Install via pip:

pip install lifelines

Kaplan-Meier Nelson-Aalen, and parametric models


For readers looking for an introduction to survival analysis, it’s recommended to start at Introduction to survival analysis

Let’s start by importing some data. We need the durations that individuals are observed for, and whether they “died” or not.

from lifelines.datasets import load_waltons
df = load_waltons() # returns a Pandas DataFrame

    T  E    group
0   6  1  miR-137
1  13  1  miR-137
2  13  1  miR-137
3  13  1  miR-137
4  19  1  miR-137

T = df['T']
E = df['E']

T is an array of durations, E is a either boolean or binary array representing whether the “death” was observed or not (alternatively an individual can be censored).


lifelines assumes all “deaths” are observed unless otherwise specified.

from lifelines import KaplanMeierFitter
kmf = KaplanMeierFitter(), event_observed=E)  # or, more succinctly,, E)

After calling the fit method, we have access to new properties like survival_function_ and methods like plot(). The latter is a wrapper around Panda’s internal plotting library.

kmf.plot_survival_function() # or just kmf.plot()

Alternatively, you can plot the cumulative density function:


By specifying the timeline keyword argument in fit, we can change how the above models are indexed:, E, timeline=range(0, 100, 2))

kmf.survival_function_   # index is now the same as range(0, 100, 2)
kmf.confidence_interval_ # index is now the same as range(0, 100, 2)

Instead of the Kaplan-Meier estimator, you may be interested in a parametric model. lifelines has builtin parametric models. For example, Weibull, Log-Normal, Log-Logistic, and more.

from lifelines import *

fig, axes = plt.subplots(2, 3, figsize=(9, 5))

kmf = KaplanMeierFitter().fit(T, E, label='KaplanMeierFitter')
wbf = WeibullFitter().fit(T, E, label='WeibullFitter')
exf = ExponentialFitter().fit(T, E, label='ExponentalFitter')
lnf = LogNormalFitter().fit(T, E, label='LogNormalFitter')
llf = LogLogisticFitter().fit(T, E, label='LogLogisticFitter')
pwf = PiecewiseExponentialFitter([40, 60]).fit(T, E, label='PiecewiseExponentialFitter')


Multiple groups

groups = df['group']
ix = (groups == 'miR-137')[~ix], E[~ix], label='control')
ax = kmf.plot()[ix], E[ix], label='miR-137')
ax = kmf.plot(ax=ax)

Alternatively, for many more groups and more “pandas-esque”:

ax = plt.subplot(111)

kmf = KaplanMeierFitter()

for name, grouped_df in df.groupby('group'):["T"], grouped_df["E"], label=name)

Similar functionality exists for the NelsonAalenFitter:

from lifelines import NelsonAalenFitter
naf = NelsonAalenFitter(), event_observed=E)

but instead of a survival_function_ being exposed, a cumulative_hazard_ is.


Similar to Scikit-Learn, all statistically estimated quantities append an underscore to the property name.


More detailed docs about estimating the survival function and cumulative hazard are available in Survival analysis with lifelines.

Getting data in the right format

Often you’ll have data that looks like::

*start_time1*, *end_time1*
*start_time2*, *end_time2*
*start_time3*, None
*start_time4*, *end_time4*

lifelines has some utility functions to transform this dataset into duration and censoring vectors. The most common one is datetimes_to_durations. The docs for it are here.

from lifelines.utils import datetimes_to_durations

# start_times is a vector or list of datetime objects or datetime strings
# end_times is a vector or list of (possibly missing) datetime objects or datetime strings
T, E = datetimes_to_durations(start_times, end_times, freq='h')

Perhaps you are interested in viewing the survival table given some durations and censoring vectors. The docs for it are here.

from lifelines.utils import survival_table_from_events

table = survival_table_from_events(T, E)

          removed  observed  censored  entrance  at_risk
0               0         0         0       163      163
6               1         1         0         0      163
7               2         1         1         0      162
9               3         3         0         0      160
13              3         3         0         0      157

Survival regression

While the above KaplanMeierFitter and NelsonAalenFitter are useful, they only give us an “average” view of the population. Often we have specific data at the individual level, either continuous or categorical, that we would like to use. For this, we turn to survival regression, specifically AalenAdditiveFitter, WeibullAFTFitter, and CoxPHFitter.

from lifelines.datasets import load_regression_dataset
regression_dataset = load_regression_dataset()


The input of the fit method’s API in a regression is different. All the data, including durations, censored indicators and covariates must be contained in a Pandas DataFrame (yes, it must be a DataFrame). The duration column and event occurred column must be specified in the call to fit. Below we model our regression dataset using the Cox proportional hazard model, full docs here.

from lifelines import CoxPHFitter

# Using Cox Proportional Hazards model
cph = CoxPHFitter(), 'T', event_col='E')

<lifelines.CoxPHFitter: fitted with 200 observations, 11 censored>
      duration col = 'T'
         event col = 'E'
number of subjects = 200
  number of events = 189
    log-likelihood = -807.62
  time fit was run = 2019-01-27 23:11:22 UTC

      coef  exp(coef)  se(coef)    z      p  -log2(p)  lower 0.95  upper 0.95
var1  0.22       1.25      0.07 2.99 <0.005      8.49        0.08        0.37
var2  0.05       1.05      0.08 0.61   0.54      0.89       -0.11        0.21
var3  0.22       1.24      0.08 2.88 <0.005      7.97        0.07        0.37
Concordance = 0.58
Likelihood ratio test = 15.54 on 3 df, -log2(p)=9.47


The same dataset, but with a Weibull Accelerated Failure Time model. This model was two parameters (see docs here), and we can choose to model both using our covariates or just one. Below we model just the scale parameter, lambda_.

from lifelines import WeibullAFTFitter

wft = WeibullAFTFitter(), 'T', event_col='E', ancillary_df=regression_dataset)

<lifelines.WeibullAFTFitter: fitted with 200 observations, 11 censored>
      duration col = 'T'
         event col = 'E'
number of subjects = 200
  number of events = 189
    log-likelihood = -504.48
  time fit was run = 2019-02-19 22:07:57 UTC

                    coef  exp(coef)  se(coef)     z      p  -log2(p)  lower 0.95  upper 0.95
lambda_ var1       -0.08       0.92      0.02 -3.45 <0.005     10.78       -0.13       -0.04
        var2       -0.02       0.98      0.03 -0.56   0.57      0.80       -0.07        0.04
        var3       -0.08       0.92      0.02 -3.33 <0.005     10.15       -0.13       -0.03
        _intercept  2.53      12.57      0.05 51.12 <0.005       inf        2.43        2.63
rho_    _intercept  1.09       2.98      0.05 20.12 <0.005    296.66        0.99        1.20
Concordance = 0.58
Log-likelihood ratio test = 19.73 on 3 df, -log2(p)=12.34


If we focus on Aalen’s Additive model, which has time-varying hazards:

# Using Aalen's Additive model
from lifelines import AalenAdditiveFitter
aaf = AalenAdditiveFitter(fit_intercept=False), 'T', event_col='E')

Along with CoxPHFitter and WeibullAFTFitter, after fitting you’ll have access to properties like cumulative_hazards_ and methods like plot, predict_cumulative_hazards, and predict_survival_function. The latter two methods require an additional argument of individual covariates:

X = regression_dataset.drop(['E', 'T'], axis=1)
aaf.predict_survival_function(X.iloc[10:12]).plot()  # get the unique survival functions of two subjects

Like the above estimators, there is also a built-in plotting method:



More detailed documentation and tutorials are available in Survival Regression.