Dear statalist experts,
I am doing research on a 205 item mental health questionnaire with a 6-point Likert-scale. When I run a GRM model on a subscale consisting of 18 items, difficulty parameters of certain items (such as the first 4 items) are missing in the output, specifically those corresponding to the 1-answer category. The '1>=' lines are absent. For some reason, the difficuly values of the 1-answer category are present on other items (such as the 5th and 6th item). See my code and output below.
Is this a familiar problem? Is there any way it can be solved? I've visually checked the distribution of answers per items in the data viewer and it's not that there are significantly less categroy 1 responses on the problematic items.
Kind regards,
Leon
I am doing research on a 205 item mental health questionnaire with a 6-point Likert-scale. When I run a GRM model on a subscale consisting of 18 items, difficulty parameters of certain items (such as the first 4 items) are missing in the output, specifically those corresponding to the 1-answer category. The '1>=' lines are absent. For some reason, the difficuly values of the 1-answer category are present on other items (such as the 5th and 6th item). See my code and output below.
Is this a familiar problem? Is there any way it can be solved? I've visually checked the distribution of answers per items in the data viewer and it's not that there are significantly less categroy 1 responses on the problematic items.
Kind regards,
Leon
Code:
. irt grm YSQ2VeIn YSQ15VeIn YSQ16VeIn YSQ18VeIn YSQ33VeIn YSQ42VeIn YSQ53VeIn YSQ61VeIn YSQ81VeIn YSQ90VeIn YSQ101VeIn YSQ104VeIn YSQ114VeIn YSQ115VeIn YSQ123VeIn YSQ124VeIn YSQ132VeIn YSQ194VeIn
Fitting fixed-effects model:
Iteration 0: Log likelihood = -5799.4164
Iteration 1: Log likelihood = -5786.6811
Iteration 2: Log likelihood = -5785.9291
Iteration 3: Log likelihood = -5785.9242
Iteration 4: Log likelihood = -5785.9242
Fitting full model:
Iteration 0: Log likelihood = -5349.9268
Iteration 1: Log likelihood = -5222.5521
Iteration 2: Log likelihood = -5212.816
Iteration 3: Log likelihood = -5212.5648
Iteration 4: Log likelihood = -5212.5639
Iteration 5: Log likelihood = -5212.5639
Graded response model Number of obs = 207
Log likelihood = -5212.5639
------------------------------------------------------------------------------
| Coefficient Std. err. z P>|z| [95% conf. interval]
-------------+----------------------------------------------------------------
YSQ2VeIn |
Discrim | 1.183237 .173658 6.81 0.000 .8428741 1.523601
Diff |
>=2 | -1.341861 .2295204 -1.791713 -.8920096
>=3 | -.5377289 .1642807 -.859713 -.2157447
>=4 | .384103 .1525669 .0850773 .6831287
>=5 | 1.099151 .1964062 .7142022 1.4841
=6 | 2.010574 .2973895 1.427702 2.593447
-------------+----------------------------------------------------------------
YSQ15VeIn |
Discrim | 1.825097 .2534155 7.20 0.000 1.328412 2.321783
Diff |
>=2 | -.0142853 .117554 -.2446869 .2161162
>=3 | .6522974 .1301012 .3973038 .907291
>=4 | 1.164916 .1646363 .8422353 1.487598
>=5 | 1.764075 .2256164 1.321875 2.206275
=6 | 2.25073 .2904651 1.681429 2.820031
-------------+----------------------------------------------------------------
YSQ16VeIn |
Discrim | 1.215856 .191247 6.36 0.000 .8410192 1.590694
Diff |
>=2 | -.3408849 .1546366 -.643967 -.0378028
>=3 | .4065747 .1511872 .1102533 .7028961
>=4 | 1.223495 .2115162 .8089306 1.638059
>=5 | 1.899616 .2920508 1.327207 2.472025
=6 | 2.427564 .3698693 1.702634 3.152495
-------------+----------------------------------------------------------------
YSQ18VeIn |
Discrim | 1.102743 .1975583 5.58 0.000 .7155362 1.489951
Diff |
>=2 | .3793554 .1615392 .0627445 .6959664
>=3 | 1.390311 .2571968 .8862143 1.894407
>=4 | 2.224517 .3806155 1.478524 2.970509
>=5 | 2.778096 .4768052 1.843575 3.712617
=6 | 3.24428 .5680259 2.13097 4.357591
-------------+----------------------------------------------------------------
YSQ33VeIn |
Discrim | 1.038255 .1652066 6.28 0.000 .7144556 1.362054
Diff |
>=1 | -5.659958 1.243188 -8.096561 -3.223354
>=2 | -.932624 .2129297 -1.349958 -.5152895
>=3 | -.0103438 .1615971 -.3270682 .3063807
>=4 | .7272172 .1834156 .3677291 1.086705
>=5 | 1.352922 .2414296 .8797286 1.826115
=6 | 2.247015 .3602571 1.540924 2.953106
-------------+----------------------------------------------------------------
YSQ42VeIn |
Discrim | 2.044644 .2624544 7.79 0.000 1.530243 2.559045
Diff |
>=1 | -3.560313 .5849178 -4.706731 -2.413895
>=2 | -.1667897 .1139891 -.3902041 .0566248
>=3 | .2210447 .110601 .0042708 .4378186
>=4 | .869207 .1339057 .6067567 1.131657
>=5 | 1.403864 .1723061 1.06615 1.741578
=6 | 1.911851 .2210754 1.478551 2.34515
-------------+----------------------------------------------------------------
YSQ53VeIn |
Discrim | .7244334 .1562731 4.64 0.000 .4181438 1.030723
Diff |
>=2 | -1.396122 .3586703 -2.099103 -.6931414
>=3 | -.6123809 .2510312 -1.104393 -.1203688
>=4 | .2070495 .2127041 -.2098429 .6239419
>=5 | 1.124761 .299138 .5384618 1.711061
=6 | 1.826042 .4175896 1.007581 2.644502
-------------+----------------------------------------------------------------
YSQ61VeIn |
Discrim | 1.289985 .1854585 6.96 0.000 .9264929 1.653477
Diff |
>=2 | -1.005695 .1869903 -1.372189 -.6392012
>=3 | .0463646 .138964 -.2259999 .318729
>=4 | .9736455 .1770352 .6266628 1.320628
>=5 | 1.507507 .2263168 1.063934 1.95108
=6 | 2.524449 .3604533 1.817974 3.230925
-------------+----------------------------------------------------------------
YSQ81VeIn |
Discrim | 1.830451 .2347685 7.80 0.000 1.370313 2.290589
Diff |
>=1 | -3.783272 .6512713 -5.05974 -2.506803
>=2 | -.4258343 .1277222 -.6761653 -.1755033
>=3 | .1066359 .1150072 -.1187741 .3320458
>=4 | .6188004 .1258847 .3720709 .8655299
>=5 | 1.229492 .166286 .9035778 1.555407
=6 | 1.689562 .2095793 1.278794 2.10033
-------------+----------------------------------------------------------------
YSQ90VeIn |
Discrim | 1.530546 .2124941 7.20 0.000 1.114065 1.947026
Diff |
>=1 | -4.235931 .7930572 -5.790295 -2.681567
>=2 | -.1293133 .1306298 -.3853431 .1267165
>=3 | .5265063 .1347078 .2624838 .7905288
>=4 | 1.301341 .1852603 .9382377 1.664445
>=5 | 1.680185 .2238568 1.241433 2.118936
=6 | 2.076679 .2731548 1.541306 2.612053
-------------+----------------------------------------------------------------
YSQ101VeIn |
Discrim | 2.365729 .327421 7.23 0.000 1.723995 3.007462
Diff |
>=2 | .2161075 .1053766 .0095731 .4226419
>=3 | .6761871 .1162671 .4483078 .9040663
>=4 | 1.150992 .1444594 .8678573 1.434128
>=5 | 1.463479 .1685939 1.133041 1.793917
=6 | 1.954179 .2189915 1.524963 2.383394
-------------+----------------------------------------------------------------
YSQ104VeIn |
Discrim | .994189 .1735118 5.73 0.000 .6541121 1.334266
Diff |
>=2 | -.5534536 .1901167 -.9260756 -.1808316
>=3 | .1361842 .1668357 -.1908078 .4631762
>=4 | 1.288854 .2574478 .7842661 1.793443
>=5 | 1.942934 .3429464 1.270771 2.615096
=6 | 2.801484 .4803396 1.860036 3.742932
-------------+----------------------------------------------------------------
YSQ114VeIn |
Discrim | 1.533086 .2127089 7.21 0.000 1.116185 1.949988
Diff |
>=2 | -.3201126 .1362505 -.5871586 -.0530666
>=3 | .4273028 .1307011 .1711334 .6834723
>=4 | .9508142 .158449 .6402598 1.261369
>=5 | 1.708071 .2273834 1.262408 2.153734
=6 | 2.12101 .2785885 1.574986 2.667033
-------------+----------------------------------------------------------------
YSQ115VeIn |
Discrim | 1.843115 .2366591 7.79 0.000 1.379272 2.306959
Diff |
>=2 | -.3919885 .1259348 -.6388162 -.1451608
>=3 | .2705601 .1160835 .0430406 .4980796
>=4 | .8056327 .1345567 .5419063 1.069359
>=5 | 1.41964 .1793796 1.068063 1.771218
=6 | 1.968522 .2362022 1.505574 2.43147
-------------+----------------------------------------------------------------
YSQ123VeIn |
Discrim | 1.742005 .2342453 7.44 0.000 1.282893 2.201118
Diff |
>=1 | -3.893444 .6896771 -5.245187 -2.541702
>=2 | -.0837331 .1213895 -.3216522 .154186
>=3 | .6852576 .1334462 .4237079 .9468073
>=4 | 1.310572 .174864 .9678453 1.6533
>=5 | 1.821536 .2249975 1.380549 2.262523
=6 | 2.378922 .2992909 1.792322 2.965521
-------------+----------------------------------------------------------------
YSQ124VeIn |
Discrim | 1.676639 .2250125 7.45 0.000 1.235622 2.117655
Diff |
>=1 | -3.971831 .7171216 -5.377364 -2.566298
>=2 | -.2009464 .1263081 -.4485058 .046613
>=3 | .5397328 .1280314 .2887958 .7906698
>=4 | 1.09208 .1588216 .7807954 1.403365
>=5 | 1.705632 .2153345 1.283585 2.12768
=6 | 2.410922 .3094841 1.804345 3.0175
-------------+----------------------------------------------------------------
YSQ132VeIn |
Discrim | 1.296905 .2101175 6.17 0.000 .8850825 1.708728
Diff |
>=1 | -4.749652 .9863589 -6.68288 -2.816424
>=2 | .3556579 .1451268 .0712145 .6401012
>=3 | 1.150485 .2000682 .7583582 1.542611
>=4 | 1.750085 .2713601 1.218229 2.281941
>=5 | 2.131524 .3253469 1.493856 2.769192
=6 | 2.78632 .4345595 1.934599 3.638041
-------------+----------------------------------------------------------------
YSQ194VeIn |
Discrim | 2.133397 .2674486 7.98 0.000 1.609207 2.657586
Diff |
>=2 | -.4302575 .1191145 -.6637177 -.1967973
>=3 | .2965771 .1102453 .0805004 .5126539
>=4 | .8296664 .1286848 .5774489 1.081884
>=5 | 1.128304 .1459861 .8421761 1.414431
=6 | 1.616986 .1857674 1.252889 1.981084
------------------------------------------------------------------------------
