Description Usage Arguments Details Value Author(s) See Also Examples

View source: R/quantile.survfit.R

Retrieve quantiles and confidence intervals for them from a survfit object.

1 2 3 4 5 6 |

`x` |
a result of the survfit function |

`probs` |
numeric vector of probabilities with values in [0,1] |

`conf.int` |
should lower and upper confidence limits be returned? |

`scale` |
optional scale factor, e.g., |

`tolerance` |
tolerance for checking that the survival curve exactly equals one of the quantiles |

`...` |
optional arguments for other methods |

The kth quantile for a survival curve S(t) is the location at which a horizontal line at height p= 1-k intersects the plot of S(t). Since S(t) is a step function, it is possible for the curve to have a horizontal segment at exactly 1-k, in which case the midpoint of the horizontal segment is returned. This mirrors the standard behavior of the median when data is uncensored. If the survival curve does not fall to 1-k, then that quantile is undefined.

In order to be consistent with other quantile functions, the argument
`prob`

of this function applies to the cumulative distribution
function F(t) = 1-S(t).

Confidence limits for the values are based on the intersection of the
horizontal line at 1-k with the upper and lower limits for the
survival curve. Hence confidence limits use the same
p-value as was in effect when the curve was created, and will differ
depending on the `conf.type`

option of `survfit`

.
If the survival curves have no confidence bands, confidence limits for
the quantiles are not available.

When a horizontal segment of the survival curve exactly matches one of
the requested quantiles the returned value will be the midpoint of the
horizontal segment; this agrees with the usual definition of a median
for uncensored data. Since the survival curve is computed as a series
of products, however, there may be round off error.
Assume for instance a sample of size 20 with no tied times and no
censoring. The survival curve after the 10th death is
(19/20)(18/19)(17/18) ... (10/11) = 10/20, but the computed result will
not be exactly 0.5. Any horizontal segment whose absolute difference
with a requested percentile is less than `tolerance`

is
considered to be an exact match.

The quantiles will be a vector if the `survfit`

object contains
only a single curve, otherwise it will be a matrix or array. In
this case the last dimension will index the quantiles.

If confidence limits are requested, then result will be a list with
components
`quantile`

, `lower`

, and `upper`

, otherwise it is the
vector or matrix of quantiles.

Terry Therneau

`survfit`

, `print.survfit`

,
`qsurvreg`

1 2 3 4 5 6 7 8 9 | ```
fit <- survfit(Surv(time, status) ~ ph.ecog, data=lung)
quantile(fit)
cfit <- coxph(Surv(time, status) ~ age + strata(ph.ecog), data=lung)
csurv<- survfit(cfit, newdata=data.frame(age=c(40, 60, 80)),
conf.type ="none")
temp <- quantile(csurv, 1:5/10)
temp[2,3,] # quantiles for second level of ph.ecog, age=80
quantile(csurv[2,3], 1:5/10) # quantiles of a single curve, same result
``` |

```
$quantile
25 50 75
ph.ecog=0 285 394 655
ph.ecog=1 181 306 550
ph.ecog=2 105 199 351
ph.ecog=3 118 118 118
$lower
25 50 75
ph.ecog=0 189 348 558
ph.ecog=1 156 268 460
ph.ecog=2 61 156 285
ph.ecog=3 NA NA NA
$upper
25 50 75
ph.ecog=0 350 574 NA
ph.ecog=1 223 429 689
ph.ecog=2 163 288 654
ph.ecog=3 NA NA NA
10 20 30 40 50
92 144 181 218 270
10 20 30 40 50
92 144 181 218 270
```

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