Cox PH with censored data

gistfile1.txt

# Data from Stata manual, p.5:
# "We wish to analyze an experiment testing the ability of emergency
# generators with a new-style bearing to withstand overloads. For this
# experiment, the overload protection circuit was disabled, and
#the generators were run overloaded until they burned up. "
# https://www.stata.com/manuals13/ststcox.pdf
df <- read.table(text =
    "failtime load bearings
100 15 0
140 15 1
97 20 0
122 20 1
84 25 0
100 25 1
54 30 0
52 30 1
40 35 0
 55 35 1
 22 40 0
 30 40 1", header = T)

# Add zero time and event status
library(tidyverse);
df <- df %>%
    mutate(time = 0, event = TRUE)

# Run Cox PH model
library(survival)
coxph(Surv(time = time, time2 = failtime, event = event) ~ load + bearings, data = df);
#Call:
#coxph(formula = Surv(time = time, time2 = failtime, event = event) ~
#    load + bearings, data = df)
#
#            coef exp(coef) se(coef)     z      p
#load      0.4309    1.5386   0.1444  2.98 0.0028
#bearings -2.8107    0.0602   1.1782 -2.39 0.0171
#
#Likelihood ratio test=24  on 2 df, p=6.09e-06
#n= 12, number of events= 12