Why Long Sessions Increase Variance
What the term means
Longer sessions can increase the absolute variability of total monetary results because they include more individual rounds. That does not mean the game's per-round variance necessarily increases or that every long session must end in a larger loss.
How the system or behaviour works
For independent identical rounds, the variance of a sum is the number of rounds times the variance of one round; the standard deviation of that sum scales with the square root of the count. Expected cumulative loss under a negative edge grows approximately in proportion to turnover.
Example and interpretation
If one round has a hypothetical result standard deviation of $5, the standard deviation of 100 independent such results is about $50, not $500. The model's per-round $5 standard deviation remains unchanged.
Limits and cautions
The distinction between total swings, average return and fatigue matters. Stateful bonus games or changing stakes require a more careful model than simple independent identical trials.
Related concept to keep separate
For a different question, see Short vs Long Gambling Sessions (/guides/short-vs-long-sessions/). The comparison helps distinguish the present topic from a neighbouring concept without assuming the two share the same rules or conclusions.