Hi,

I'm want run a regression analysis on panel data from a time period 1997-2015. I ran the following two codes (one with vce robust option, one without)
Code:
xtreg returnoninvestment pat_yr maactive maactive#c.pat_yr totalma2 emp at i.fyear i.sic2d, re
Code:
xtreg returnoninvestment pat_yr maactive maactive#c.pat_yr totalma2 emp at i.fyear i.sic2d, vce (robust)
but both codes gave different results. And I'm not really sure when to use the vce robust option. I thought it is appropriate to adjust for clusters in panel data because all variables are company (gvkey) and year (fyear) specific, but I am not sure about. Could someone give me some advice about whether to use the vce robust option or not? Here you can find the dataset I ran my regression on:

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input long gvkey float returnoninvestment double pat_yr float(maactive totalma2) double(emp at fyear) byte sic2d
1078   48.01397              195.5 0  0 54.487 12061.068 1997 38
1078   44.20482              172.5 0  0 56.236 13216.213 1998 38
1078  38.035473              152.5 1  1   57.1 14471.044 1999 38
1078  33.813366              130.5 1  1 60.571 15283.254 2000 38
1078   26.41248                136 1  2 71.426 23296.423 2001 38
1078  26.380386              134.5 1  3 71.819 24259.102 2002 38
1078   25.20349              128.5 1  4 72.181 26715.342 2003 38
1078  23.309574               99.5 1  5   60.6 28767.494 2004 38
1078  25.467196               64.5 1  5 59.735 29141.203 2005 38
1078  23.073746                178 1  6 66.663 36178.172 2006 38
1078   20.25775              153.5 1  7     68 39713.924 2007 38
1078   24.72846              170.5 1  7     69 42419.204 2008 38
1078   20.20704              212.5 1  8     73 52416.623 2009 38
1078   22.96796                480 1  9     90 59462.266 2010 38
1078  24.603336                630 1 10     91 60276.893 2011 38
1078  21.790276              692.5 1 11     91 67234.944 2012 38
1078   9.331705                674 1 12     69     42953 2013 38
1078   11.68603              573.5 1 13     77     41275 2014 38
1078  12.398426              465.5 1 14     74     41247 2015 38
1104  12.102242                  6 1  1   .303    79.308 2014 34
1104  11.211775                  0 1  1   .342    81.421 2015 34
1161  -3.367206              273.5 0  0   12.8  3515.271 1997 36
1161  -4.845113                556 0  0   13.8  4252.968 1998 36
1161  -8.298296                830 0  0 13.387  4377.698 1999 36
1161  20.479486               1053 0  0 14.696  5767.735 2000 36
1161   .8975403             1091.5 0  0 14.415  5647.242 2001 36
1161 -20.079834 1153.8333740234375 1  1 12.146  5619.181 2002 36
1161  -4.891128                908 1  2   14.3  7094.345 2003 36
1161  4.1479573              812.5 1  2   15.9   7844.21 2004 36
1161   5.039897              539.5 1  2   9.86  7287.779 2005 36
1161  4.1140866                510 1  3   16.5     13147 2006 36
1161 -14.615013              339.5 1  3  16.42     11550 2007 36
1161  -20.21299                235 1  3   14.7      7675 2008 36
1161  -8.584337              246.5 1  3   13.4      9078 2009 36
1161  15.370196  230.8333282470703 1  3   11.1      4964 2010 36
1161  15.880654                253 1  3 11.093      4954 2011 36
1161  1.2038835                262 1  4  10.34      4000 2012 36
1161   3.343824                287 1  4 10.671      4337 2013 36
1161   6.705671  297.8333282470703 1  4  9.687      3767 2014 36
1161  -19.69136                246 1  4  9.139      3109 2015 36
1209  13.899348               72.5 1  1   16.4    7244.1 1997 28
1209  16.468351                 82 1  2   16.7    7489.6 1998 28
1209  14.267895                 87 1  2   17.4    8235.5 1999 28
1209  15.012787                105 1  2   17.5    8270.5 2000 28
1209  15.262887                 87 1  3   17.8    8084.1 2001 28
1209   13.40005                 77 1  4   17.2      8495 2002 28
1209   11.73117              105.5 1  5   18.5    9431.9 2003 28
1209  12.459675                 77 1  6   19.9   10040.4 2004 28
1209  14.033336               61.5 1  6   20.2   10408.8 2005 28
1209   13.72897               71.5 1  7   20.7   11180.7 2006 28
1209   15.47853               73.5 1  8   22.1   12659.5 2007 28
1209   17.16711                 74 1  9   21.1   12571.3 2008 28
1209  13.350144                 62 1  9   18.9   13029.1 2009 28
1209   15.42095                 62 1 10   18.3   13505.9 2010 28
1209   16.50078               58.5 1 11   18.9   14290.7 2011 28
1209   12.84138                 87 1 12   21.3   16941.8 2012 28
1209  11.941256                 92 1 13   21.6   17850.1 2013 28
1209   12.69443                 81 1 14   21.2   17779.1 2014 28
1209  16.507212                 63 1 14   19.7   17438.1 2015 28
1234  3.0086274                  1 0  0   .386    60.942 1997 38
1234   3.198363                4.5 0  0   .461    60.415 1998 38
1234   5.880355                5.5 0  0   .443     64.64 1999 38
1234  8.2584505                  5 0  0   .441     63.69 2000 38
1234  10.337478                  2 0  0   .443    64.287 2001 38
1234  11.113247                  1 0  0    .42    60.807 2002 38
1234   14.16007                  0 0  0   .427     60.05 2003 38
1234  16.055586                  2 0  0   .456    67.408 2004 38
1234  19.710047                  1 0  0   .493     78.47 2005 38
1234  17.422897                  1 0  0   .486    95.772 2006 38
1234   21.99972                  0 0  0   .492    99.313 2007 38
1234  22.781183                  1 0  0   .485   115.353 2008 38
1234  22.267435                  0 0  0   .465   132.749 2009 38
1234   26.56302                  1 0  0   .437   134.652 2010 38
1234   27.55534                  2 0  0   .458   161.895 2011 38
1234   24.93992                  7 0  0   .459    155.81 2012 38
1234  25.466797                  0 0  0   .485   172.066 2013 38
1234  27.289564                  0 0  0   .483   171.514 2014 38
1234  29.500755                  0 0  0   .489   164.336 2015 38
1300   29.40546                191 0  0   70.5     13707 1997 99
1300   28.96796                241 0  0   70.4     15560 1998 99
1300  29.585747                408 0  0    120     23527 1999 99
1300   26.58998                496 1  1    125     25175 2000 99
1300   19.76836                535 1  2    115     24226 2001 99
1300  18.381706                558 1  3    108     27559 2002 99
1300   13.98343                531 1  4    108     29344 2003 99
1300  15.312317                641 1  5    109     31062 2004 99
1300   19.86607                535 1  6    116     32294 2005 99
1300  26.003376                650 1  7    118     30941 2006 99
1300   29.14418                640 1  8    122     33805 2007 99
1300  34.791603                799 1  9    128     35490 2008 99
1300   25.39474              873.5 1 10    122     36004 2009 99
1300  22.572845               1111 1 11    130     37834 2010 99
1300  18.652647               1059 1 11    132     39808 2011 99
1300   23.45232               1126 1 12    132     41853 2012 99
1300   25.11101             1039.5 1 13    131     45435 2013 99
1300   26.73292             1095.5 1 14    127     45451 2014 99
1300   30.39321               1176 1 15    129     49316 2015 99
1327   20.34571                  2 0  0    .84    76.929 1997 36
1327   23.92722                  0 0  0    .94   106.681 1998 36
1327   13.62236                  1 0  0   1.09   259.104 1999 36
end