mrtab: error message if by variable is constant

[Dirk Enzmann]

When using the SSC program mrtab by Ben Jann using the by() option with the by variable being constant, mrtab will issue the error message

'/' invalid observation number
r(198);

The user may wonder how to avoid the error message -- it will be issued if the by variable is constant. It would be helpful if mrtab would issue the error message "by variable is constant", instead.

Here a reproducible example:

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(hackmot1 hackmot2 hackmot3 hackmot4 hackmot5 hackmot6 hackmot7 hackmot8 hackmot9 hackltp)
0 0 0 0 0 0 0 0 1 1
0 0 0 0 1 0 0 0 0 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
0 1 1 0 1 1 0 0 0 1
1 1 1 0 1 0 0 0 0 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 0 0 0 1 1
1 0 1 0 0 0 1 0 0 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 1 0 0 0 0 1
0 0 0 0 0 1 0 0 0 1
1 0 0 0 1 0 0 0 0 1
1 0 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 1 1 0 0 0 1
0 1 0 0 1 0 1 0 1 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 1 0 0 0 0 1
0 0 0 0 0 0 1 0 0 1
0 1 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 1
1 0 1 0 0 0 1 0 0 1
0 1 0 0 0 0 0 0 0 1
0 1 0 0 0 0 0 0 0 1
0 0 0 0 0 0 0 0 1 1
0 0 0 0 0 1 0 0 0 1
end
label values hackmot1 hackmot1
label def hackmot1 0 "not selected", modify
label def hackmot1 1 "yes", modify
label values hackmot2 hackmot2
label def hackmot2 0 "not selected", modify
label def hackmot2 1 "yes", modify
label values hackmot3 hackmot3
label def hackmot3 0 "not selected", modify
label def hackmot3 1 "yes", modify
label values hackmot4 hackmot4
label def hackmot4 0 "not selected", modify
label values hackmot5 hackmot5
label def hackmot5 0 "not selected", modify
label def hackmot5 1 "yes", modify
label values hackmot6 hackmot6
label def hackmot6 0 "not selected", modify
label def hackmot6 1 "yes", modify
label values hackmot7 hackmot7
label def hackmot7 0 "not selected", modify
label def hackmot7 1 "yes", modify
label values hackmot8 hackmot8
label def hackmot8 0 "not selected", modify
label values hackmot9 hackmot9
label def hackmot9 0 "not selected", modify
label def hackmot9 1 "yes", modify
label values hackltp hackltp
label def hackltp 1 "yes", modify

mrtab hackmot1-hackmot9, by(hackltp)

The result will be

. mrtab hackmot1-hackmot9, by(hackltp)
'/' invalid observation number
r(198);