An Introduction to SAS University Edition

Chapter 16: Working with Multiple Observations per Subject

Introduction

Useful Tools for Working with Longitudinal Data

Describing First. and Last. Variables

Computing Visit-to-Visit Differences

Computing Differences Between the First and Last Visits

Counting the Number of Visits for Each Patient

Conclusion

Problems

Introduction

Because SAS processes one observation at a time, you need special programming tools when you want to analyze data where there are multiple observations per subject or any other grouping variable. For example, suppose you have a data set in which each observation represents a patient visit to a clinic. You record the patient ID, the date of the visit, and some health-related values. Some common programming tasks would ...