In the game Call of Cthulhu, the game design is very well thought out and realistic. There is, however, a few players who want to have the option of improving characters through experience and it is for these individuals that I offer the following rules.
In standard CoC you improve skills in the following manner: For every successful use of a skill in a game, you allowed to roll against the skill to see if it improves. Thus if you use the following skills successfully in a game Handgun (56%) and Spot Hidden (77%) you are able to roll percentile dice against the skill to see if you roll OVER the percentage that you already have. Thus for Handgun you roll an 86 and for Spot Hidden you roll a 23. The roll for Handgun (86) is over 56 and thus successful, while the roll for Spot Hidden is 23 and less than the 77% that you already possess. For Handgun you then roll a 1d10 and add the result, let us say that you rolled a 7, to the 56% and the total becomes your new Handgun skill (56 + 7 = 63%).
This is fine for a thoroughly realistic campaign, although for some groups this amount of progression is entirely unsatisfactory.
The following system is based on Experience Points (EP's) and they are gained in the standard way. The EP's are then spent to buy rolls to improve skills. Each EP that you spend allows you ONE roll against your current skill level with percentile dice and is handled exactly the same way as the standard system (roll d100, if the result is over your current level, roll a 1d10 and add it to the skill to get the new skill level.) with one addition. If you roll lower than your skill level, you still gain 1% to the skill. Please note that the EP system REPLACES the standard one and you do not get to roll for all successful skill use and use EP's to improve skills. You have the option of using the EP system AND the standard system together, but beware that characters will soon progress at a very rapid rate which has the potential to turn CoC into a super-heroic game rather than a horror game.
To improve attributes (Strength, Dexterity, Power, etc.) each EP that you spend allows you a d6 to roll against your current attribute level. If your strength is a 13 and you wish to improve it you would expend 4 EP's to buy 4d6 to roll against a 13. If the roll is below 13 you gain nothing. If the roll is above 13, however, you gain 1 point to your attribute. Notice that the higher your attributes are the more expensive it is to raise them because each point can only buy 1d6, so to raise a 14 you would have to spend a minimum of 3 EP's to buy 3d6 and hope that you roll higher than a 14. The more EP's you spend, the higher chance that you will roll over the target number and raise your stats, but there is no guarantee other than spending more EP's than the attribute value thus ensuring you cannot roll lower than the current stat level.
I realize that this is very expensive, but improving statistics such as Strength and Dexterity, etc. is very difficult in reality.
Gaining EP's during game play should be based on the following scale:
A word of caution is needed, in that this system may be used to easily create munchkin characters in CoC which is not in the spirit of the game. If this becomes a problem, scrap this system and use the official one. Also, remember that the HUMAN MAXIMUM for all statistics is 18, with the only exception being Education which has a maximum value of 21. All skills have a maximum value of 99%. DO NOT let players exceed these limits.