ACR DC9 ENTERED CTR AIRSPACE AT WRONG ALT AFTER RPTR AND DEVELOPMENTAL CTLR MISSED WRONG ALT READBACK OF 11000 FT VICE 10000 FT BY FLC.

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I WAS WORKING RADAR E CTL POS AND WAS DIRECTLY INVOLVED WITH AN OPDEV IN WHICH A READBACK ERROR OCCURRED. OJT WAS BEING CONDUCTED. I WAS INSTRUCTING. WORKLOAD MODERATE FOR THE DEVELOPMENTAL (TRAINEE) CTLR. ACR X CALLED DEP AIRBORNE (RWY 36); R TURN HDG 040 DEGS; CLBING TO 5000 FT. ACR X WAS RADAR IDENTED; ADVISED 'TURN R HDG 110 DEGS; JOIN VICTOR 2; RESUME OWN NAV; CLB AND MAINTAIN 10000 FT.' ACR X READ BACK THE CLRNC. DURING THE READBACK; THE DEVELOPMENTAL CTLR; WHILE SPEAKING ALOUD; INDICATED THAT SHE MUST INITIATE A POINTOUT TO ZAU LONE ROCK SECTOR AS THIS ACFT WOULD CROSS A STRATIFIED SECTION OF ZAU AIRSPACE APPROX 10 MI WIDE. THE POINTOUT WAS IMMEDIATELY ISSUED AND APPROVED BY THE ZAU CTLR. PRIOR TO ACR X REACHING A CRUISING ALT OF 10000 FT; THE DEVELOPMENTAL CTLR XFERRED RADIO COMS TO MKE APCH AFTER SUCCESSFULLY COMPLETING AN AUTOMATED RADAR HDOF. WITHIN 1-2 MINS OF COM RELEASE; I OBSERVED ACR X AT AN ALT OF 10800 FT AND IMMEDIATELY CONTACTED THE ZAU CTLR WITH REGARDS TO THE DEV. I OBSERVED ACR X LEVEL AT 11000 FT; INFORMED THE ZAU CTLR OF SUCH. HE ADVISED US THAT THE TFC WAS 'ALRIGHT' AND FURTHER DISCUSSION WAS TERMINATED. I PROMPTLY NOTIFIED THE AREA SUPVR IN CHARGE AND WAS RELIEVED OF MY CTL POS. FURTHER REVIEW OF THE AUDIO TAPES INDICATE THAT ACR X PROPERLY READ BACK THE TURN TO 110 DEGS AND TO JOIN THE ASSIGNED AIRWAY; HOWEVER; THE AIR CREW READ BACK AN ASSIGNED ALT OF 11000 INSTEAD OF THE ACTUAL ASSIGNED ALT OF 10000 FT. NEITHER THE DEVELOPMENTAL CTLR; MYSELF; NOR THE SIC (OR PLT; PENDING WHO WAS COMMUNICATING AND WHO WAS FLYING); CAUGHT THE ERROR WHICH LED TO THE DEV. PRIMARY CONTRIBUTING FACTOR: ACR X INITIALLY FILED FOR 11000 FT. BY LOA WITH MKE APCH CTL; MKE ARRS MUST BE AT 7000 FT; 9000 FT; OR 10000 FT. THE ASIC AMENDED THE REQUESTED ALT TO COMPLY WITH THE LOA. OTHER FACTOR: MY FAILURE TO PRIORITIZE PROPERLY -- COMPREHEND READBACKS FIRST; THEN COMPREHENSION OF TRAINEE INTENTIONS TO MAKE THE POINTOUT; AS IN THIS CASE. I PLACED A HIGHER PRIORITY ON THE VERBALLY SPOKEN INTENTION OF FUTURE ACTIONS BY THE DEVELOPMENTAL. MY BELIEF OF THE PROB: AS AN OJT INSTRUCTOR; I HAVE MADE MY PRIORITY; WHILE TRAINING; TO JOT DOWN NOTES AS TOPIC OF DISCUSSION FOR LATER DEBRIEF. NOT HAVING FLT PROGRESS STRIPS DIRECTLY IN FRONT OF ME LEAVES A LARGER THAN DESIRABLE MARGIN OF ERROR. I HAVE NO REF EXCEPT MEMORY (NO JOGGERS) TO AID ME IN THE REMEMBRANCE OF INSTRUCTIONS. BOTTOM LINE -- I MISSED THE READBACK.