Hello everyone,

I am using unbalanced panel data on individual respondents (T = 11 and N = 83687). I am running a conditional logit model with fixed effects, the dependent variable it's a dummy (at time t) and I run most of the covariates with a lag of one year in (t-1).
I'm using regional (i.region) and year's effects (i.syear). My problem is due to convergence. If I run the fixed effects conditional logit model with all the year's effects (i.syear) and the covariates Age and Age2 (using squared Age to model the age effect) I get no convergence. Therefore, considering that I got no standard errors (.) in the year effect 2019, I tried to create myself a dummy variable for each year I'm considering and I excluded the year 2019 effect (because I thought there were not enough observations to achieve the convergence). In that case, it works fine and the convergence it's achieved.

Moreover, I tried to consider instead of Age and Age2 some Age Categories ( 5 categories). With the Age categories everything it's working fine and the convergence it's achieved considering all the year's effects (by using i.syear).

I think I will probably consider the Age categories model for my final version but I was wondering why was the convergence not achieved when considering the Age and Age2 and all the year's effects (i.syear) and on the contrary, it was when considering the Age categories and all the year's effects?

here there is the code:

Code:
. xtlogit resigning L.insecurity L.ln_income i.region i.syear L.age L.age2 i.L.married i.L.health_status1    L.ye
> ars_of_education L.n_of_children L.annual_working_hours, fe iterate(20)
note: multiple positive outcomes within groups encountered.
note: 12,229 groups (50,752 obs) omitted because of all positive or
all negative outcomes.

Iteration 0:   log likelihood = -4954.8824  (not concave)
Iteration 1:   log likelihood = -4923.8604  (not concave)
Iteration 2:   log likelihood = -4918.2668  (not concave)
Iteration 3:   log likelihood = -4917.9681  (not concave)
Iteration 4:   log likelihood = -4917.8448  (not concave)
Iteration 5:   log likelihood = -4917.7838  (not concave)
Iteration 6:   log likelihood = -4917.6602  (not concave)
Iteration 7:   log likelihood = -4917.6032  (not concave)
Iteration 8:   log likelihood = -4917.3976  (not concave)
Iteration 9:   log likelihood = -4917.3279  (not concave)
Iteration 10:  log likelihood = -4917.3174  (not concave)
Iteration 11:  log likelihood = -4917.3072  (not concave)
Iteration 12:  log likelihood = -4917.2688  (not concave)
Iteration 13:  log likelihood = -4917.2554  (not concave)
Iteration 14:  log likelihood = -4917.2542  (not concave)
Iteration 15:  log likelihood = -4917.2531  (not concave)
Iteration 16:  log likelihood =  -4917.252  (not concave)
Iteration 17:  log likelihood = -4917.2511  (not concave)
Iteration 18:  log likelihood = -4917.2502  (not concave)
Iteration 19:  log likelihood = -4917.2493  (not concave)
Iteration 20:  log likelihood = -4917.2485  (not concave)
convergence not achieved

Conditional fixed-effects logistic regression        Number of obs    = 13,872
Group variable: pid                                  Number of groups =  2,474

Obs per group:
min =      2
avg =    5.6
max =     10

LR chi2(34)      = 196.67
Log likelihood = -4917.2485                          Prob > chi2      = 0.0000


resigning  Coefficient  Std. err.      z    P>z     [95% conf. interval]

insecurity 
L1.    .1586778   .0395872     4.01   0.000     .0810883    .2362673

ln_income 
L1.   -.0959634   .1089586    -0.88   0.378    -.3095183    .1175914

region 
2    -.0029459   .6049219    -0.00   0.996    -1.188571    1.182679
3     .0213726   .8443094     0.03   0.980    -1.633444    1.676189
4     54.71097          .        .       .            .           .
5     -1.05323   .7851114    -1.34   0.180     -2.59202    .4855604
6    -.3901004   .8363038    -0.47   0.641    -2.029226    1.249025
7     .6840851   1.003371     0.68   0.495    -1.282485    2.650655
8     .5695272   .8315001     0.68   0.493    -1.060183    2.199237
9     .0330805   .8980872     0.04   0.971    -1.727138    1.793299
10    -43.49452   1.72e+09    -0.00   1.000    -3.38e+09    3.38e+09
11     .9676775   .9933413     0.97   0.330    -.9792356    2.914591
12     .7399395   1.002769     0.74   0.461    -1.225452    2.705332
13     .8720612   1.059855     0.82   0.411    -1.205217    2.949339
14    -.1655239   1.044938    -0.16   0.874    -2.213564    1.882517
15    -1.174527   1.385044    -0.85   0.396    -3.889163    1.540109
16    -1.920376   1.317821    -1.46   0.145    -4.503258    .6625068

syear 
2011     .1142141   .0922802     1.24   0.216    -.0666517    .2950799
2012      .162089   .0917376     1.77   0.077    -.0177134    .3418914
2013     .2067589   .0895892     2.31   0.021     .0311674    .3823505
2014     .1426668   .0795742     1.79   0.073    -.0132957    .2986292
2015     .0924091   .0750384     1.23   0.218    -.0546635    .2394818
2016     .0959823   .0722217     1.33   0.184    -.0455696    .2375341
2017     .0659704   .0703078     0.94   0.348    -.0718303    .2037711
2018     .1146478   .0704689     1.63   0.104    -.0234688    .2527643
2019     .0564252          .        .       .            .           .

age 
L1.   -.1092868   .0493051    -2.22   0.027     -.205923   -.0126507

age2 
L1.    .0007596   .0005432     1.40   0.162    -.0003051    .0018242

L.married 
1    -.1836457   .1240138    -1.48   0.139    -.4267083    .0594169

L.health_status1 
2     .4764549   .2225177     2.14   0.032     .0403282    .9125817
3     .1984931   .2217544     0.90   0.371    -.2361375    .6331237
4     .1046584   .2238471     0.47   0.640    -.3340738    .5433906
5     .2330741   .2369465     0.98   0.325    -.2313325    .6974807

years_of_education 
L1.   -.0513078   .0984258    -0.52   0.602    -.2442188    .1416031

n_of_children 
L1.   -.1159357   .0600321    -1.93   0.053    -.2335965    .0017251

annual_working_hours 
L1.   -.0003312   .0000471    -7.03   0.000    -.0004236   -.0002388
Code:
. xtlogit resigning L.insecurity L.ln_income $yearsdummy i.region L.age L.age2 i.L.sex i.L.married    i.L.health_
> status1 L.years_of_education L.n_of_children L.annual_working_hours, fe iterate(20)
note: multiple positive outcomes within groups encountered.
note: 12,229 groups (50,752 obs) omitted because of all positive or
all negative outcomes.
note: 1L.sex omitted because of no within-group variance.

Iteration 0:   log likelihood = -4953.8719  
Iteration 1:   log likelihood = -4917.7094  
Iteration 2:   log likelihood = -4917.2589  
Iteration 3:   log likelihood = -4917.2287  
Iteration 4:   log likelihood = -4917.2222  
Iteration 5:   log likelihood = -4917.2207  
Iteration 6:   log likelihood = -4917.2204  
Iteration 7:   log likelihood = -4917.2203  
Iteration 8:   log likelihood = -4917.2203  

Conditional fixed-effects logistic regression        Number of obs    = 13,872
Group variable: pid                                  Number of groups =  2,474

Obs per group:
min =      2
avg =    5.6
max =     10

LR chi2(35)      = 196.73
Log likelihood = -4917.2203                          Prob > chi2      = 0.0000


resigning  Coefficient  Std. err.      z    P>z     [95% conf. interval]

insecurity 
L1.    .1595486   .0395919     4.03   0.000       .08195    .2371473

ln_income 
L1.   -.0939751   .1089648    -0.86   0.388    -.3075422     .119592

year11    .1067305   .0922694     1.16   0.247    -.0741142    .2875752
year12    .1484747   .0917286     1.62   0.106      -.03131    .3282595
year13    .1870052   .0895873     2.09   0.037     .0114174    .3625931
year14    .1159637   .0795739     1.46   0.145    -.0399984    .2719258
year15    .0599535    .075038     0.80   0.424    -.0871184    .2070253
year16    .0574416   .0722222     0.80   0.426    -.0841113    .1989945
year17    .0212388   .0703086     0.30   0.763    -.1165636    .1590412
year18    .0639982   .0704692     0.91   0.364     -.074119    .2021153

region 
2     -.010283   .6049746    -0.02   0.986    -1.196011    1.175445
3     .0200304   .8440641     0.02   0.981    -1.634305    1.674366
4     23.77858   26989.53     0.00   0.999    -52874.74     52922.3
5    -1.056672   .7848169    -1.35   0.178    -2.594885    .4815409
6    -.3918029    .836058    -0.47   0.639    -2.030447    1.246841
7     .6866278   1.003021     0.68   0.494    -1.279258    2.652514
8     .5670053   .8313347     0.68   0.495    -1.062381    2.196391
9     .0289339   .8978407     0.03   0.974    -1.730801    1.788669
10    -14.35951   810.0542    -0.02   0.986    -1602.037    1573.318
11      .966391   .9924808     0.97   0.330    -.9788356    2.911618
12     .7433985   1.002162     0.74   0.458    -1.220804    2.707601
13       .87058   1.059877     0.82   0.411    -1.206741    2.947901
14    -.1643945   1.044952    -0.16   0.875    -2.212463    1.883674
15    -1.177462   1.385052    -0.85   0.395    -3.892115     1.53719
16    -1.923164   1.317747    -1.46   0.144    -4.505901    .6595729

age 
L1.   -.1143711   .0492992    -2.32   0.020    -.2109957   -.0177464

age2 
L1.     .000883   .0005431     1.63   0.104    -.0001814    .0019474

L.sex 
1            0  (omitted)

L.married 
1    -.1807483   .1240086    -1.46   0.145    -.4238007    .0623041

L.health_status1 
2     .4649026   .2221624     2.09   0.036     .0294722    .9003329
3     .1864222   .2213913     0.84   0.400    -.2474967    .6203412
4     .0925622   .2234899     0.41   0.679    -.3454699    .5305943
5     .2207287   .2366089     0.93   0.351    -.2430162    .6844736

years_of_education 
L1.   -.0487996   .0984007    -0.50   0.620    -.2416614    .1440622

n_of_children 
L1.    -.112988   .0600387    -1.88   0.060    -.2306617    .0046858

annual_working_hours 
L1.   -.0003305   .0000471    -7.01   0.000    -.0004229   -.0002381
and with Age categories

Code:
. xtlogit resigning L.insecurity L.ln_income i.syear i.region i.L.age_cate i.L.sex i.L.married    i.L.health_stat
> us1 L.years_of_education L.n_of_children L.annual_working_hours, fe iterate(20)
note: multiple positive outcomes within groups encountered.
note: 12,207 groups (50,651 obs) omitted because of all positive or
all negative outcomes.
note: 1L.sex omitted because of no within-group variance.

Iteration 0:   log likelihood = -4954.8564  
Iteration 1:   log likelihood =    -4914.5  
Iteration 2:   log likelihood = -4914.0655  
Iteration 3:   log likelihood = -4914.0341  
Iteration 4:   log likelihood = -4914.0284  
Iteration 5:   log likelihood = -4914.0271  
Iteration 6:   log likelihood = -4914.0269  
Iteration 7:   log likelihood = -4914.0268  
Iteration 8:   log likelihood = -4914.0268  

Conditional fixed-effects logistic regression        Number of obs    = 13,866
Group variable: pid                                  Number of groups =  2,474

Obs per group:
min =      2
avg =    5.6
max =     10

LR chi2(37)      = 201.15
Log likelihood = -4914.0268                          Prob > chi2      = 0.0000


resigning  Coefficient  Std. err.      z    P>z     [95% conf. interval]

insecurity 
L1.    .1568622   .0395197     3.97   0.000      .079405    .2343195

ln_income 
L1.   -.0901518   .1090069    -0.83   0.408    -.3038015    .1234978

syear 
2011     .0686782   .0977131     0.70   0.482    -.1228359    .2601924
2012     .0736324   .1028235     0.72   0.474    -.1278979    .2751627
2013      .080782   .1058594     0.76   0.445    -.1266987    .2882627
2014    -.0216384   .1040958    -0.21   0.835    -.2256624    .1823856
2015    -.1144291   .1060272    -1.08   0.280    -.3222386    .0933803
2016    -.1529156   .1085844    -1.41   0.159    -.3657372     .059906
2017    -.2264816   .1120058    -2.02   0.043     -.446009   -.0069543
2018    -.2151943   .1162956    -1.85   0.064    -.4431294    .0127408
2019     -.319139   .1217926    -2.62   0.009     -.557848   -.0804299

region 
2    -.0053282   .6031277    -0.01   0.993    -1.187437     1.17678
3    -.0144088   .8435746    -0.02   0.986    -1.667785    1.638967
4      23.9593   29766.11     0.00   0.999    -58316.55    58364.47
5    -1.077494   .7862965    -1.37   0.171    -2.618607    .4636191
6    -.4071589   .8362864    -0.49   0.626     -2.04625    1.231932
7      .658063   1.002816     0.66   0.512     -1.30742    2.623546
8     .5285258   .8313712     0.64   0.525    -1.100932    2.157983
9     .0023187   .8976535     0.00   0.998     -1.75705    1.761687
10    -14.84499   1041.706    -0.01   0.989    -2056.552    2026.862
11     .9501312   .9967343     0.95   0.340    -1.003432    2.903694
12     .6816164   1.005483     0.68   0.498    -1.289094    2.652327
13     .8610184   1.057921     0.81   0.416    -1.212469    2.934506
14    -.2024885   1.044481    -0.19   0.846    -2.249633    1.844656
15    -1.178271   1.384005    -0.85   0.395     -3.89087    1.534328
16    -1.944648   1.316166    -1.48   0.140    -4.524285    .6349899

L.age_cate1 
2    -.3518934   .1504093    -2.34   0.019    -.6466902   -.0570965
3    -.3433212   .1950824    -1.76   0.078    -.7256756    .0390332
4    -.2346913   .2379139    -0.99   0.324     -.700994    .2316115

L.sex 
1            0  (omitted)

L.married 
1    -.1878148   .1241472    -1.51   0.130    -.4311388    .0555092

L.health_status1 
2     .4630734    .222156     2.08   0.037     .0276556    .8984911
3     .1842551   .2213972     0.83   0.405    -.2496754    .6181855
4     .0916305   .2234936     0.41   0.682     -.346409    .5296699
5     .2184561   .2366326     0.92   0.356    -.2453354    .6822476

years_of_education 
L1.   -.0569611   .0980777    -0.58   0.561    -.2491898    .1352676

n_of_children 
L1.   -.1095408   .0597734    -1.83   0.067    -.2266946     .007613

annual_working_hours 
L1.   -.0003309   .0000471    -7.02   0.000    -.0004233   -.0002385
Thank you for your help.

Alessandro