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## Final Review

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**Final Review**EPI 809 / Spring 2008**Ch11 Regression and correlation**• Linear regression • Model, interpretation. • Model Coefficient calculation. • b = Lxy / Lxx (slope), b0 = Y – b x • Assumption, goodness-of-fit, validity. • Independent error, Gaussian dist. Const. var. • Test and inference (t-test). • Multiple regression. F-test vs T-test. • Pearson correlation • Interpretation and inference • T-test and Fisher’s z-test (transformation). 1. t = r (n-2)1/2 /(1-r2)1/2 ~ t n-2 2. Z = ½ ln [(1+r) / (1-r)] ~ Normal mean=Z(r0) and var =1/(n-3) - - EPI 809 / Spring 2008**Learning Objectives**• Describe the Linear Regression Model • State the Regression Modeling Steps • Explain Ordinary Least Squares • Compute Regression Coefficients • Understand and check model assumptions • Predict Response Variable • Comments of SAS Output EPI 809 / Spring 2008**Learning Objectives…**• Correlation Models • Link between a correlation model and a regression model (one indep. Var): b = rSy/Sx, and Sy2 = Lyy /(n-1) • Test of coefficient of Correlation EPI 809 / Spring 2008**ANOVA**• Continuous response, categorical explanatory (indep) var. • Assumption. (Gauss-Markov condition). • Decomposition SS SS total = SS trt + SS error or SS total = SS trt + SSblk + SS error or SS total = SSA + SSB + SSAB + SS error • Estimation vs Prediction (diff. var.) EPI 809 / Spring 2008**Multiple comparison**• Contrast for multiple levels of var. construct contrast according to aim. • Adjustment for multiple comparison • LSD, Bonferroni, Sheffe. EPI 809 / Spring 2008**Ch 9 Non-parametric tests**• Mainly interested in ranking (distribution) Normality of data may be violated. • Sign test, rank sum test, signed-rank test, Kruskal-Wallis test EPI 809 / Spring 2008**Summary**EPI 809 / Spring 2008**Ch 10 Categorical Data Analysis**EPI 809 / Spring 2008**Learning Objectives**• Comparison of binomial proportion using Z and 2 Test. • Explain 2 Test for Independence of 2 variables • Explain The Fisher’s test for independence • McNemar’s tests for correlated data • Kappa Statistic • Use of SAS Proc FREQ EPI 809 / Spring 2008**Z Test for Difference in Two Proportions**1. Assumptions • Populations Are Independent • Populations Follow Binomial Distribution • Normal Approximation Can Be Used for large samples (All Expected Counts 5) • Z-Test Statistic for Two Proportions EPI 809 / Spring 2008**Sample Distribution for Difference Between Proportions**EPI 809 / Spring 2008**2 Test of Independence Hypotheses & Statistic**1. Hypotheses H0: Variables Are Independent Ha: Variables Are Related (Dependent) • Test Statistic • Degrees of Freedom: (r - 1)(c - 1) O: Observed count E: Expected count r Rows & C Columns EPI 809 / Spring 2008**Fisher’s Exact Test**a b M1 c d M2 N1 N2 N • Hypergeometric distribution • Example: 2x2 table (cell counts a, b, c, d). Assuming fixed marginal totals: M1 = a+b, M2 = c+d, N1 = a+c, N2 = b+d. for convenience assume N1<N2, M1<M2. possible value of a are: 0, 1, …min(M1,N1). • Probability distribution of cell count a follows a hypergeometric distribution: N = a + b + c + d = N1 + N2 = M1 + M2 • Pr (x=a) = N1! N2! M1! M2! / [N! a! b! c! d!] • Mean (x) = M1 N1 / N • Var (x) = M1 M2 N1 N2 / [N2 (N-1)] EPI 809 / Spring 2008**Fisher’s Exact Test**• Fisher exact test is based on hypergeometric distr. • Probability of observing this specific table given fixed marginal totals is Pr (a=3,b=7, c=5, d=10) = 10!15!8!17!/[25!3!7!5!10!] = 0.3332 • Note the above is not the p-value. Why? • Not the accumulative probability, or not the tail probability. • Notice range of a: [0, min(M1, N1)] for M1<M2 and N1<N2 • Tail prob = sum of all values (a = 3, 2, 1, 0). EPI 809 / Spring 2008**po - pe** = 1 - pe Kappa ( )Measures of Association • Cohen’s Kappa ( ) • Cohen’s measures the agreement between two variables and is defined by Kappa >.75 excellent reproducibility; [.4, .75] good reproducibility; <.4 marginal reproducibility. EPI 809 / Spring 2008**{ |B – C| - 1 }2** 2 = B + C McNemar’s Test for Correlated (Dependent) Proportions • H0: 1 = 2 : discordant probabilities. • Ha: 1 2 • Test Statistic: Chi-squares with df = 1. EPI 809 / Spring 2008**Chapter 13**Design and Analysis Techniques for Epidemiologic Studies EPI 809 / Spring 2008**Learning Objectives**• Define study designs • Measures of effects for categorical data • Confounders and effects modifications • Stratified analysis (Mantel Haenszel statistic, multiple logistic regression) • Use of SAS Proc FREQ and Proc Logistic EPI 809 / Spring 2008**Experimental Study**• Randomization protects against bias in assignment to groups. • Blinding protects against bias in outcome assessment or measurement. • Control for (major) sources of variability, although not necessarily reflecting real life conditions • Expensive in terms of time and money EPI 809 / Spring 2008**Observational Study most likely used in Epidemiology**• Types of study • Cross-sectional study Both expos & outcome random; • Case-control study (retrospective) Random expos, fixed outcome; • Cohort study (Prospective) Fixed expos, random outcome. EPI 809 / Spring 2008**Measures of effects**• Depends on study design • Prospective study: Incidence of disease (risk difference, relative risk, odds ratio of disease) • Cross-sectional: Prevalence of disease (risk difference, relative risk, odds ratio of disease) • Case-cohort: study of exposure (odds ratio of exposure) EPI 809 / Spring 2008**Risk difference**Only for cross-sectional and cohort studies Measured the attributable risk due to exposure EPI 809 / Spring 2008**Relative Risk**Only for cross-sectional and cohort studies: Ratio of the probability that the outcome characteristic is present for one group, relative to the other The range of RR is [0, ). By taking the logarithm, we have (- , +) as the range for ln(RR) and a better approximation to normality for the estimated EPI 809 / Spring 2008**Odds Ratio - Disease**• Odds ratio is the odds of the event for exposed divided by the odds of the event for unexposed • Sample odds of the outcome for each group: EPI 809 / Spring 2008**Odds Ratio-Exposure**we fixed the number of cases and controls then ascertained exposure status. The relative risk is therefore not estimable from these data alone. Instead of the relative risk we can estimate the exposure OR which Cornfield (1951) showed equivalent to the disease OR: In other words, the odds ratio can be estimated regardless of the sampling scheme. EPI 809 / Spring 2008**Odds Ratio-Relative risk**For rare diseases, the disease odds ratio approximates the relative risk: Since with case-control data we are able to effectively estimate the exposure odds ratio we are then able to equivalently estimate the disease odds ratio which for rare diseases approximates the relative risk. EPI 809 / Spring 2008**Odds Ratio**The odds ratio has [0, ) as its range. The log odds ratio has (- , +) as its range and the normal approximation is better as an approximation to the estimated log odds ratio. Confidence intervals are based upon: Therefore, a (1 - ) confidence interval for the odds ratio is given by exponentiating the lower and upper bounds. EPI 809 / Spring 2008**Summary**• RD = p1 - p2 = risk difference (null: RD = 0) • also known as attributable risk or excess risk • measures absolute effect – the proportion of cases among the exposed that can be attributed to exposure • RR = p1/p2 = relative risk (null: RR = 1) • measures relative effect of exposure • bounded above by 1/p2 • OR = [p1(1-p2)]/[p2 (1-p1)] = odds ratio (null: OR = 1) • range is 0 to • approximates RR for rare events • invariant of switching rows and cols • key parameter in logistic regression EPI 809 / Spring 2008**Effect modifier**• Variation in the magnitude of measure of effect across levels of a third variable. • Effect modification is not a bias but useful information Happens when RR or OR is different between strata (subgroups of population) EPI 809 / Spring 2008**Confounding**• Distortion of measure of effect because of a third factor • Should be prevented or Needs to be controlled for EPI 809 / Spring 2008**Confounding**Exposure Outcome Third variable Be associated with exposure - without being the consequence of exposure Be associated with outcome - independently of exposure EPI 809 / Spring 2008**Confounding and Control**• Positive confounding • - positively or negatively related to both • the disease and exposure • Negative confounding • - positively related to disease but is • negatively related to exposure or the • reverse • Prevention (Design Stage) • Restriction to one stratum or Matching • Control (Analysis Stage) • Stratified analysis – Mantel Haenszel • Multivariable analysis – logistic regression. EPI 809 / Spring 2008**Mantel Haenszel Methods common odds ratio**• The Mantel-Haenszel estimate of the odds ratio assumes there is a common odds ratio: • ORpool = OR1 = OR2 = … = ORK • To estimate the common odds ratio we take a weighted average of the stratum-specific odds ratios: • MH estimate: EPI 809 / Spring 2008**Mantel Haenszel Methods**• (2) Test of common odds ratio • Ho: common OR is 1.0 vs. Ha: common OR 1.0 • - A standard error is available for the MH common odds • - Standard CI intervals and test statistics are based on the standard normal distribution. • (3) Test of effect modification (heterogeneity, interaction) • Ho: OR1 = OR2 = … = ORK • Ha: not all stratum-specific OR’s are equal • Breslow-Day (SAS) homogeneity test can be used EPI 809 / Spring 2008**Multiple Logistic Regression**EPI 809 / Spring 2008**Multiple Logistic Regression-Formulation**The relationship between π and x is S shaped The logit (log-odds) transformation (link function) EPI 809 / Spring 2008**Interpretation of the parameters**• If π is the probability of an event and O is the odds for that event then • The link function in logistic regression gives the log-odds EPI 809 / Spring 2008