My apologies; I had modified the appearance to try to make it more clear what I was attempting. I've fixed it now and the code runs as written.

Yes, you are correct about the intermediate piddles (the ones of which I am finding the median) being different lengths. I was thinking perhaps there's some way to expand into extra dimensions, where each dimension would contain the values in a given $b range, and then take medover the entire array to condense it into the form of $d. Is this possible?

Like this, only without having to manually type all of the $mask(..) .= 1 lines:

pdl> $e = $a(,,*3)->copy

pdl> p $e

[
 [
  [ 0  6 18  7 19  3 10  2 12  4  8  9  1 15 11 11 19 17  0  9]
 ]
 [
  [ 0  6 18  7 19  3 10  2 12  4  8  9  1 15 11 11 19 17  0  9]
 ]
 [
  [ 0  6 18  7 19  3 10  2 12  4  8  9  1 15 11 11 19 17  0  9]
 ]
]

pdl> $mask = $e->zeroes

pdl> $mask(,,0)->where(($b>0)*($b<=5)) .= 1

pdl> $mask(,,1)->where(($b>5)*($b<=10)) .= 1

pdl> $mask(,,2)->where(($b>10)*($b<=15)) .= 1

pdl> p $mask

[
 [
  [0 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
 ]
 [
  [0 0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 0 0 0]
 ]
 [
  [0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1]
 ]
]

pdl> $mask = $mask->setbadif($mask==0)

pdl> p $mask

[
 [
  [BAD   1   1   1   1   1 BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD
+ BAD BAD BAD]
 ]
 [
  [BAD BAD BAD BAD BAD BAD   1   1   1   1   1 BAD BAD BAD BAD BAD BAD
+ BAD BAD BAD]
 ]
 [
  [BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD   1   1   1   1   1   1
+   1   1   1]
 ]
]


pdl> $f = $e*$mask

pdl> p $f

[
 [
  [BAD   6  18   7  19   3 BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD
+ BAD BAD BAD]
 ]
 [
  [BAD BAD BAD BAD BAD BAD  10   2  12   4   8 BAD BAD BAD BAD BAD BAD
+ BAD BAD BAD]
 ]
 [
  [BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD BAD   9   1  15  11  11  19
+  17   0   9]
 ]
]

pdl> $g = $f->medover

pdl> p $g

[
 [ 7]
 [ 8]
 [11]
]
[download]

Thank you very much for your help!