a) Have a very high tpa,
b) Have many Inns, because Inns give you protection, increases your thievery defense by 1% if you have built inns on 1% of your acres,
c) Cast Alarm on your own province (20% more defense against thieves),
d) Hire a councilor (Melangult or Goliath),
e) Race bonus, like Giants or Humans,
No, there are more factors that affect your chances to be succesful:
a) Inns, because Inns from Age 6 also increases your chance to do thievery operations by 1% if you have built inns on 1% of your acres,
b) Cast Silence on your own province (20% better chance to succeed with a thievery operation),
c) Racial Bonuses (Human & Giants).
d) Counsellor.
e) The Modifier of the specific operation (Greater Modifier = Easiest Operation = More chances to succeed),
f ) There is a Size modifier, (which only affects the strength though, not the actual success of the operation)
g) There is a randomness, because "# base chance = random number between 1 and 100".
One thief can steal 15 gold, 15 food and 15 metal in one rob and the maximum is 2% of a province's total resources. There is however also a size-protection. The bigger the difference in acres, the fewer one single thief can rob. Try robbing close to your own acres.
Influence doesn't have any impact on the strength of the operation (e.g. how much you rob, how many troops you kill), but it has a (small) effect on the success chance of the operation. If you are close to 100% you have more chance of succeeding then if you are on 40%. How is the thievery rank measured ? This is only known to The Gods. What us mortals do know however is that the more operations you make, the more 'points' you get. The more points, the higher the rank. You get negative points for failed operations.
Each time someone succeeds in a thievery operation against you, you also lose some points.
Peasant (worst)
Bandit
Thief Lord
Thief Baron
Viscount
Thief King
Decent (best)
You only know who is robbing you when he fails an operation. In 2/3 of the cases you get a message ( "we have found thieves from xxx (xx) in our Province. They were all executed.").
There is a maximum of 5% deviation of the statistics when spying on a province. This can be either positive (adding troops that don't exist, e.g. if a player has 1,000 rangers, the SOP will show 1,050) or negative (showing 950 rangers instead of 1,000).
The same deviation can also occur with Spy on Military.