Incremental increase method combines both arithmetical increase and geometrical increase methods in population forecasting. This method takes into account the current population size and applies a fixed increment or growth rate to estimate future population figures.

Arithmetical increase method:
- The arithmetical increase method assumes a constant increment in the population size over a given time period.
- It is based on the assumption that the number of individuals added to the population remains constant over time.
- For example, if the population increases by 100 individuals every year, the arithmetical increase method would project the same increase every year without considering any changes in growth rate.

Geometrical increase method:
- The geometrical increase method assumes a constant growth rate applied to the population size.
- It is based on the assumption that the growth rate remains constant over time.
- For example, if the population grows by a consistent percentage (e.g., 2%) every year, the geometrical increase method would project the same growth rate every year without considering any changes in the increment.

Incremental increase method:
- The incremental increase method combines the arithmetical and geometrical increase methods.
- It recognizes that population growth is influenced by both a fixed increment and a growth rate.
- This method takes into account the current population size and applies a fixed increment and a growth rate to estimate future population figures.
- The fixed increment represents the arithmetical increase, while the growth rate represents the geometrical increase.
- By considering both factors, the incremental increase method provides a more realistic and accurate forecast of population growth.

In summary, the incremental increase method is a combination of the arithmetical increase method (fixed increment) and the geometrical increase method (growth rate). It considers both factors to estimate future population figures, providing a more comprehensive and accurate population forecast.