# 18.6 - Concordance Correlation Coefficient for Measuring Agreement

18.6 - Concordance Correlation Coefficient for Measuring Agreement

How well do two diagnostic measurements agree? Many times continuous units of measurement are used in the diagnostic test. We may not be interested in correlation or linear relationship between the two measures, but in a measure of agreement.

The concordance correlation coefficient, $$r_c$$ , for measuring agreement between continuous variables X and Y (both approximately normally distributed), is calculated as follows:

$$r_c=\dfrac{2S_{XY}}{S_{XX}+S_{YY}+(\bar{X}-\bar{Y})^2}$$

Similar to the other correlation coefficient, the concordance correlation satisfies $$-1 ≤ r_c ≤ +1$$. A value of $$r_c = +1$$ corresponds to perfect agreement. A value of $$r_c = - 1$$ corresponds to perfect negative agreement, and a value of $$r_c = 0$$ corresponds to no agreement. The sample estimate, $$r_c$$ , is an estimate of the population concordance correlation coefficient:

$$\rho_c=\dfrac{2\sigma_{XY}}{\sigma_{XX}+\sigma_{YY}+(\mu_{X}-\mu_{Y})^2}$$

Let's look at an example that will help to make this concept clearer.

## SAS® Example

### SAS PROC FREQ option for constructing Cohen's kappa and weighted kappa statistics

*******************************************************************************
*  This program indicates how to construct a bivariate scatterplot with an    *
*  overlay of the line of identity.                                           *
*                                                                             *
*  This program also provides an IML module for calculating point and         *
*  interval estimates of the Pearson correlation coefficient and the          *
*  concordance correlation coefficient.                                       *
*******************************************************************************;

data dice_baseline;
input subject cort_auc1 cort_auc2;
cards;
61001    5.28170    5.37851
61002    5.58796    5.33628
61003    6.47607    6.59770
61005    6.36019    6.39746
61007    5.81121    5.82528
61008    6.03036    6.21147
61009    5.84549    6.10434
61014    6.80349    6.90689
61015    6.28977    6.27369
61023    5.88446    5.94352
61024    5.79701    6.04876
61026    5.01302    4.72154
61027    6.48824    6.37891
61028    5.30862    5.53405
61033    5.70905    5.77803
61034    5.98545    5.77613
61035    6.19924    6.20880
61036    6.00639    5.98313
61037    6.27793    6.44342
61038    6.57390    6.62936
61039    5.69639    5.43509
61042    5.96588    6.01282
61044    6.04803    6.11529
61046    6.39423    6.03876
62002    7.44584    7.58421
62003    5.90813    5.87230
62004    6.05483    5.94695
62005    5.65735    5.64983
62006    6.44815    6.44280
62007    6.28611    6.45374
62009    6.40863    6.14994
62012    5.62564    5.58142
62013    6.68375    6.58815
62014    5.76951    5.84802
62015    5.94383    5.95489
62016    5.66024    5.73711
62017    4.77492    4.57465
62018    5.60468    5.43495
62019    6.82819    6.86652
62020    5.18986    5.05725
62021    6.48810    6.59655
62022    6.08867    5.76965
62023    5.91400    5.89672
62024    5.58217    5.57651
62026    6.32857    6.48921
62027    7.67703    7.76541
62028    5.92411    5.66689
62029    6.15313    6.16558
62030    5.20392    5.42481
62032    6.43962    6.46171
62033    6.20661    6.28542
63001    6.04767    5.82528
63002    6.46923    6.51728
63003    5.68370    5.79701
63004    5.11719    5.47363
63005    6.10993    6.13541
63006    4.91744    5.06968
63008    5.35972    5.56605
63010    6.73016    6.76285
63011    5.93700    5.94092
63012    6.07716    5.92548
63014    6.58185    6.52781
63015    5.84317    5.85030
63016    5.98144    6.22389
63017    5.77452    5.81662
63018    5.46142    5.84898
63021    5.44920    5.43688
63022    5.96519    6.01302
63026    6.29258    6.42339
63027    6.86608    6.92806
63028    5.47875    5.72634
63030    6.16190    6.16608
63032    5.66707    5.97114
63033    5.80634    5.63640
63034    6.37256    6.24416
63035    5.65755    5.98070
64001    6.07596    6.06010
64002    6.57898    6.54552
64003    6.60733    6.89724
64004    5.69611    5.82963
64005    6.60331    6.62972
64007    5.89963    5.83195
64008    6.19731    6.07044
64009    5.88875    6.03427
64010    5.64912    5.46135
64011    6.99962    7.10324
64012    6.61282    6.68520
64015    6.60477    6.76468
64016    5.35161    5.55307
64017    6.63249    6.77868
64020    5.37717    5.19760
64023    5.58781    5.87044
64025    5.74499    5.77090
64027    5.92655    6.09265
64028    5.01060    5.17112
64030    7.12400    7.17368
64031    5.79909    5.37603
64032    5.75609    5.85174
64033    6.79275    6.78255
64035    6.02198    6.03915
64036    5.43960    5.82229
64037    5.20163    5.12928
65001    6.18838    6.66593
65002    6.13860    6.26224
65003    6.98807    6.97658
65005    5.54628    5.50547
65006    4.47249    4.59256
65007    5.04034    5.07775
65008    5.42025    5.46227
65009    5.26772    5.41463
65011    6.43019    6.38438
65013    6.56323    6.46607
65014    5.06134    4.70619
65016    6.32666    6.31564
65017    5.90235    6.05890
65018    6.05800    5.99467
65020    5.96388    6.01204
65021    5.57324    5.61324
65022    6.21017    6.15262
65025    6.01934    6.00318
65026    5.52082    5.54575
65027    5.89237    5.67469
65028    6.24592    6.37106
65031    6.31524    6.44334
65032    6.19602    6.29576
65033    6.07305    6.07966
65037    5.60960    5.38492
65038    5.39806    5.18466
65040    5.95464    6.20802
66003    5.96880    5.86128
66004    5.89707    5.69116
66005    6.27067    6.35294
66006    5.39744    5.23236
66010    6.64408    6.64990
66011    6.29430    6.32724
66012    5.22507    5.28754
66013    5.15840    5.05580
66014    5.85385    5.54974
66018    6.49611    5.87795
66019    5.43285    5.50871
66021    6.32416    6.31612
66023    5.77253    5.74469
66024    5.89920    5.95774
;
run;

proc means data=dice_baseline noprint;
var cort_auc1 cort_auc2;
output out=dice_baselinemin min=var1 var2;
run;

proc means data=dice_baseline noprint;
var cort_auc1 cort_auc2;
output out=dice_baselinemax max=var1 var2;
run;

data dice_baselineall;
set dice_baselinemin dice_baselinemax;
drop _type_ _freq_;
if _N_=1 then var1=floor(min(var1,var2));
if _N_=1 then var2=floor(min(var1,var2));
if _N_=2 then var1=ceil(max(var1,var2));
if _N_=2 then var2=ceil(max(var1,var2));
run;

data dice_baselineall;
set dice_baseline dice_baselineall;
run;

proc gplot uniform data=dice_baselineall;
plot cort_auc1*cort_auc2 var1*var2/overlay vaxis=axis1 haxis=axis2 nolegend frame;
axis1 label=(a=90 'Cortisol Every Hour') minor=none;
axis2 label=('Cortisol Every Two Hours') minor=none;
symbol1 value=star color=black interpol=none;
symbol2 value=none color=black interpol=join;
title "Concordance Correlation Coefficient";
run;

proc iml;
*******************************************************************************
*  Enter the appropriate SAS data set name in the use statement and enter the *
*  appropriate variable names in the read statements.                         *
*******************************************************************************;
use dice_baseline;
read all var {cort_auc1} into var1;
read all var {cort_auc2} into var2;
*******************************************************************************
*  The IML module, labeled concorr, starts next.                              *
*******************************************************************************;
start concorr;
nonmiss=loc(var1#var2^=.);
var1=var1[nonmiss];
var2=var2[nonmiss];
free nonmiss;
n=nrow(var1);
mu1=sum(var1)/n;
mu1=round(mu1,0.0001);
mu2=sum(var2)/n;
mu2=round(mu2,0.0001);
sigma11=ssq(var1-mu1)/(n-1);
sigma11=round(sigma11,0.0001);
sigma22=ssq(var2-mu2)/(n-1);
sigma22=round(sigma22,0.0001);
sigma12=sum((var1-mu1)#(var2-mu2))/(n-1);
sigma12=round(sigma12,0.0001);
lshift=(mu1-mu2)/((sigma11#sigma22)##0.25);
rho=sigma12/sqrt(sigma11#sigma22);
rho=round(rho,0.0001);
z=log((1+rho)/(1-rho))/2;
se_z=1/sqrt(n-3);
t=tinv(0.975,n-3);
z_low=z-(se_z#t);
z_upp=z+(se_z#t);
rho_low=(exp(2#z_low)-1)/(exp(2#z_low)+1);
rho_low=round(rho_low,0.0001);
rho_upp=(exp(2#z_upp)-1)/(exp(2#z_upp)+1);
rho_upp=round(rho_upp,0.0001);
crho=(2#sigma12)/((sigma11+sigma22)+((mu1-mu2)##2));
crho=round(crho,0.0001);
z=log((1+crho)/(1-crho))/2;
if sigma12^=0 then do;
t1=((1-(rho##2))#(crho##2))/((1-(crho##2))#(rho##2));
t2=(2#(crho##3)#(1-crho)#(lshift##2))/(rho#((1-(crho##2))##2));
t3=((crho##4)#(lshift##4))/(2#(rho##2)#((1-(crho##2))##2));
se_z=sqrt((t1+t2-t3)/(n-2));
end;
else se_z=sqrt(2#sigma11#sigma22)/((sigma11+sigma22+((mu1-mu2)##2))#(n-2));
t=tinv(0.975,n-2);
z_low=z-(se_z#t);
z_upp=z+(se_z#t);
crho_low=(exp(2#z_low)-1)/(exp(2#z_low)+1);
crho_low=round(crho_low,0.0001);
crho_upp=(exp(2#z_upp)-1)/(exp(2#z_upp)+1);
crho_upp=round(crho_upp,0.0001);
Results=n//mu1//mu2//sigma11//sigma22//sigma12//rho_low//rho//rho_upp//
crho_low//crho//crho_upp;
r_name={'SampleSize' 'Mean_1' 'Mean_2' 'Variance_1' 'Variance_2' 'Covariance' 'Corr LowerCL'
'Corr' 'Corr UpperCL' 'ConcCorr LowerCL' 'ConcCorr' 'ConcCorr UpperCL'};
print 'The Estimated Correlation and Concordance Correlation (and 95% Confidence Limits)';
print Results [rowname=r_name];
finish concorr;
*******************************************************************************
*  The IML module, labeled concorr, is finished.                              *
*******************************************************************************;
run concorr;


The ACRN DICE trial was discussed earlier in this course. In that trial, participants underwent hourly blood draws between 08:00 PM and 08:00 AM once a week in order to determine the cortisol area-under-the-curve (AUC). The participants hated this! They complained about the sleep disruption every hour when the nurses came by to draw blood, so the ACRN wanted to determine for future studies if the cortisol AUC calculated on measurements every two hours was in good agreement with the cortisol AUC calculated on hourly measurements. The baseline data were used to investigate how well these two measurements agreed. If there is good agreement, the protocol could be changed to take blood every two hours.

Note for this SAS program - Run the program to view the output. This is higher level SAS than you are expected to program yourself in this course, but some of you may find the programming of interest.

The SAS program yielded $$r_c = 0.95$$ and a 95% confidence interval = (0.93, 0.96). The ACRN judged this to be excellent agreement, so it will use two-hourly measurements in future studies.

What about binary or ordinal data? Cohen's Kappa Statistic will handle this...

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