If your stringers are more than 10’ long we recommend installing a rigid beam at mid span to add support to the structure.  Stair stringers strength is considerably weakened by the cuts made to create the stair surfaces and stairs must be strong to withstand very stressful conditions.  It is preferable to break long stair runs with intermediate landings both visually and structurally but in some cases you may need to use one long run.  A brace will dampen the shear stress that can compromise the stringers integrity in the center of the stairs.  Most deck stairs are under built and as a result feel bouncy and are a risk to collapse especially after weathering can propagate cracks in the stringers.  You may also consider doubling your stair stringers by nailing two 2x12’s together like a beam in order to increase strength. Adding blocking between stringers can also strengthen the entire stair structure.