Flag Changes in a Group with Some Missing Observations

[Marley Mass]

Dear all,

This is my first time using statalist so any help would be much appreciated. I have a dataset where the id variable should uniquely identify individuals however there are some errors where this is not the case. There are some instances where 2 people of 2 different genders have the same id so I'd like to create a different id for these individuals.

I have sorted my data by group and date and would like to create the flag_change variable in the example below, where a value of 1 is given if there is a change in the gender within a group, while taking into account the fact that there are some missing observations. I have tried the following code but this didn't work and just created missing values for the flag_change variable.

bysort id: gen flag_change = (gender != gender[_n-1]) if _n > 1 & !missing(gender) & !missing(gender[_n-1])

id	gender	date	flag_change
1	.	22 May	0
1	Man	23 May	0
1	Woman	15 Sep	1
1	.	16 Sep	0
2	Man	1 Dec	0
2		3 Dec	0
3	Man	12 Apr	0
3	.	16 Apr	0
3	.	15 Jun	0
3	Woman	22 Jun	1
4	Woman	9 Jan	0

Any advice? Thanks in advance for your help!

[Andrew Musau]

Assuming gender is a numeric variable with two levels and date is a proper Stata date variable:

gen tag= missing(gender)
bys id tag (date): gen wanted= (gender!=gender[_n-1]) & _n>1 
drop tag
sort id date

[Marley Mass]

This worked perfectly. Thank you very much Andrew!