Getting stronger is really tough

All my ideas lately to improve the playing strength of my chess engine did not work out. Sometimes it looked promising but something that seemed like a small progress vanished if more games were played.

I have not the resources to run hundreds of thousand of games. Playing 1000 games is already quite a commitment and the error bar after 1000 games is still very big. So my latest version v2589 shows something like a small improvement but actually it can even be worse than the last one in the list. They are all very close together and I kept the code changes not for a proven ELO gain but because they fixed some small bug or simplified the code a bit.  

Rank Name                 Elo    +    - games score oppo. draws
   1 gaviota v0.85.1      199   35   35   368   78%   -20   17%
   2 cheng3 1.07 JA        52   14   14  1860   60%   -21   17%
   3 Daydreamer 1.75 JA    30   12   12  2358   57%   -19   29%
   4 iCE 0.3 v2589        -20   16   16  1295   52%   -31   37%
   5 iCE 0.3 v2459        -24   20   20   766   50%   -25   36%
   6 iCE 0.3 v2441        -25    9    9  3768   47%    -1   33%
   7 iCE 0.3 v2489        -25   18   18  1014   50%   -25   32%
   8 iCE 0.3 v2457        -25    8    8  5160   50%   -22   33%
   9 iCE 0.3 v2559        -27   10   10  3493   49%   -17   31%
  10 iCE 0.3 v2574        -31    9    9  4162   48%   -16   33%
  11 iCE 0.3 v2413        -34   12   12  2161   49%   -28   33%
  12 iCE 0.3 v2571        -35   11   11  2768   48%   -18   32%
  13 iCE 0.3 v2437        -35    9    9  4249   47%   -15   31%


So I'm still looking for something that adds some ELO. But it seems most of the low hanging fruits have been picked.