AN ACR LGT CREW MISUNDERSTOOD THEIR ALT CLRNC; THEN OVERSHOT THE NEXT ASSIGNED ALT AND CAUSED A TCASII ALERT.

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I'VE FLOW WITH THIS CAPT SEVERAL TIMES IN THE PAST. BECAUSE OF THIS EXPERIENCE; I HAVE TRIED TO MAKE GOOD USE OF X AIRLINE'S CREW COORD CONCEPT TRAINING. HE FAILED HIS LAST PROFICIENCY CHK WHICH ADDED TO MY VIGILANCE. THE CAPT WAS FLYING THIS LEG FROM IAH TO ORD. THE WX WAS IMC AND THE LATEST ATIS INDICATED AN IFR APCH WAS GOING TO BE MADE EITHER TO 14R OR 9R. THE CAPT BRIEFED THE APCH. WHEN WE CHANGED TO APCH CTL FREQ; A DIFFERENT RWY WAS ASSIGNED. WITH THIS NEW INFO; A NEW APCH BRIEFING WAS INITIATED BY THE CAPT; AT THE SAME TIME HDG AND ALT CHANGES WERE BEING MADE. TO KEEP UP WITH THE FLOW OF INSTRUCTIONS AND ACCOMPLISH MY DUTIES AS FO; I ASKED THE SO TO IDENT THE OM. ABOUT THAT SAME TIME; WE WERE GIVEN INSTRUCTIONS TO SLOW OUR AIRSPD TO '170 KTS AND DSND TO ?? ALT.' MY CONCENTRATION WAS DIVIDED BTWN 3 THINGS AT THAT TIME: MY DUTIES; THE CAPT'S FLYING AND MONITORING THE SO'S PROGRESS WITH HER DUTIES. I REMEMBER ACKNOWLEDGING THE HDG CHANGE; AND SETTING 7000 FT IN THE WINDOW FOR OUR DSCNT. I LOOKED AT THE CAPT FOR ACKNOWLEDGEMENT THAT HE UNDERSTOOD THE INSTRUCTIONS AND HE NODDED BACK. SHORTLY AFTER REACHING 7000 FT; THE APCH CTLR SAID OUR CLRNC LIMIT WAS 8000 FT. I RESPONDED WE COPIED 7000 AND ASKED IF HE WANTED US TO CLB BACK UP? HE SAID 'THE DAMAGE HAS ALREADY BEEN DONE; STAY AT 7.' THE CAPT RESPONDED; 'THAT'S WHAT I HEARD AND IT'S SET IN THE WINDOW.' ANOTHER DSCNT WAS GIVEN TO 6000 FT. THE CAPT WAS HAND-FLYING THE ACFT; AS HE USUALLY DOES ON THE DSCNT; APCH; AND LNDG. HE DSNDED TO 5775 FT IMMEDIATELY CORRECTED WHEN HE REALIZED HE HAD OVERSHOT HIS ALT. A Y AIRLINES PLT THEN RPTED TO APCH CTL THAT HE HAD RECEIVED AN RA ON HIS TCASII TO DSND. APCH ASKED WHAT OUR ALT WAS; AND I RESPONDED THAT WE WERE AT 6000; WHICH WE WERE BY THAT TIME. THE REMAINDER OF THE APCH AND LNDG WAS UNEVENTFUL. ORD IS; BY NATURE; AN INTENSE ARPT TO OPERATE INTO AND OUT OF. BECAUSE THERE IS USUALLY MORE THAN 1 RWY AVAILABLE FOR LNDG; IT WOULD BE HELPFUL IF THERE WOULD BE A WAY TO KNOW AHEAD OF TIME TO PROPERLY BRIEF FOR THE APCH ON A SPECIFIC RWY.