This example shows how to do it.
Names Default to Here( 1 );
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
test = dt << New Column( "Test", Character, Nominal,
Values({
"Mark",
"Jane",
"5.37",
"3.14",
"Linda",
"1.732"
})
);
Wait( 2 );
test << Set Data Type( Numeric );