Events

This page deals with event mechanics. For an overview of all events in the game, see list of event lists.

Events occur throughout the course of play of EU4. There are a whole range of events in the game, which can result in positive, negative and mixed outcomes for a player's country. They take the form of a pop-up notification on the player's screen, which may present a player with a choice, or may simply inform the player of the consequences and require they acknowledge the event has occurred. After four months on the same date, the first choices gets autoselected, if the player did not choose an option.

Triggered-only events

Triggered-only events are events that occur as a result of a nation's actions. The events can happen to that same nation or to another nation altogether. The needed trigger can be as simple as one nation choosing a particular option in an event or complex as taking over a certain province, thereby enabling an event at another nation. The events are usually fired immediately or within a short amount of time from being triggered.

Pulse events

Pulse events are random events that happen within regular intervals of time. One of their main purposes is to provide interactions and a sense of randomness within the game. That being said, while the events are bound to happen they are still based on player's actions; for example, for an exploration event to fire, the player needs to have an explorer/conquistador in the process of exploration.

The events are grouped together into different sets depending on their purpose (regular gameplay or mechanic related) and assigned weights within their respective sets. Each set is programmed to fire in regular time intervals. The time intervals range from yearly intervals and up-to 5 year intervals. It is important to note however that there are several same-year intervals and it does not mean they'll fire at the same date. When the interval arrives an event is picked at random from within the set and fired. Events with higher weights have higher chances to fire than the lower ones. Having certain modifiers, idea groups, provinces etc. can increase or decrease some events' weights. Most pulses also have a weight for getting no event at all.

Date calculations for x-year pulses

The 2 year pulses always happen in years which are divisible by 2 and the 3 year pulses happen in years divisible by 3. The years in which the 4 and 5 year pulses happen depend on the tag order id of the country which gets the event. They happen if $({\text{year}}-{\text{tag order id}})$ is divisible by 4 (5 for 5 year pulses). The day of the year on which the pulse happens for a country is calculated with the following formula(this applies to all x-year pulses):

$({\text{year}}+{\text{tag order id}}+{\text{pulse offset}}){\bmod {3}}65$

Day 0 is January 1st and day 364 is December 31st.

The pulse offset is different for each pulse:

Pulse	Offset
2 year pulse I	0
2 year pulse II	30
2 year pulse III	60
2 year pulse IV	90
2 year pulse V	120
3 year pulse I	94
3 year pulse II	124
3 year pulse III	154
3 year pulse IV	184
4 year pulse I	130
4 year pulse II (unused)	160
4 year pulse III (unused)	190
4 year pulse IV (unused)	220
5 year pulse I	167
5 year pulse II	197
5 year pulse III	227
5 year pulse IV (only used for fail-safes/cleanups)	257

Mean time to happen

The chance of an event with an MTTH of 365 days firing within x days.

Other events are not triggered but have a chance of happening at any time if their conditions are met. These events are controlled by a mean time to happen (MTTH).

This mechanic determines the time it takes for an event to trigger, measured in months. Yet since events are probabilistic, there is a small chance an event may happen very quickly, or possibly never at all during the game. For example, if an event had a MTTH of 120 months, it means that usually it would take around a decade for the event to trigger. Certain factors may increase or decrease MTTH for an event, such as country's leader's quality, national stability, and so on.

Assuming the probabilities are calculated in the same manner as in EU3, then despite the name MTTH is actually the median time, or half-life, for the event to trigger, i.e., the amount of time during which there is a 50% chance of the event occurring by that time, rather than the mean (average).

The engine checks whether an event occurs every EVENT_PROCESS_OFFSET days, set to 20 at default. The chance per check is

$1 - 2^{-t_c / t_{1/2} }$

where $t_c$ is the EVENT_PROCESS_OFFSET and $t_{1/2}$ is the median.

This approximates the "ideal" probability

$1 - 2^{-t / t_{1/2}}$

where $t$ is the amount of time that passes. The approximation is best for a long MTTH.

The median $t_{1/2}$ and mean $\overline{t}$ are related by

$\overline{t} = \frac{t_{1/2}}{\ln 2} \approx 1.44 t_{1/2}$

In terms of the mean, the ideal probability is:

$1 - e^{-t / \overline{t}}$

For a repeating event the number of occurrences within a given number of checks is given by a binomial distribution, which may be approximated by a Poisson distribution for a long MTTH.

See also

• List of events files

Mechanics

Customization

Nation designer • Random New World

Ages and Institutions

Ages • Institutions

Innovativeness and Technology

Innovativeness • Technology

Defense

Fort • Zone of control