Calculates weighted concordance statistics, which, depending on the chosen
weighting method (`weight_meth`

) and tied times parameter (`tiex`

), are
equivalent to several proposed methods.
By default, no weighting is applied and this is equivalent to Harrell's C-index.

## Details

For the Kaplan-Meier estimate of the **training survival** distribution (\(S\)),
and the Kaplan-Meier estimate of the **training censoring** distribution (\(G\)),
we have the following options for time-independent concordance statistics
(C-indexes) given the weighted method:

`weight_meth`

:

`"I"`

= No weighting. (Harrell)`"GH"`

= Gonen and Heller's Concordance Index`"G"`

= Weights concordance by \(1/G\).`"G2"`

= Weights concordance by \(1/G^2\). (Uno et al.)`"SG"`

= Weights concordance by \(S/G\) (Shemper et al.)`"S"`

= Weights concordance by \(S\) (Peto and Peto)

The last three require training data. `"GH"`

is only applicable to LearnerSurvCoxPH.

The implementation is slightly different from survival::concordance. Firstly this implementation is faster, and secondly the weights are computed on the training dataset whereas in survival::concordance the weights are computed on the same testing data.

## Dictionary

This Measure can be instantiated via the dictionary mlr_measures or with the associated sugar function msr():

## Parameters

Id | Type | Default | Levels | Range |

t_max | numeric | - | \([0, \infty)\) | |

p_max | numeric | - | \([0, 1]\) | |

weight_meth | character | I | I, G, G2, SG, S, GH | - |

tiex | numeric | 0.5 | \([0, 1]\) | |

eps | numeric | 0.001 | \([0, 1]\) |

## Parameter details

`eps`

(`numeric(1)`

)

Very small number to substitute zero values in order to prevent errors in e.g. log(0) and/or division-by-zero calculations. Default value is 0.001.

`t_max`

(`numeric(1)`

)

Cutoff time (i.e. time horizon) to evaluate concordance up to.`p_max`

(`numeric(1)`

)

The proportion of censoring to evaluate concordance up to in the given dataset. When`t_max`

is specified, this parameter is ignored.`weight_meth`

(`character(1)`

)

Method for weighting concordance. Default`"I"`

is Harrell's C. See details.`tiex`

(`numeric(1)`

)

Weighting applied to tied rankings, default is to give them half (0.5) weighting.

## References

Peto, Richard, Peto, Julian (1972).
“Asymptotically efficient rank invariant test procedures.”
*Journal of the Royal Statistical Society: Series A (General)*, **135**(2), 185–198.

Harrell, E F, Califf, M R, Pryor, B D, Lee, L K, Rosati, A R (1982).
“Evaluating the yield of medical tests.”
*Jama*, **247**(18), 2543–2546.

Goenen M, Heller G (2005).
“Concordance probability and discriminatory power in proportional hazards regression.”
*Biometrika*, **92**(4), 965–970.
doi:10.1093/biomet/92.4.965
.

Schemper, Michael, Wakounig, Samo, Heinze, Georg (2009).
“The estimation of average hazard ratios by weighted Cox regression.”
*Statistics in Medicine*, **28**(19), 2473–2489.
doi:10.1002/sim.3623
.

Uno H, Cai T, Pencina MJ, D'Agostino RB, Wei LJ (2011).
“On the C-statistics for evaluating overall adequacy of risk prediction procedures with censored survival data.”
*Statistics in Medicine*, n/a–n/a.
doi:10.1002/sim.4154
.

## See also

Other survival measures:
`mlr_measures_surv.calib_alpha`

,
`mlr_measures_surv.calib_beta`

,
`mlr_measures_surv.chambless_auc`

,
`mlr_measures_surv.dcalib`

,
`mlr_measures_surv.graf`

,
`mlr_measures_surv.hung_auc`

,
`mlr_measures_surv.intlogloss`

,
`mlr_measures_surv.logloss`

,
`mlr_measures_surv.mae`

,
`mlr_measures_surv.mse`

,
`mlr_measures_surv.nagelk_r2`

,
`mlr_measures_surv.oquigley_r2`

,
`mlr_measures_surv.rcll`

,
`mlr_measures_surv.rmse`

,
`mlr_measures_surv.schmid`

,
`mlr_measures_surv.song_auc`

,
`mlr_measures_surv.song_tnr`

,
`mlr_measures_surv.song_tpr`

,
`mlr_measures_surv.uno_auc`

,
`mlr_measures_surv.uno_tnr`

,
`mlr_measures_surv.uno_tpr`

,
`mlr_measures_surv.xu_r2`

## Super classes

`mlr3::Measure`

-> `mlr3proba::MeasureSurv`

-> `MeasureSurvCindex`

## Methods

## Inherited methods

### Method `new()`

This is an abstract class that should not be constructed directly.

#### Usage

`MeasureSurvCindex$new()`

## Examples

```
library(mlr3)
task = tsk("rats")
learner = lrn("surv.coxph")
part = partition(task) # train/test split, stratified on `status` by default
learner$train(task, part$train)
p = learner$predict(task, part$test)
# Harrell's C-index
p$score(msr("surv.cindex")) # same as `p$score()`
#> surv.cindex
#> 0.7620216
# Uno's C-index
p$score(msr("surv.cindex", weight_meth = "G2"),
task = task, train_set = part$train)
#> surv.cindex
#> 0.8030636
# Harrell's C-index evaluated up to a specific time horizon
p$score(msr("surv.cindex", t_max = 97))
#> surv.cindex
#> 0.7439024
# Harrell's C-index evaluated up to the time corresponding to 30% of censoring
p$score(msr("surv.cindex", p_max = 0.3))
#> surv.cindex
#> 0.7439024
```