Hallo,

I am working with ordinal scales for cerebral atrophy (ratings 0=no atrophy

to 4=severe atrophy) and vascular burden (ratings 0=no burden to 3=severe

burden) assessed by two raters and I want to calculate intra- and

inter-rater reliability. I am having a few problems:

1) Since I am dealing with an ordinal scale I have decided to use weighted

kappa. I have installed the extension bundle for weighted kappa from SPSS

(version 25) but I keep getting the error message "some ratings are less

than one this command is not executed". I have looked through the threads in

stats forums and in the Internet but as far as I can see nobody has come up

with this issue. I know from the literature that intra- and inter-rater

reliability are commonly calculated for these scales using weighted kappa.

Also, weighted kappa seems to be applicable to ratings of zero

inSPSS(e.g.http://www.spssusers.co.uk/Events/2015/ALDRIDGE2015.pdf). The

only solution I see is recoding the ratings to avoid the value of zero but,

since I would have to keep in my dataset (which is very large!) the original

ratings (which are the ones that are clinically meaningful), I would end up

having so many variables that it would all get very confusing and it seems a

pity, especially if the error message is due to me making some mistake I am

unaware of.

2) I then thought I would see if one can use ratings of zero with the IBM

SPSS syntax for weighted kappa

(

http://www-01.ibm.com/support/docview.wss?uid=swg21477357). I first tried

the syntax out on a simplified "dummy dataset" in which ratings go from 1 to

4 instead of 0 to 4, with rater 1 and rater 2 rating atrophy on the right

and left side of the brain and 4 variables (atrophyright1, atrophyright2,

atrophyleft1, atrophyleft2). However, I never got to the next step of trying

with ratings of zero because I keep getting error messages. I have trouble

understanding the syntax and I must be putting variables in the wrong place.

I have looked at other examples of syntax for weighted kappa

(

http://www.spssusers.co.uk/Events/2015/ALDRIDGE2015.pdf, Valiquette et al

1994 Computing kappa coefficients using SPSS matrix) but it hasn't helped me

get it right.

I have used the following syntax for inter-rater reproducibility for atrophy

on the right side of the brain and my ratings matrix is underneath here. I

am showing in italics the bits I put in the syntax.

Rater 1

1 2 3 4

1 3 2 1 1

2 2 4 2 0 Rater 2

3 1 2 3 2

4 1 2 2 3

GET x /var=/atrophyright1 atrophyright 2/

DATA LIST LIST / /x1 to x4/

BEGIN DATA

/3211

2420

1232

1223/

END DATA.

matrix.

GET x /var=/x1 to x4/.

compute wt1=make(nrow(x),ncol(x),0).

compute wt2=make(nrow(x),ncol(x),0).

compute prop=x/msum(x).

loop i=1 to nrow(x).

loop j=1 to ncol(x).

compute wt1(i,j)=1-(abs(i-j)/(nrow(x)-1)).

compute wt2(i,j)=1-((i-j)/(nrow(x)-1))**2.

end loop.

end loop.

compute

wk1num=msum(wt1&*prop)-msum(mdiag(rsum(prop))*wt1*mdiag(csum(prop))).

compute wk1den=1-msum(mdiag(rsum(prop))*wt1*mdiag(csum(prop))).

compute wk1=wk1num/wk1den.

print wk1.

compute

wk2num=msum(wt2&*prop)-msum(mdiag(rsum(prop))*wt2*mdiag(csum(prop))).

compute wk2den=1-msum(mdiag(rsum(prop))*wt2*mdiag(csum(prop))).

compute wk2=wk2num/wk2den.

print wk2.

end matrix.

Thanks in advance for any help you can give me!

Paola

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