Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Handbook for Health Care Research, Second Edition Chapter 11 CHAPTER 11 Statistical Methods for Nominal Measures © 2010 Jones and Bartlett Publishers, LLC Handbook for Health Care Research, Second Edition Chapter 11 Describing the Data • Contingency table - used to display counts or frequencies of two or more nominal variables. • Proportion- the number of objects of a particular type divided by the total number of objects in the group • Percentage- proportion multiplied by 100% • Ratio- number of objects in a group with a particular characteristic of interest divided by the number of objects in the same group without the characteristic • Odds- A ratio of the probabilities of the two possible states of a binary event. • Rate-number of objects occurring per unit of time © 2010 Jones and Bartlett Publishers, LLC 2 Handbook for Health Care Research, Second Edition Chapter 11 Characteristics of a Diagnostic Test • One of the most common sources of nominal data is diagnostic testing • True- and False-Positive Rates (sensitivity)probability that a test will be positive when the condition of interest is present • True- and False-Negative Rates (specificity)probability that the test will be negative when the condition of interest is absent • Sensitivity is the ability of a test to correctly identify patients with the condition of interest © 2010 Jones and Bartlett Publishers, LLC 3 Handbook for Health Care Research, Second Edition Chapter 11 Characteristics of a Diagnostic Test • Specificity is the ability of a test to correctly identify patients who do not have the condition of interest • Positive predictive value of a test is the probability that the condition of interest is present when the test is positive • Negative predictive value of a test (or predictive value of a negative test) is the probability that the condition of interest is absent when the test is negative © 2010 Jones and Bartlett Publishers, LLC Handbook for Health Care Research, Second Edition Chapter 11 Characteristics of a Diagnostic Test • Diagnostic Accuracy- proportion of correct results out of all results • Likelihood ratio-combines sensitivity and specificity into a single number expressing the odds that the test result occurs in patients with the condition versus those without the condition: likelihood ratio = sensitivity/false positive rate • Receiver Operating Characteristic (ROC) Curvehelps compare two diagnostic tests to see which would be most useful © 2010 Jones and Bartlett Publishers, LLC 5 Handbook for Health Care Research, Second Edition Chapter 11 ROC Receiver operating characteristic curves for two weaning tests, the RSBI and the CROP index. 0.00.20.40.60.81.0 False Positive Rate The CROP is a better test because there is more area under its curve compared with the RSBI curve. © 2010 Jones and Bartlett Publishers, LLC 6 Handbook for Health Care Research, Second Edition Chapter 11 Characteristics of a Diagnostic Test • Intrarater reliability index-person measures the same variable twice and the measurements are compared • Inter-rater reliability index- two or more people measure the same variable and their measurements are compared • Kappa- allows evaluation of the inter-rater reliability: =observed agreement-�chance agreement K 1-chance agreement • Phi -is an index of agreement independent of chance: Φ= ad-bc © 2010 Jones and Bartlett Publishers, LLC 7 Handbook for Health Care Research, Second Edition Chapter 11 Comparing a Single Sample with a Population • Binomial Test- the plotted probability of each outcome from an experiment Binomial distribution showing the probability of getting various numbers of heads in 10 coin tosses. X, Number of Heads This assumes the probability of getting heads on a single toss is 0.50. • Z test: z= p-po © 2010 Jones and Bartlett Publishers, LLC 8 Handbook for Health Care Research, Second Edition Chapter 11 Comparing Two Samples, Unmatched Data • Unpaired data-data from two independent groups • Fisher Exact Test-used for 2 × 2 contingency tables (which have exactly two rows and two columns) – Null Hypothesis: There is no significant difference between the proportion of patients that lived (or died) in ICU A compared with ICU B. © 2010 Jones and Bartlett Publishers, LLC 9 Handbook for Health Care Research, Second Edition Chapter 11 Comparing Two or More Samples, Matched Data • Paired data-patients act as their own controls • McNemar test- is an analysis of contingency tables that have repeated observations of the same individuals. Conditions for use: • determining whether or not individuals responded to treatment • comparing results of two different treatments on the same people © 2010 Jones and Bartlett Publishers, LLC 10 Handbook for Health Care Research, Second Edition Chapter 11 Comparing Three or More Samples, Unmatched Data • Chi-squared test- can be used for analyzing contingency tables that are larger than 2 × 2. -Suppose we wanted to test the effectiveness of three different drugs -Null Hypothesis-The proportions are all equal Contingency Table for a Chi-Squared Test Outcome Effective Not effective Drug A Drug B Drug C 11 2 6 6 3 7 Report from Statistics Program Power of performed test with alpha = 0.05: 0.671. The power of the performed test (0.671) is below the desired power of 0.800. © 2010 Jones and Bartlett Publishers, LLC 11