Having trouble with confidence bands on Kaplan-Meier curve

I am trying to add 95% confidence bands to kaplan meier curves. I have attached a sample data file. The code I am using is as follows:

datatable=sampledata;

timeDays =datatable.TimeDays;

probability = datatable.Cohort1SurvivalProbability;

lower =datatable.Cohort1SurvivalProbability95CILower;

upper =datatable.Cohort1SurvivalProbability95CIUpper;

timeDays = timeDays';

lower = lower';

upper = upper';

probability = probability';

plot(timeDays,probability,'b', 'LineWidth', 5);

xlabel('Time');

ylabel('Survival probability');

title('Kaplan-Meier curve with confidence bands');

hold on

%% upper and lower confidence bands (shaded)

patch([timeDays, fliplr(timeDays)], [lower, fliplr(upper)],'b','FaceAlpha',0.1,'EdgeColor', 'none');

%% second curve now

timeDays2 =datatable.TimeDays;

probability2 = datatable.Cohort2SurvivalProbability;

lower2 =datatable.Cohort1SurvivalProbability95CILower;

upper2 =datatable.Cohort2SurvivalProbability95CIUpper;

timeDays2 = timeDays2';

lower2 = lower2';

upper2 = upper2';

probability2 = probability2';

plot(timeDays2,probability2,'r', 'LineWidth', 5);

hold off

patch([timeDays2, fliplr(timeDays2)], [lower2, fliplr(upper2)],'r','FaceAlpha',0.1,'EdgeColor', 'none');

legend('Cohort 1','','Cohort 2');

The output I get through this code is also attached. As can be seen, the confidence band for cohort 2 is above the km curve for cohort 2, rather than being around it, as is the case for cohort 1. In the dataset, the upper and lower 95% confidence interval values are above and below the corresponding probability values, so I do not understand why the confidence band is coming out this way. I shall be highly grateful for any help in this regard.

Answer 1

[Star Strider]

You have one typographical error.

This assignment:

lower2 =datatable.Cohort1SurvivalProbability95CILower;

should instead be:

lower2 =datatable.Cohort2SurvivalProbability95CILower;

then it works!

The code —

F = openfig('kmcurve.fig');

datatable=readtable('sampledata.xlsx')

datatable = 2056×7 table
    TimeDays    Cohort1SurvivalProbability    Cohort1SurvivalProbability95CILower    Cohort1SurvivalProbability95CIUpper    Cohort2SurvivalProbability    Cohort2SurvivalProbability95CILower    Cohort2SurvivalProbability95CIUpper
    ________    __________________________    ___________________________________    ___________________________________    __________________________    ___________________________________    ___________________________________

        0                0.99928                            0.99807                                0.99973                           0.99711                            0.99529                                0.99823              
        1                0.99873                            0.99734                                 0.9994                           0.99711                            0.99529                                0.99823              
        2                0.99837                            0.99686                                0.99915                           0.99657                            0.99463                                0.99781              
        3                0.99764                            0.99594                                0.99863                           0.99639                            0.99441                                0.99767              
        4                0.99745                            0.99571                                0.99849                           0.99639                            0.99441                                0.99767              
        5                0.99691                            0.99503                                0.99807                           0.99639                            0.99441                                0.99767              
        6                0.99691                            0.99503                                0.99807                           0.99639                            0.99441                                0.99767              
        7                0.99691                            0.99503                                0.99807                           0.99621                            0.99419                                0.99753              
        8                0.99654                            0.99458                                0.99779                           0.99621                            0.99419                                0.99753              
        9                0.99635                            0.99435                                0.99765                           0.99621                            0.99419                                0.99753              
       10                0.99598                            0.99391                                0.99735                           0.99603                            0.99397                                0.99738              
       11                0.99561                            0.99346                                0.99706                           0.99603                            0.99397                                0.99738              
       12                0.99561                            0.99346                                0.99706                           0.99603                            0.99397                                0.99738              
       13                0.99543                            0.99324                                0.99691                           0.99585                            0.99376                                0.99724              
       14                0.99543                            0.99324                                0.99691                           0.99549                            0.99333                                0.99695              
       15                0.99543                            0.99324                                0.99691                           0.99549                            0.99333                                0.99695              

timeDays =datatable.TimeDays;
probability = datatable.Cohort1SurvivalProbability;
lower =datatable.Cohort1SurvivalProbability95CILower;
upper =datatable.Cohort1SurvivalProbability95CIUpper;
timeDays = timeDays';
lower = lower';
upper = upper';
probability = probability';
plot(timeDays,probability,'b', 'LineWidth', 5);
xlabel('Time');
ylabel('Survival probability');
title('Kaplan-Meier curve with confidence bands');
hold on
%% upper and lower confidence bands (shaded) 
patch([timeDays, fliplr(timeDays)], [lower, fliplr(upper)],'b','FaceAlpha',0.1,'EdgeColor', 'none');
%% second curve now
timeDays2 =datatable.TimeDays;
probability2 = datatable.Cohort2SurvivalProbability;
lower2 =datatable.Cohort2SurvivalProbability95CILower;
upper2 =datatable.Cohort2SurvivalProbability95CIUpper;
timeDays2 = timeDays2';
lower2 = lower2';
upper2 = upper2';
probability2 = probability2';
plot(timeDays2,probability2,'r', 'LineWidth', 5);
hold off
patch([timeDays2, fliplr(timeDays2)], [lower2, fliplr(upper2)],'r','FaceAlpha',0.1,'EdgeColor', 'none');
legend('Cohort 1','','Cohort 2');

.

[Muhammad]

Oh I missed that typo 😅 Thank-you so much!

[Star Strider]

As always, my pleasure!