Comparing Dates, Create new variable

[Jonathan Edwin]

Hi there,

Essentially, what I'm trying to do is compare two dates: 'Treatment Date 1' and 'Treatment Date 2'.
I would like to create a new variable if Date 1>Date 2.

I tried the very basic code below. However, when there is a missing value for Date 1 it is creating the new variable as if Date 1>Date 2.

gen byte txendrecent=1 if Date1>Date 2


Any suggestions greatly appreciated.

Jonathan Edwin
Epidemiologist, Population and Public Health Dept.
Government of Northwest Territories

[Jonathan Edwin]

To note, I tried re-formatting the dates, but the same issue with use of missing dates.

[William Lisowski]

The output of help missing tells us

    Stata has 27 numeric missing values:

        ., the default, which is called the "system missing value" or sysmiss

    and

        .a, .b, .c, ..., .z, which are called the "extended missing values".

    Numeric missing values are represented by large positive values.  The ordering is

                    all nonmissing numbers < . < .a < .b < ... < .z

    Thus, the expression age > 60 is true if variable age is greater than 60 or
    missing.

    To exclude missing values, ask whether the value is less than ".".  For instance,

        . list if age > 60 & age < .

    To specify missing values, ask whether the value is greater than or equal to ".".
    For instance,

        . list if age >=.

Note however that in your code, txendrecent will be missing when it is not 1. A better formulation would be

generate byte txendrecent=0
replace txendrecent=1 if Date1>Date2 & Date1<.

which can be shortened to

generate byte txendrecent = (Date1>Date2) & (Date1<.)

These both will generate txendrecent as 0 or 1, consistent with Stata treating 0 as false and all other values, including missing values, as true.

[Jonathan Edwin]

That nuance about missing values is all coming back to me now. Thanks William!

Best,
Jonathan