Dear Statalist forum members,

I am interested in performing a test for unit roots in my panel dataset. I am using the Im-Pesaran-Shin (IPS) test, as I am working with an unbalanced panel dataset which looks as follows:
Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input int wbcode2 str46 country double year float(y z)
  4 "Albania"             1981        . 700.2622
 4 "Albania"             1982        . 701.2576
 4 "Albania"             1983        . 700.4279
 4 "Albania"             1984        . 697.0951
 4 "Albania"             1985        . 696.5898
 4 "Albania"             1986        . 699.5364
 4 "Albania"             1987        . 696.0583
 4 "Albania"             1988        . 692.1292
 4 "Albania"             1989        . 699.6201
 4 "Albania"             1990        .  688.527
 4 "Albania"             1991        . 653.3777
 4 "Albania"             1992        . 646.6589
 4 "Albania"             1993        . 657.1221
 4 "Albania"             1994        . 666.5153
 4 "Albania"             1995        . 680.2153
 4 "Albania"             1996 759.1942 689.8364
 4 "Albania"             1997        . 679.7451
 4 "Albania"             1998        . 692.1179
 4 "Albania"             1999        .  701.945
 4 "Albania"             2000        . 709.0192
 4 "Albania"             2001        . 715.6054
 4 "Albania"             2002 755.0409  718.062
 4 "Albania"             2003        . 723.0593
 4 "Albania"             2004        . 728.2089
 4 "Albania"             2005 770.4127 733.0224
 4 "Albania"             2006        . 737.4312
 4 "Albania"             2007        . 742.7509
 4 "Albania"             2008 777.7005 749.7994
 4 "Albania"             2009        . 752.6907
 4 "Albania"             2010        . 755.7471
 6 "Argentina"           1981        . 885.4302
 6 "Argentina"           1982        . 878.8189
 6 "Argentina"           1983        . 881.0916
 6 "Argentina"           1984        . 881.7568
 6 "Argentina"           1985        . 872.3582
 6 "Argentina"           1986 893.8957 878.4457
 6 "Argentina"           1987 880.8799  879.836
 6 "Argentina"           1988        . 875.7877
 6 "Argentina"           1989        . 866.5623
 6 "Argentina"           1990        . 862.7296
 6 "Argentina"           1991 840.1109 873.2861
 6 "Argentina"           1992 844.5835  883.226
 6 "Argentina"           1993 846.8036 887.6558
 6 "Argentina"           1994  843.349 892.0464
 6 "Argentina"           1995  835.522 887.9047
 6 "Argentina"           1996 831.3823 892.0524
 6 "Argentina"           1997 838.1978 898.6461
 6 "Argentina"           1998 829.3229 901.2557
 6 "Argentina"           1999 828.6542 896.6932
 6 "Argentina"           2000 823.6368 894.8403
 6 "Argentina"           2001   812.23 889.3307
 6 "Argentina"           2002 774.5782  876.848
 6 "Argentina"           2003  811.662  884.407
 6 "Argentina"           2004   827.03 892.1624
 6 "Argentina"           2005 837.4753 900.0604
 6 "Argentina"           2006 850.8935 907.3051
 6 "Argentina"           2007 854.4684 914.7261
 6 "Argentina"           2008 860.5446 920.3903
 6 "Argentina"           2009 866.9901 920.3641
 6 "Argentina"           2010 868.5862 928.2607
 7 "Armenia"             1981        .        .
 7 "Armenia"             1982        .        .
 7 "Armenia"             1983        .        .
 7 "Armenia"             1984        .        .
 7 "Armenia"             1985        .        .
 7 "Armenia"             1986        .        .
 7 "Armenia"             1987        .        .
 7 "Armenia"             1988        .        .
 7 "Armenia"             1989        .        .
 7 "Armenia"             1990        . 667.8819
 7 "Armenia"             1991        . 656.3641
 7 "Armenia"             1992        . 604.0199
 7 "Armenia"             1993        . 597.1417
 7 "Armenia"             1994        . 604.7981
 7 "Armenia"             1995        .  613.539
 7 "Armenia"             1996        . 620.8334
 7 "Armenia"             1997        . 625.2849
 7 "Armenia"             1998        . 633.1551
 7 "Armenia"             1999 703.0751 636.9979
 7 "Armenia"             2000        . 643.2015
 7 "Armenia"             2001 699.0588 652.6636
 7 "Armenia"             2002 703.8257  665.205
 7 "Armenia"             2003 709.0377 678.3604
 7 "Armenia"             2004 723.7893 688.2485
 7 "Armenia"             2005 735.6433 701.1245
 7 "Armenia"             2006 742.7406 713.3945
 7 "Armenia"             2007 751.8933 726.1359
 7 "Armenia"             2008 755.9017 732.6488
 7 "Armenia"             2009 744.7542 717.2007
 7 "Armenia"             2010 744.1578 719.0481
 9 "Antigua and Barbuda" 1981        . 849.0743
 9 "Antigua and Barbuda" 1982        . 850.4492
 9 "Antigua and Barbuda" 1983        . 856.5016
 9 "Antigua and Barbuda" 1984        . 867.4309
 9 "Antigua and Barbuda" 1985        . 876.7612
 9 "Antigua and Barbuda" 1986        . 890.3862
 9 "Antigua and Barbuda" 1987        .  899.933
 9 "Antigua and Barbuda" 1988        . 906.5459
 9 "Antigua and Barbuda" 1989        . 912.9224
 9 "Antigua and Barbuda" 1990        . 915.4196
 9 "Antigua and Barbuda" 1991        .  916.552
 9 "Antigua and Barbuda" 1992        . 915.7404
 9 "Antigua and Barbuda" 1993        . 918.7645
 9 "Antigua and Barbuda" 1994        . 922.3918
 9 "Antigua and Barbuda" 1995        . 915.5472
 9 "Antigua and Barbuda" 1996        . 919.4695
 9 "Antigua and Barbuda" 1997        . 921.6172
 9 "Antigua and Barbuda" 1998        . 923.3804
 9 "Antigua and Barbuda" 1999        . 925.0287
 9 "Antigua and Barbuda" 2000        . 924.3232
 9 "Antigua and Barbuda" 2001        . 924.5861
 9 "Antigua and Barbuda" 2002        .  925.397
 9 "Antigua and Barbuda" 2003        .  928.853
 9 "Antigua and Barbuda" 2004        . 934.2868
 9 "Antigua and Barbuda" 2005        . 937.0965
 9 "Antigua and Barbuda" 2006        . 948.3361
 9 "Antigua and Barbuda" 2007        . 957.3103
 9 "Antigua and Barbuda" 2008        . 956.9407
 9 "Antigua and Barbuda" 2009        . 943.9459
 9 "Antigua and Barbuda" 2010        . 938.3828
10 "Australia"           1981  926.277 960.0023
10 "Australia"           1982        . 961.5115
10 "Australia"           1983        . 957.9136
10 "Australia"           1984        . 961.2773
10 "Australia"           1985 927.9258 964.9133
10 "Australia"           1986        . 967.7253
10 "Australia"           1987        . 968.7257
10 "Australia"           1988        . 972.7957
10 "Australia"           1989  930.525 974.9674
10 "Australia"           1990        . 976.9996
10 "Australia"           1991        . 975.4856
10 "Australia"           1992        . 974.7252
10 "Australia"           1993        . 977.7516
10 "Australia"           1994        . 980.5826
10 "Australia"           1995 922.2964 983.2389
10 "Australia"           1996        . 986.0215
10 "Australia"           1997        . 988.7983
10 "Australia"           1998        . 992.2396
10 "Australia"           1999        . 996.1283
10 "Australia"           2000        . 998.8112
end
The problem is the following. The two variables which I want to investigate for unit roots are the following: y=100*log(median), where median represents median household income within a country, and z=100*log(gdppercapita). When I run the IPS test, where I use the following command
Code:
xtunitroot ips z,  lags(aic)
for the variable z everything works fine. But when I run this test for the variable y, I get the following error message from Stata: no observations r(2000). Stata is telling me that I want to do a certain statistical calculation and there are no observations on which to perform it.
Some descriptive statistics of the two variables z and y:
Code:
    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
           y |        965    795.3641       102.6   530.2708    1003.75
           z |      4,909    756.8494    155.4241   405.6728   1094.003
Might anyone have a clue why the IPS command works for the variable z, but not the variable y? Any advice on this would be greatly appreciated.

I thank you for your time.

Best,

Ryan Marapin