There are certain rules which must be followed for the labels used so that expressions will be parsed correctly. In general any combination of letters and digits can be used for the column titles and the labels of variables, but there are certain exceptions. Also note that there is no difference between upper and lower case letters, and that spaces in expressions are ignored.
a) Don't start any label with a digit e.g. 1stname, 2A, etc.
b) Don't start any label with a C or a V or a VV followed by a digit e.g. C2D, c40, v2, VV4xyz, etc.
c) Don't call any column or variable ROW, P, G, E, RANKS, ZED, XNUMBER, XTOTAL or XMEAN.
d) Don't start any label with the name of a mathematical function e.g. sint (uses SIN), pname (uses pN), etc. See the full list of arithmetic operators above.
e) Don't start any label with the name of another column or variable, i.e. if one column is called DA, then no column or variable label must begin with the letters DA. (Strictly speaking this rule only applies to columns to the right of the first one, that is to say if that one column is titled DA then no column to the right of it should begin with DA.)
If you get any problems with syntax errors or expressions not behaving as you think they should then come back and check these rules. For example if you did have a column called PNAME, then the expression sin(PNAME) would give you a syntax error with AME, because the parser would think that the PN referred to the normal probability function. Similarly if you had a column labelled 4 then the expression <4 would not mean "less than 4" but "less than the value in the column labelled 4".