise estimate of the values in the overall population, but fortunately the imprecision can itself be estimated and incorporated into the presentation of findings.

  5. --- Presenting study findings directly on the scale of original measurement together with information on the inherent imprecision due to sampling variability, has distinct advantages over just giving ‘p-values’ usually dichotomized into “significant” or “non-significant”. “THIS IS THE RATIONALE FOR USING CI”

  6. Confidence Intervals for Reporting Results • “[Hypothesis tests] are sometimes overused and their results misinterpreted.” • “Confidence intervals are of more than philosophical interest, because their broader use would help eliminate misinterpretations of published results.” • “Frequently, a significance level or pvalue is reduced to a ‘significance test’ by saying that if the level is greater than 0.05, then the difference is ‘not significant’ and the null hypothesis is ‘not rejected’….The distinction between statistical significance and clinical significance should not be confused.”

  7. But why do we always see 95% CI’s? • “Duality” between confidence intervals and pvalues • Example: Assume that we are testing that for a significant change in QOL due to an intervention, where QOL is measured on a scale from 0 to 50. • 95% confidence interval: (-2, 13) • pvalue = 0.07 • It is true that if the 95% confidence interval overlaps 0, then a t-test testing that the treatment effect is 0 will be insignificant at the alpha = 0.05 level. • It is true that if the 95% confidence interval does not overlap 0, then a t-test testing that the treatment effect is 0 will be significant at the alpha = 0.05 level.

  8. -THE BRITISH MEDICAL JOURNAL -THE LANCET -THE MEDICAL JOURNAL OF AUSTRALIA -THE AMERICAN JOURNAL OF PUBLIC HEALTH -THE BRITISH HEART JOURNAL ---------------------------------------------- ---------------------------------------------- ---------------------------------------------- ----------------------------------------------

  9. Different Interpretations of the 95% confidence interval • “We are 95% sure that the TRUE parameter value is in the 95% confidence interval” • “If we repeated the experiment many many times, 95% of the time the TRUE parameter value would be in the interval” • “Before performing the experiment, the probability that the interval would contain the true parameter value was 0.95.”

  10. -- In a representative sample of 100 observations of heights of men, drawn at random from a large population, suppose the sample mean is found to be 175 cm (sd=10cm) . -- Can we make any statements about the population mean ? -- We cannot say that population mean is 175 cm because we are uncertain as to how much sampling fluctuation has occurred. -- What we do instead is to determine a range of possible values for the population mean, with 95% degree of confidence. -- This range is called the 95% confidence interval and can be an important adjuvant to a significance test.

  11. In general, the 95% confidence interval is given by: Statistic ± confidence factor x S.Error of statistic In the previous example, n =100 ,sample mean = 175, S.D., =10, and the S.Error =10/√100 = 1. Therefore, the 95% confidence interval is, 175± 1.96 * 1 = 173 to 177” That is, if numerous random sample of size 100 are drawn and the 95% confidence interval is computed for each sample, the population mean will be within the computed intervals in 95% of the instances.

  12. Sampling Distributions sem = 0.47 sem = 0.23 sem = 0.10 sem = 0.17

  13. General formula for 95% confidence interval for single mean • Notes: • sample size must be sufficiently large for non-normal variables. • how large is large? depends on skewness of variable • VERY often people use 2 instead of 1.96.

  14. Example: A study was carried out to determine the effect, if any, of pesticide exposure on blood pressure. A random sample of 100 men was selected from a group of agriculture workers known to have been exposed to pesticides. 100 randomly selected workers with no such exposure comprised the control group. Their mean SBP and Sd., were given as 145 mm Hg, 20 mm hg (exposed group) and 120 mm hg, 15 mm Hg (non exposed group). Calculate the 95% and 90% confidence intervals for the true difference in mean SBP between the exposed and non exposed populations .

  15. 95 % C.I. for Difference in Means

  16. The 90% confidence interval for the difference in means : 25 ± 1.645 (2.5) = 20.9 to 29.1 The 95 % confidence interval for the difference in means: 25 ± 1.96 (2.5) = 20.1 to 29.9 Thus we can be 90% (or 95%) certain that the true mean difference in systolic blood pressure between the exposed and non-exposed populations lies between 21 and 29 mm Hg (20 and 20 mm Hg). Notice that the both the intervals does not include zero so the data are not compatible with no difference in mean systolic blood pressure.

  17. 95% Confidence Intervals for Proportions • Socinski et al., Phase III Trial Comparing a Defined Duration of Therapy versus Continuous Therapy Followed by Second-Line Therapy in Advanced-Stage IIIB/IV Non-Small-Cell Lung Cancer JCO, March 1, 2002. • Patients and Methods: Arm A (4 cycles of carboplatin at an AUC of 6 and paclitaxel), Arm B (continuous treatment with carboplatin/ paclitaxel until progression). At progression, patients from each arm receive second-line weekly paclitaxel at 80mg/m2/week. • Results: 230 Patients were randomized (114 in arm A and 116 in Arm B). Overall response rates were 22% and 24% for arms A and B. Grade 2 to 4 neuropathy was seen in 14% and 27% of Arm A and B patients, respectively.

  18. 95% Confidence Intervals for Proportions • What are 95% confidence intervals for the response rates in the two arms? • standard error of a sample proportion is • An equation for confidence interval for a proportion: • Assumptions: • n is reasonably large • p is not “too” close to 0 or 1 • rule of thumb: pn > 5

  19. Example: Response Rate to Treatment • Arm A: • Arm B:

  20. Example: Grade 2 to 4 Neuropathy • Arm A: • Arm B:

  21. 95% Confidence Interval for Difference in Proportions What is the 95% confidence interval for the difference in rates of neuropathy in arms A and B?


  23. Example:The following finding of non-significance in a clinicaltrial on 178 patients.

  24. Chi-square value = 1.74 ( p > 0.1) (non –significant) i.e. there is no difference in efficacy between the two treatments. --- The observed difference is: 75% - 66% = 9% and the 95% confidence interval for the difference is: - 4% to 22% - -- This indicates that compared to treatment B, treatment A has, at best an appreciable advantage (22%) and at worst , a slight disadvantage (- 4%). --- This inference is more informative than just saying that the difference is non significant.

  25. Example:

  26. The chi-square value = 6.38 p = 0.01 (highly significant) The observed difference in efficacy is 82 % - 60% = 22% 95% C .I. = 6 % to 38% This indicates that changing from treatment B to treatment A can result in 6% to 38% more patients being cured. Again, this is more informative than just saying that the two treatments are significantly different.

  27. Example: Disease Control ProgramConsider the following findings pertaining to case-holding in the National TB control program

  28. The chi square = 4.32, p-value = 0.04 (significant) The impact of special motivation program = 52 % - 46% = 6% in terms of improved case-holding. The 95% C.I. = 0.4 % to 11.6%, which indicates that the benefit from the special motivation program is not likely to be more than 11.6%. This information may helps the investigator to conclude that the special program was not really worthwhile, and that other strategies need to be explored, to provide a greater magnitude of benefit.

  29. CHARACTERISTICS OF CI’S --The (im) precision of the estimate is indicated by the width of the confidence interval. --The wider the interval the less precision THE WIDTH OF C.I. DEPENDS ON: ---- SAMPLE SIZE ---- VAIRABILITY ---- DEGREE OF CONFIDENCE

  30. EFFECT OF VARIABILITY ·Properties of error 1.Error increases with smaller sample size For any confidence level, large samples reduce the margin of error 2.Error increases with larger standard Deviation As variation among the individuals in the population increases, so does the error of our estimate 3.Error increases with larger z values Tradeoff between confidence level and margin of error

  31. Not only 95%…. • 90% confidence interval: NARROWER than 95% • 99% confidence interval: WIDER than 95%

  32. Interval width (error) increases with Increased confidence level Higher confidence levels have Higher z values Figure 8-10 and 8-11 Error is high in small samples

  33. Other Confidence Intervals • Differences in means • Response rates • Differences in response rates • Hazard ratios • median survival • difference in median survival • OR, RR, Correlation, Regression • Non – parametric tests

  34. Recap • 95% confidence intervals are used to quantify certainty about parameters of interest. • Confidence intervals can be constructed for any parameter of interest (we have just looked at some common ones). • The general formulas shown here rely on the central limit theorem • You can choose level of confidence (does not have to be 95%). • Confidence intervals are often preferable to pvalues because they give a “reasonable range” of values for a parameter.

  35. Excellent References on Use of Confidence Intervals in Clinical Trials • Richard Simon, “Confidence Intervals for Reporting Results of Clinical Trials”, Annals of Internal Medicine, v.105, 1986, 429-435. • Leonard Braitman, “Confidence Intervals Extract Clinically Useful Information from the Data”, Annals of Internal Medicine, v. 108, 1988, 296-298. • Leonard Braitman, “Confidence Intervals Assess Both Clinical and Statistical Significance”, Annals of Internal Medicine, v. 114, 1991, 515-517.


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