From: Third rock from the Sun.
From the Manual:
Each Event has a chance to have an effect. In
most cases, this will be 100%. If an effect does not
occur because it fails this check, it is considered
“cancelled”. If the Event passes the check, it is
considered “activated”. This can in turn cause other
Events to be activated or cancelled. Each Event has
a Turn Range. In most cases, this will be one Turn.
If a larger Range is specified, the actual occurrence
of the Event can activate (with equal chance) in
any Turn from the Turn of triggering to that Turn
plus the Turn Range. The check for activation of
an Event with less than 100% chance to occur only
happens once, regardless of the Turn range. The
Activation Check is made first, and the actual Turn
of Occurrence is calculated after it is determined
that the Event is to occur.
Example: An Event scheduled to be triggered
on Turn 5, with a Turn Range of 4 and a chance of
activation of 75%. There is a 75% chance (one-time
check) that the Event will occur. If it does occur, it
will actually be activated on Turn 5, 6, 7, or 8.