The following is a brief insight on how to gauge and balance your games for your current group. The example given is based on the Pathfinder system.
You’ve started a new campaign. Your players have brought their new PC sheets with them. And despite knowing your players sheet by heart (or not, some of us work long hours). There really is no guarantee to know if your players will be engaged and challenged enough during your first encounter. Keep in mind however, that not every player may use their PC to it’s full potential. Despite them possibly being a horrible min maxer. It doesn’t matter if the PC is a potential boss killer if the PC isn’t played right or even dies cause of bad decisions & tactics. On the flip side, another player in the group may be a horrible min maxer and play that PC to terrifying effect. Leaving the other party members in their shadow.
You will need to rely on knowing your group and appealing to them during creation to keep things balanced as it is. Like most things in life, good communication is required.
Now that you have a ‘somewhat balanced’ group, you are still confronted with that first encounter. It’s easy to go overboard and have them fight a murder machine, thinking everything will be ok. At the same time, most GM’s will fear wiping their party during the first game and will end up with weak mobs that can’t hit to save their lives.