# Simulation with AnyLogic/System Dynamics/Step 5. Adding constants

**Step 5. Adding Constants**

Now we will define constants of our model. In AnyLogic you define a constant by creating a parameter.

**Define a constant representing total population**

- Drag the
**Parameter**element from the**System Dynamics**page of the**Palette**view onto the diagram of active object class. - On the
**General**page of the**Properties**view, set up the parameter properties. - Change the name of the constant. Type
`TotalPopulation`in the**Name**edit box. - In the
**Default Value**edit box, type`100000`. This will be the total population in our model.

The rate, with which potential adopters come into contact with adopters, is assumed to be constant. So, we will define a constant to represent contact rate.

**Define the ContactRate constant**

- Define the constant in the same way. Enter the
**Name**:`ContactRate`. - Assume a contact rate of 100 per person per year. In the
**Default Value**, type`100`.

In this model the volume of advertising and the probability that a potential adopter will adopt as the result of exposure to a given amount of advertising are assumed to be constant each period. So, we will add a constant to model the advertising effectiveness — the fractional adoption rate from advertising.

**Define a constant representing advertising effectiveness**

Define one more constant to specify the adoption fraction — the proportion of contacts that are sufficiently persuasive to induce the potential adopter to purchase the product.