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Getting Casual about Cause. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

Rothman’s causal pie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

Bradford Hill’s criteria of causality . . . . . . . . . . . . . . . . . . . . . . . . . . 298

PART 6: ANALYZING SURVIVAL DATA. . . . . . . . . . . . . . . . . . . . . . . 299

CHAPTER 21: Summarizing and Graphing Survival Data. . . . . . . . . 301

Understanding the Basics of Survival Data. . . . . . . . . . . . . . . . . . . . . . 302

Examining how survival times are intervals . . . . . . . . . . . . . . . . . . 302

Recognizing that survival times aren’t normally distributed. . . . . 303

Considering censoring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

Looking at the Life-Table Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Making a life table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307

Interpreting a life table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

Graphing hazard rates and survival probabilities

from a life table. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

Digging Deeper with the Kaplan-Meier Method. . . . . . . . . . . . . . . 313

Heeding a Few Guidelines for Life-Tables and the

Kaplan-Meier Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

Recording survival times correctly. . . . . . . . . . . . . . . . . . . . . . . . . . 315

Miscoding censoring information. . . . . . . . . . . . . . . . . . . . . . . . . . . 316

CHAPTER 22: Comparing Survival Times. . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

Comparing Survival between Two Groups with the

Log-Rank Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

Understanding what the log-rank test is doing. . . . . . . . . . . . . . . . 319

Running the log-rank test on software. . . . . . . . . . . . . . . . . . . . . . . 320

Looking at the calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 320

Assessing the assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323

Considering More Complicated Comparisons . . . . . . . . . . . . . . . . . . . 324

Estimating the Sample Size Needed for Survival Comparisons. . . . . 324

CHAPTER 23: Survival Regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327

Knowing When to Use Survival Regression. . . . . . . . . . . . . . . . . . . . . . 328

Grasping the Concepts behind Survival Regression. . . . . . . . . . . . . . . 329

The steps to perform a PH regression. . . . . . . . . . . . . . . . . . . . . . . 330

Hazard ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334

Executing a Survival Regression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335

Interpreting the Output of a Survival Regression. . . . . . . . . . . . . . . . . 337

Testing the validity of the assumptions. . . . . . . . . . . . . . . . . . . . . . 339

Checking out the table of regression coefficients. . . . . . . . . . . . . .340

Homing in on hazard ratios and their confidence intervals. . . . . 341

Assessing goodness-of-fit and predictive ability of the model. . . 342

Focusing on baseline survival and hazard functions. . . . . . . . . . . 342