Finding First and Last Observations

• In a drug dataset, records are available of patients taken drugs on different days. 
• We want to find out what was the first visit of the patient and last visit of the patient and check his weight.
• Steps to find out first and last observations:
• Sort the drug dataset by patient isr.
• Use Set and By and find the first and last observations.
• The First and Last observations will be available as new columns with values 1 in the newly created dataset.
• Remember, You have to "use a BY statement, in conjunction with a SET statement, to identify the first and last observations among a group of observations".

Read more on this topic: https://onlinecourses.science.psu.edu/stat481/node/9