Gov. Rick Scott portrays U.S. Sen. Bill Nelson as an "empty chair" who often skips meetings on the important topic of security."Nelson skipped 45 percent of the hearings on national security," said a TV ad from Scottís campaign. "National security! Bill Nelson doesnít write laws ó he doesnít even show up."The ad refers to Nelsonís tenure on the Senate Armed Services Committee. Nelson is defending his seat in a close battle with Scott.Attacks for missed hearings are common. But these attacks omit several caveats about committee attendance."I have never been one that put much stock in the Ďattendanceí statistics to measure a senatorís work" on the Senate Armed Services Committee, said Arnold Punaro, who served as the committeeís Democratic staff director from 1983 to 1997.Scottís campaign published a spreadsheet showing that between 2001 and 2018, Nelson missed 45 percent (or 284 out of 636) of Senate Armed Service Committee hearings. (He wasnít on the committee in 2011-12.) The campaign said it counted full committee hearings and subcommittees on which Nelson served and was expected to attend.Nelsonís attendance varied with each two-year congressional session, ranging from 44 percent in 2005-06 to 76 percent for the current 2017-18 session.Scottís campaign pulled attendance records from the Senate Armed Services Committee website and the Government Printing Office. However, attendance isnít published for closed or classified hearings.Since there arenít public attendance records for all types of hearings, that means we donít get the full picture of Nelsonís service on the committee.Dan McLaughlin, Nelsonís spokesman, said by his count Nelson attended 104 of 126 Armed Services hearings, informal meetings, sessions and briefings ó including more than two dozen that were closed or classified ó in 2017-18.PolitiFact canít verify the Nelson campaignís figures since that includes meetings that lack a public attendance record.Experts on Congress agreed that a straight-up attendance record doesnít tell us much."A hearing about a national security threat, for example, is much more significant than routine budget hearings the (committee) holds every fiscal year," said Georgetown University professor Joshua Huder, who studies Congress.It is common for senators, who serve on multiple committees and subcommittees, to miss hearings. Sometimes, multiple committees will hold hearings at the same time, forcing the senator to split their time or prioritize one hearing over another.Roll Call, a Washington publication that regularly covers Congress, wrote in 2014 that unless the senator wields the gavel, he or she may only show up for five minutes, or when it is their turn to ask questions."The results include guffaw-inducing scenes where even senior lawmakers enter the wrong hearing room, misidentify a witness and question the wrong person on the other side of the dais," Roll Call wrote.Because the Scott campaignís data omits that for some types of hearings there is no publicly available attendance record, this count is not perfect.We rate this statement Mostly True.Read more rulings at PolitiFact.com/florida.