I'm attempting to estimate the effect of 2 drugs (
drug2) on the likelihood of a patient falling (
event). The patients can fall more than once and can be put on or taken off of the the drugs at any point.
My question is how the data should be structured with regard to the time period (days), specifically whether there needs to be overlap between the days. There are two reasons why I think my structure is wrong, the first being a seemingly incorrect
N. I am also getting some errors where the time period is a single day (i.e.
time2=4) and am unsure how these should be coded. Should the start time of subsequent entries be the stop time of the previous entry? I've tried it both ways (with and without overlap), and while having overlap gets rid of the warning, the
N is still incorrect.
Warning message: In Surv(time = c(0, 2, 7, 15, 20, 0, 18, 27, 32, 35, 39, 46, 53, : Stop time must be > start time, NA created
Right now I have the data set up where the beginning of the next entry is the next day. Unique patients are identified by their
Time1 Time2 Drug1 Drug2 Event ChartNo 0 2 1 0 0 123 3 10 1 1 1 123 11 14 1 1 1 123 0 11 0 1 0 345 0 19 1 0 1 678 0 4 0 1 0 900 5 18 1 1 0 900
Patient 123 was on drug1 at the start to day 2, after which point they had drug2 added. They went from day 3 to day 10 on both drugs before falling the first time, then fell a second time on day 14 while still on both drugs. Patient 345 went 11 days on drug2 without falling (then was censored), etc.
The actual estimation looks like this:
S <- Srv(time=time1, time2=time2, event=event) cox.rms <- cph(S ~ Drug1 + Drug2 + cluster(ChartNo), surv=T)
My main concern is that the
n for my analysis is reported to be
2017 (the number of rows in the data), when in actuality I only have
314 unique patients. I am unsure if this is normal or the result of some error I've made along the way.
> cox.rms$n Status No Event Event 1884 133
The same is true when using
coxph() from the survival package.
n= 2017, number of events= 133
The number of events is correct however.
This Post seems to have it set up with the 'overlap' I described, but I am unsure about the
N, and they don't seem to be clustering by