[Vero]

pawn Код:

new rand = random(10); // How many cases


                switch(rand)
                {
                    case 0, 3, 5, 7: //Case number = a chance 
                    {
                        SendClientMessage(playerid, W, "You robbed the bank!" );
                    }
                    case 1, 4, 6, 8: // The more the case numbers the bigger the chance.
                    {
                                  SendClientMessage(playerid, W, "You have been caught!" );

That might do the trick.

EDIT: Changes some of the // to make it more useful.