Congress is preparing for its first major debate on federal gun laws in nearly a decade, but first both sides will need to figure out whose facts to use.

In the wake of the deadly school shootings in Newtown, Conn., gun-control advocates point to figures that seem to show a correlation between stricter laws and lower crime and homicide rates. Pro-gun groups, though, say the data show just the opposite — that violence and crime drops where concealed-carry laws are allowed.

The press and the public are caught in the middle, searching for concrete conclusions that are tough to come by.

Sen. Pat Roberts, Kansas Republican, said that while everything should be on the table for discussion in the gun debate, the sheer volume of data presents a challenge for moving legislation forward — beginning with the 1994 assault weapons ban, which either helped or hindered crime-fighting efforts, depending on whose numbers are to be believed.

“There’s been an awful lot of studies with regards to what a ban did and what happened after that,” Mr. Roberts said. “The use of those kind of weapons actually went up after the ban, so you’ve got to figure that out.”