Estimating the rate of decline in a variable over time

[mantubiradar19]

I'm working with longitudinal data, wherein the person has multiple measurements at different time points as follows. I would like to how I can estimate the rate of decline in this variable over time? Also, what is the best way to visualize this data?

Date          GFR
1/1/15         53
4/7/15         53
8/18/15       54
12/20/15     50
5/13/16       48
10/25/16     49
3/9/17          47
7/15/17        46
1/4/18          48

I'm interested in looking at the distribution of decline in GFR over time. 

[maguiremq]

You can subtract differences with the LAG function. I then used PROC SGPLOT along with the SERIES statement to graph the difference over time. Since it's longitudinal and involves time, I think this is the best option. Notice how I omitted the missing from the PROC SGPLOT statement, too.

data have;
input Date :mmddyy10. GFR;
format date mmddyy10.;
datalines;
1/1/15         53
4/7/15         53
8/18/15       54
12/20/15     50
5/13/16       48
10/25/16     49
3/9/17          47
7/15/17        46
1/4/18          48
;

data have_2;
	set have;
		gfr_lag = lag(gfr);
		if not missing(gfr_lag) then do;
			diff = gfr_lag - gfr;
		end;
run;

proc sgplot
	data = have_2 (where = (gfr_lag ~= .));
	series x = date y = diff;
	format date mmddyy10.;
run; 

Kind of zoomed through this. Let me know if there are issues.

[Sajid01]

Hello @mantubiradar19 
One way is to plot the the GFR as a function of number of days from the start.

The slope of the line will give the rate of decline.
The following code should work.

data src;
input Date_ :mmddyy8.  GFR;
format date_  mmddyy8.;
days=coalesce(lag(days),0)+date_-lag(date_);
if _n_=1 then days=0;
datalines;
1/1/15         53
4/7/15         53
8/18/15       54
12/20/15     50
5/13/16       48
10/25/16     49
3/9/17          47
7/15/17        46
1/4/18          48
;
run;
proc orthoreg data=src;
   model gfr =days;
   effectplot fit / obs;
run;

The  following will be the output. The slope -0.031327867107 is the rate of decline in units of GFR per day.