* make a fake data file
clear
input id month var1
1 1 0
1 2 0
1 3 1
1 4 0
1 5 0
1 6 0
1 7 1
1 8 1
1 9 1
1 10 0
1 11 1
1 12 1
1 13 0
1 14 0
1 15 0
1 16 1
1 17 1
1 18 1
1 19 1
1 20 1
2 1 0
2 2 0
2 3 1
2 4 1
2 5 1
2 6 1
2 7 1
2 8 1
2 9 1
2 10 0
2 11 1
2 12 1
2 13 0
2 14 0
2 15 0
2 16 1
2 17 1
2 18 1
2 19 1
2 20 1
3 1 0
3 2 0
3 3 0
3 4 0
3 5 0
3 6 0
3 7 0
3 8 0
3 9 1
3 10 1
3 11 1
3 12 1
3 13 1
3 14 1
3 15 1
3 16 1
3 17 1
3 18 1
3 19 1
3 20 1
4 1 0
4 2 0
4 3 0
4 4 0
4 5 0
4 6 0
4 7 1
4 8 0
4 9 1
4 10 1
4 11 0
4 12 1
4 13 1
4 14 1
4 15 1
4 16 1
4 17 1
4 18 1
4 19 1
4 20 1
end
* set line where spell must start by
local line = 6
* set minimum spell length (for it to be a spell)
local minspel = 2
* can do find and replace to change these variable names
* id variable is named "id"
* month variable is named "month"
* binary variable is called "var1"
sort id month
* make f_id which is 0/1 if first id of group
generate f_id = 0
quietly by id : replace f_id=1 if _n == 1
* make l_id which is 0/1 if last id of group
generate l_id = 0
quietly by id : replace l_id=1 if _N == _n
* initialize variables
gen sp_strt = 0 /* start of spell */
gen sp_end = 0 /* end of spell */
gen sp_len = 0 /* spell length */
* cycle through obs, 1 to _N, examining spells
local obs = 1
while (`obs' <= _N) {
* initialize vars at start of ID
if f_id[`obs']==1 {
local start = 0 /* macro var with start of spell */
local stop = 0 /* macro var with end of spell */
local inspel = 0 /* 0/1 if in spell */
local hadspel = 0 /* 0/1 if already found a spell for this id */
}
* if 1, try to identify start of spell
if var1[`obs'] == 1 {
* start of spell
if (`inspel' == 0) & (`hadspel' == 0) {
local inspel = 1
local start = month[`obs']
replace sp_strt = 1 in `obs'
}
}
* if 0, try to identify end of spell
if var1[`obs'] == 0 {
* are we in a spell
if (`inspel' == 1) & (`hadspel' == 0) {
replace sp_end = -1 in `obs'
* does the spell start after the line,
* or end after the line, and is spell long enough
if ( (`start' >= `line') | (month[`obs'-1] >= `line') ) & ///
((month[`obs'-1]-`start'+1) >= `minspel') {
local stop = month[`obs'-1]
replace sp_end = 1 in `obs'
local hadspel = 1
}
else {
* otherwise, reset spell info
local inspel = 0
local start = 0
}
}
}
* if at last observation for id
if (l_id[`obs'] == 1) {
* check to see if in a spell not completed
if (`inspel' == 1) & (`hadspel' == 0) {
local stop = month[`obs']
}
* store spell info in last observation for id
replace sp_strt = `start' in `obs'
replace sp_end = `stop' in `obs'
replace sp_len = `stop' - `start' + 1 in `obs'
}
* add 1 to obs
local obs = `obs' + 1
}
* have a look at results to see if OK
clist month var1 sp*
* keep just last obs
quietly by id: keep if _N == _n
keep id sp*
* look at final values
clist