[Gretl-users] Re: 100-based indices with panel data (fwd)

---------- Forwarded message ---------- Date: Sat, 1 Jul 2023 19:39:27 +0000 From: Allin Cottrell <cottrell(a)wfu.edu&gt; To: "Riccardo (Jack) Lucchetti" <r.lucchetti(a)univpm.it&gt; Cc: Josué Martínez-Castillo <jota3mc(a)gmail.com&gt;, Subject: Re: 100-based indices with panel data On Sat, 1 Jul 2023, Riccardo (Jack) Lucchetti wrote: Yes, quite instructive! In case anyone's interested let's unpack Jack's formulation. First consider: base = cum(ok(EMP)) == 1 ? EMP : NA We're looking at what gretl calls series here. The inner expression, "ok(EMP)" creates a series with value 1 for valid values of its series argument and 0 for NAs (missing values). This addresses a problem with the first variant I posted, where I just took the base of the indices to be the first observation for each unit. That's OK with the grunfeld data that I referenced because it has no missing values. But if the first observation for a unit were NA, the whole index series for that unit would be NA via my method (since NAs propagate in arithmetical calculation). Not accidentally, Jack chose the supplied abdata dataset (Arellano and Bond), which contains missing values, to illustrate his calculation, and I'll work with it here. Now cum() is gretl's cumulation function, and it works "properly" for panel data: it cumulates in the time dimension, starting over for each unit. So "cum(ok(EMP))" gives a series holding the count of valid values "to date" for each unit. OK, so far? Then "cum(ok(EMP)) == 1 ? EMP : NA" is an instance of the very handy ternary operator. It has the form: result = condition ? one_thing : other_thing which can be spelled out a bit as if (condition is true) result is one_thing, otherwise result is other_thing So, "cum(ok(EMP)) == 1 ? EMP : NA" gives a series holding the value of EMP for each first-valid-observation per panel unit, and NA for all other observations. To see what's happening up to this point one could open the abdata dataset in gretl and execute these commands (in a script or via the console): series eok = ok(EMP) series cumeok = cum(eok) series base = cum(ok(EMP)) == 1 ? EMP : NA print EMP eok cumeok base --byobs Next comes the line: EMP_b100 = EMP/pexpand({base}) * 100 On the left-hand side is the final indices series. On the right-hand side we're using the original EMP (employment), multiplying by 100 (as per convention), and dividing by "pexpand({base})". What the heck is this last thing? Well, notice the curly brackets around "base". These turn a series into a vector (special case of a matrix) and the thing you need to know here is that in gretl by default this conversion skips any missing values. [Note: you can prevent this via the command "set skip_missing off".] So in a panel with N units {base} will be an N-vector holding just the first valid observation of EMP for each unit. Then the pexpand ("panel-expand") function turns this N-vector into a series by repeating each of the N values T times, for each unit. Which is (probably) just what we want to divide EMP by, to create the per-unit indices. In a panel dataset with no missing values it's exactly equivalent to the more pedestrian formulations I posted earlier. Now for a couple of missing-data complications we'd want to deal with in a built-in version of this functionality. 1) What if some units have NO valid values for the variable we're working with? Then {base} will not be an N-vector and Jack's method will not work unmodified. 2) What if the date of the first valid observation differs across units, but we want a set of indices that start in the same period? Again, some fancier footwork would be needed. We'd need to look for the first period with a common non-missing observation across all units that had more than one non-missing observation. Allin Cottrell