In the provided DATA step, the merge statement is used to combine the PATIENT and VISIT data sets
by the variable Id. The number of observations in the resulting ALLVISITS data set will equal the
number of unique Id values that appear in both the PATIENT and VISIT data sets, because the merge
in SAS is a one-to-one merge by default when the by statement is used without additional options
like in=.
Looking at the provided data sets, each Id in the PATIENT data set has corresponding entries in the
VISIT data set. Since there are 5 unique Id values and the VISIT data set contains multiple
observations for some Id values (specifically, Id 2 and 5 have more than one visit), the ALLVISITS data
set will have a total of 7 observations (1 for each patient plus the additional visits for Id 2 and 5).
Reference:
SAS documentation on the merge statement.
VISIT
VISIT
How many observations are created in the ALLVISITS data set?