-Use the @code{VARIABLE LEVEL} command to change a variable's
-measurement level (@pxref{VARIABLE LEVEL}). To treat a variable as
-categorical or scalar only for one use on @code{CTABLES}, add
-@samp{[C]} or @samp{[S]}, respectively, after the variable name. The
-following example shows how to analyze the scalar variable @code{qn20}
-as categorical:
+When @pspp{} reads data from a file in an external format, such as a
+text file, variables' measurement levels are often unknown. If
+@code{CTABLES} runs when a variable has an unknown measurement level,
+it makes an initial pass through the data to guess measurement levels
+using the rules described earlier in this manual (@pxref{Measurement
+Level}). Use the @code{VARIABLE LEVEL} command to set or change a
+variable's measurement level (@pxref{VARIABLE LEVEL}).
+
+To treat a variable as categorical or scalar only for one use on
+@code{CTABLES}, add @samp{[C]} or @samp{[S]}, respectively, after the
+variable name. The following example shows the output when variable
+@code{qn20} is analyzed as scalar (the default for its measurement
+level) and as categorical: