1. Please Don't Lag Behind LAG!
Anjan Matlapudi and J. Daniel Knapp
Pharmacy Informatics and Finance
PerformRx

2. Introduction
Lag is a common word meaning to fail to keep up or to fall behind.
- From Wikipedia
Lag: fail to maintain a desired pace or to keep up; fall or stay behind: After five minutes of hard running, some of them began to lag.
A person who lags behind, is the last to arrive, etc From Dictionary.com

3. Simple Example
Day
Temp
Lag
Lead
Yesterday 70
Today 60 80
Tomorrow

4. Why We need the Lag Function ?
Useful function while computing across observations.
Compute observations with reference to previous date and time functions.
Lag is the powerful function when we use with other SAS functions such as DIF,INTCK, RETAIN etc..
Simplify our code and easy to manipulate DATA.

5. Syntax
data TempTbl; set Dailytemp; lag_qty = lag(qty); run;

6. Table 1 Member ID Drug Fill Date Drug Name Qty 1111 01/01/2010 ABILIFY
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

7. Example 1
data Example1; set Table1; lag_qty = lag(qty); run;

8. Example 1 Member ID Drug Fill Date Drug Name Qty Lag Qty 1111
01/01/2010
ABILIFY 10 .
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

9. Example 1
data Example1; set Table1; lag_qty = lag(qty); by MemberId; run;

10. Table 1 Member ID Drug Fill Date Drug Name Qty Lag Qty 1111 01/01/2010
ABILIFY 10 .
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

11. Example 1 data Example1; set Table1; lag_qty = lag(qty); by MemberId;
If not first.MemberId then lag_qty = lag(qty);
run;

12. Wrong Results Member ID Drug Fill Date Drug Name Qty Lag Qty 1111
01/01/2010
ABILIFY 10 .
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

13. What is the Reason Behind
Question
What is the Reason Behind ?

14. Memory Allocation in Queue
LAG 1
LAG 2
line 1
line .
line 2
line 3
line 4
line 5
line 1
line .
line 2
line 3
line 4
line 5
Input data set
output data set
Input data set
output data set
Observation 1
Observation 2
Observation 3
Observation 4
Observation 5
Observation .
Observation 1
Observation 2
Observation 3
Observation 4
Observation 1
Observation 2
Observation 3
Observation 4
Observation 5
Observation .
Observation 1
Observation 2
Observation 3

15. Example 1
data Example1; set Table1; lag_qty = lag(qty); by MemberId; If first.MemberId then Qty = lag_qty; run;
. ;

16. Correct Results Member ID Drug Fill Date Drug Name Qty Lag Qty 1111
01/01/2010
ABILIFY 10 .
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

17. Iterations
data Example1; set Table1; prevQty = lag(qty); prevQty1 = lag(qty); prevQty2 = lag(qty); prevQty3 = lag(qty); prevQty4 = lag(qty); prevQty5 = lag(qty); prevQty6 = lag(qty); prevQty7 = lag(qty); prevQty8 = lag(qty); run;

18. Output Results Member ID Drug Fill Date Drug Name Qty PrevQty PrevQty17
1111
01/01/2010
ABILIFY 10 .
01/16/2010 20
03/16/2010 30
03/30/2010 40
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

19. Best Practice
To understand data and make sure to deal with missing values.
Proc Freq is the best way to check missing values in the data.
Proc Freq data = Example1;
Table MemberId*FillDate/Missing List;
Run;

20. Does LEAD Functions Exists in SAS®
Question
Does LEAD Functions Exists in SAS® ?

21. LEAD Example
data LeadTbl; merge Table1 Table1(firstobs=2 keep=qty rename=(qty=lead_qty)); run;

22. LEAD Output Member ID Drug Fill Date Drug Name Qty Lead Qty 1111
01/01/2010
ABILIFY 10 20
01/16/2010 30
03/16/2010 40
03/30/2010
2222
01/04/2010
GLEEVEC
02/10/2010
02/15/2010
07/01/2010

23. Does LAG/LEAD Functions Works in PROC SQL
Question
Does LAG/LEAD Functions Works in PROC SQL ?

24. LAG in Proc SQL
proc sql; select DrugName,qty, LAG(qty,1) as prevQty from Example1; quit;

25. Log Output
520 proc sql; 521 select drug, qty, 522 LAG(qty,1) as prevQty 523 from demo; ERROR: The LAG function is not supported in PROC SQL, it is only valid within the DATA step. 524 quit; NOTE: The SAS System stopped processing this step because of errors.*;

26. LEAD in Proc SQL
proc sql; select DrugName,qty, LEAD(qty,1) as LeadQty from Example1; quit;

27. Log Output
520 proc sql; 521 select drug, qty, 522 LEAD(qty,1) as NextQty 523 from Example1; ERROR: The LEAD Could not be Located. 524 quit; NOTE: The SAS System stopped processing this step because of errors.*;

28. Proc Expand
proc expand data = Table1 out = Leadtbl method=none; by MemberId; convert Qty =lead_qty / transformout= (lead); convert Qty = lag_qty / transformout= (lag); convert Qty =lead2_qty / transformout= (lead 2); convert Qty =lag2_qty / transformout= (lag 2); run;

29. LAG/Lead Output Results
Member ID
Drug Name
Qty
Lag1
Lag2
Lead
Lead2
1111
ABILIFY 10 . 20 30 40
2222
GLEEVEC

30. Table -2 LAG With INTCK function Example
Member ID
Drug Fill Date
Drug Name
Qty
1111
01/01/2010
ABILIFY 10
01/15/2010
AMBIEN 20
03/16/2010
ALORA 5
2222
ASPRIN 1
3333
01/04/2010
TYLENOL
4444
02/10/2010
02/15/2010
AMOXIL
07/01/2010
DIXOL

31. INTCK Function lag_memberid = lag(MemberID);
data singlelPrescriptions DualPrescriptions;
set Table2;
by MemberID DrugFill_Date;
*----Output single prescriptions----*;
if first.MemberID and last.MemberID then output singlelPrescriptions;
else
do;
lag_memberid = lag(MemberID);
lag_drugFill_date = lag(DrugFill_Date);
if lag_memberid = MemberID then
numdays= intck('DAY',lag_drugFill_date, DrugFill_Date);
if numdays < 30 then star = "*";-Output with '*' flag;
end;
output DualPrescriptions;
run;

32. LAG with INTCK Output * Member ID Drug Fill Date Drug Name Qty
Num Days
Star
1111
01/01/2010
ABILIFY 10 .
01/15/2010
AMBIEN 20 14 *
03/16/2010
ALORA 5 60
2222
02/10/2010
02/15/2010
AMOXIL
07/01/2010
DOXIL
136

33. Table -3 LAG With DIFF function Example
Member ID
Drug Fill Date
Drug Name
Price
1111
01/01/2007
ABILIFY
$30
01/15/2008
$60
10/16/2009
$80
07/01/2010
$90
2222
01/04/2007
GLEEVEC
02/10/2008
$40
02/15/2009

34. DIF Function lag_price = lag(price); dif_price = dif(price);
data DifTbl;
set Table3;
by MemberID DrugFill_Date;
diff_drugFill_date = dif(DrugFill_Date);
lag_price = lag(price);
dif_price = dif(price);
if first.memberid then do;
diff_drugFill_date = .;
lag_price = .;
dif_price = .;
end;
if diff_drugFill_date > 365 then do;
percent_increase = round((dif_price/lag_price)*100) ;
run;

35. LAG with DIF Output Member ID Drug Name Price Fill Date Lag Price
Dif Price
Per Increase
1111
ABILIFY
$30 .
$60
379 30
100%
$80
640 60 20
33%
$90
259 80 10
2222
GLEEVEC
$40
402
674 40
50%
198

36. Table – 4 LAG With RETAIN function Example
Member ID
Drug Fill Date
Drug Name
Qty
Lag Price
1111
01/01/2007
ABILIFY 10
$6.25
01/15/2008 20
10/16/2009 30
07/01/2010 40
2222
01/04/2007
GLEEVEC
$5.55
02/10/2008
02/15/2009

37. RETAIN Function
data RetainTbl(rename=(Old_price=price)); set Table4; by MemberID ; if first.MemberID then old_price=price; retain old_price; if price EQ . then old_price=old_price; else old_price=price; lag_qty = lag (qty); if first.MemberID then lag_qty = .; else avgprice = mean(old_price*lag_qty); run;

38. LAG with RETAIN Output Member ID Drug Fill Date Drug Name QTY
Lag Price
Price Var
Lag Qty
Avg Price
1111
01/01/2007
ABILIFY 10
$6.25 .
01/15/2008 20
$62.50
10/16/2009 30
$125.00
07/01/2010 40
$187.50
2222
01/04/2007
GLEEVEC
$5.55
02/10/2008
$55.55
02/15/2009
$111.00
$166.50

39. Conclusions
We demonstrated the basic use the of LAG function. We hope The LAG Code example with other functions in this paper will be useful to the Base SAS® programmers.
We suggest careful look at the data structure and make sure to deal with null values and assign correctly when you use LAG function.

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44. Thank you

45. Acknowledgements
Mr. Shimels Afework, Sr. Director of our company for his support.
SAS ® indicates USA registration. Other brand and product names are registered trademarks or trademarks of their respective companies.

46. Questions? Anjan Matlapudi and J. Daniel Knapp PerformRx
The Next Generation PBM, 200 Stevens Drive, Philadelphia, PA 19113

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