Communication efficiency

Communication Efficiency is a metric that measures the effectiveness at which a state can exert its will from its capital to a given province. It represents at what cost and at which capacity a state can move its troops and taxes to and from the provinces. It is measured by finding a minimum cost path from the capital to every provinces. If the cost of its minimum cost path is high, then a given province will receive bad CE. If not, it will receive good CE. A path is a set of connected provinces that starts from the capital and ends at the destination. A province that is further down the path is always worse off than a province that comes before. Cost of traversing a province is determined by factors such as terrain cost, road reduction on terrain cost, loading cost, harbourage reduction on loading cost, absolute distance between two provinces, and river connections. A province is traversed either through river connection, sea connection, or through land connection. Use the debug_mode functionality in the console (press ^ to open and close it) to find out which provinces is getting CE from where by hovering over the CE icon

Land
Traversal through land connection can occur if and only if two provinces are neighbors of each other. When that does occur, the cost of traversal will be determined by 'absolute distance between the provinces' cities' * ('terrain cost' * 'road reduction' + 'base').

Terrain cost can be checked by hovering over province's terrain type, but in general it is higher in provinces that are more mountainous and/or forested.

Road reduction works by simply multiplying the terrain cost with some factor determined by pathing rank, then adding a flat base on top of it. Its effect is that the cost of traversing through the province becomes more and more similar as pathing rank increases, while still ensuring that lower terrain cost provinces are easier to traverse through.

Sea
Traversal through sea connections works by including sea provinces within a path. As paths are calculated, some paths will include a set of neighboring sea zone provinces within its path toward its destination. As all destinations are land provinces, once a path goes out into the sea it must again come back to land at some point.

Key factors that determine the cost of traversing through sea provinces are embark cost and ship speed.

Embark cost is incurred when a path moves from the land to the sea or from the sea to the land. It is determined by land province's harbourage rank, whose exact details can be found by hovering over the building.

Ship speed is used when a path is moving from a sea province to another sea province. The cost of doing that is calculated by 'absolute distance between two sea zone provinces' centers' / 'ship speed'. Ship speed can be increased by dip tech.

River
Traversal through river connections can be seen as a modification of sea connection traversal. Key difference is that ship speed for river connection traversal is set as ('current ship speed' + 'max ship speed') / 2.

Capitol
The capitol building improves global CE, if capitol is in capital province, or regional CE if build in provinces not capital. The capitol decreases the CE by certain % and therefore can lead to significant changes if an pathing or harborage infrastructure is present. The capitol itself is an expensive and situational building.

Advice for improving CE

 * There are other ways of decreasing Local autonomy than improving CE.
 * If you are a land based country be aware that improving CE via pathing improvement can be very expensive, cash that you maybe want to put into the economy first.
 * Plan ahead. Think about countries you want to conquer, lands you want to colonize, cities you want to build.
 * If you conquer provinces in regions, where Local autonomy is already at 100% and CE is high. Consider to vassalize and integrate at later stages when the CE situation has improved.