Create a Variable in stata that is a value minus it's last month value

[Lindsey Buck]

Hi all,

I put some data below, but I have a question about making a varialbe that is it's value minus the value of the last month. I have a variable activemonth_final that is in the format 2020m1, 2020m2, 2020m3, etc. The variable ccda_all_act_ind has a different value for each month. Is there a way I can make a variable that shows ccda_all_act_ind minus it's value from last month? For example, in the 2020m1 case it would show 2020m1-2019m12. Thanks!

* Example generated by -dataex-. For more info, type help dataex
clear
input float(activemonth_final ccda_all_act_ind)
611    .
647 2206
587    .
671 2486
659 2241
623    .
719 3084
695 2970
599    .
707 3079
683 2793
635    .
684 2875
708 3209
672 2517
612    .
588    .
720 3213
660 2230
624    .
636 2077
696 3160
600    .
648 2178
697 3160
721 3200
649 2196
685 2816
613    .
589    .
709 3147
661 2213
625    .
637 2047
673 2658
601    .
710 3178
638 2182
650 2297
686 2980
698 3226
602    .
674 2746
626    .
614    .
494    .
662 2250
590    .
722 3195
615    .
663 2231
627    .
603    .
699 3198
639 2299
579    .
723 2941
675 2637
711 3182
591    .
651 2256
687 2917
712 3172
652 2230
676 2743
700 3215
616    .
724 2973
628    .
688 2990
604    .
640 2295
592    .
664 2227
677 2699
701 3126
689 3067
629    .
593    .
725 3051
617    .
641 2231
653 2194
713 3051
665 2299
605    .
642 2276
678 2672
594    .
654 2231
702 3076
630    .
606    .
666 2349
726 2983
690 2981
714 3180
582    .
618    .
703 3147
end
format %tm activemonth_final

------------------ copy up to and including the previous line ------------------

Listed 100 out of 152 observations
Use the count() option to list more

.

[Nick Cox]

tsset activemonth 
gen wanted = D.ccda

[Ken Chui]

tsset activemonth_final
gen wanted = ccda_all_act_ind - l.ccda_all_act_ind

[Lindsey Buck]

Thank you so much!

[Lindsey Buck]

Is there a way to do this if I wanted the value from last *year*? For instance, if I wanted ccda of august 2019-ccda of august 2018?

[Ken Chui]

Is there a way to do this if I wanted the value from last *year*? For instance, if I wanted ccda of august 2019-ccda of august 2018?

This should work:

gen wanted = ccda_all_act_ind - l12.ccda_all_act_ind

[Lindsey Buck]

I am running into issues using tsset because activemonth_final is repeated in my data (but ccda is the same across the repeated observations.) any idea?

[Nick Cox]

How does that arise? Do you have panel data? If duplicate values make sense, then differences across time aren't uniquely defined. For example, if there were two values 12 months earlier, what would be the right answer to #5?