Intraclass Correlation - Missing Values - SPSS

Hello,

I am attempting to run a mixed ICC (3,k)/(3,1) with multiple raters

from surveys. I am assessing the reliability of 6 raters, 12 subjects,

and 27 items. I want to assess reliability for each survey item

between raters, a problem exists in that I have missing values for certain unrated subjects (the same 6 raters rated 8 of the subjects, 5 of them rated 1, 4 rated 1, and 3 rated 2).

(see below for a sample item to be assessed for reliability)

I ran preliminary ICCs without with the missing values and SPSS (version 20.0) runs it, but how does it deal with the missing values? Am I artificially

inflating my reliability results (which I don't want to do)? Does SPSS

ignore those subjects altogether? Muller and Buttner (1994) state that

missing values makes a mixed model ICC an invalid reliability test, is this correct?

If so, I was going to substitute the highest and lowest extreme values in order to underestimate the true reliability value, thus creating a more conservative reliability value, however, I would rather not do this as I want a 'true' value for reliability. I hope this all makes sense, I have tried to be as clear as possible.

I am really trying to move forward in

my data analysis, so help would be extremely appreciated.

Sincerely,

Alex

EXAMPLE:

A B C D E F

A 1 2 1 2 2 2

B 1 2 1 1 1 1

C 4 2 4 4 5 6

D 5 6 5 5 4 5

E 7 5 6 7 7 7

F 4 2 1 - - -

G 3 2 2 - - -

H 3 3 3 - 3 -

I 6 4 6 6 6 6

J 1 3 1 - 2 -

K 4 3 1 5 7 5

L 7 6 7 6 - 5