AN MD80 CREW OVERSHOT ASSIGNED CRUISE ALT; WHEN THE WRONG ALT WAS ENTERED IN THE ALT ALERT WINDOW.

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FLT WAS FILED AT FL280 FROM STL TO MSP. CAPT FLYING. DURING CLB ON AUTOPLT; WE WERE CLRED BY ZAU TO CLB TO FL280 AND HEAD 330 DEGS FOR IOW VOR. I SELECTED THE HDG OF 330 DEGS AND THE FO SET FL280 IN THE ALT ALERT. AS I WAS VERIFYING THE ALT AND HDG SELECTIONS; A FLT ATTENDANT CALLED ON THE INTERPHONE. I REACHED DOWN FOR THE INTERPHONE AND AM NOT SURE I CHKED THE ALT SETTING. WE CHANGED RADIO FREQS AND THE CTLR ONCE AGAIN GAVE US A HDG OF 330 DEGS. AS WE WERE APCHING FL280; THE FLT ATTENDANT SAID SHE HAD OUR MEALS. THE FO HAD TO STAND AND UNBOLT THE DOOR. AT ABOUT FL285; THE CTLR ASKED IF WE WERE GOING TO MAINTAIN FL280. I LOOKED AT FL330 SET IN THE ALT ALERT AND ADVISED/REPLIED THAT WE WERE CLRED TO FL330. HE TOLD US TO LEVEL AT FL310. WHEN I LOOKED AT OUR FILED ALT OF FL280; THE FACT THAT WE WERE FLYING NW AND THAT THE ASSIGNED HDG WAS 330 DEGS; IT SEEMS VERY UNLIKELY THAT WE WERE ASSIGNED THAT ALT. ON THE MD80 THE METHOD FOR SELECTING THE HDG AND SETTING THE ALT ARE SIMILAR AND IN THE SAME GENERAL AREA OF THE GLARE SHIELD. THIS INCREASES THE NEED TO AVOID DISTRS AND VERIFY SETTINGS.