**Step 7. Adding Auxiliaries**

We need to add two auxiliaries representing adoptions resulting from word of mouth and from advertising.

**Create the AdoptionFromAd auxiliary**

- Drag the
**Flow Aux Variable**element from the**System Dynamics**page of the**Palette**view onto the diagram of active object class (right to the place where you want to locate a variable). - In the
**Properties**view, change the**Name**to`AdoptionFromAd`. - Define the formula expression. In the
**AdoptionFromAd**= edit box, type:`AdEffectiveness*PotentialAdopters`

**Create the AdoptionFromWOM auxiliary**

- Do it in the same way except name the auxiliary
`AdoptionFromWOM`and specify the following formula:`ContactRate*AdoptionFraction*PotentialAdopters*Adopters/TotalPopulation`

Now we can formulate the adoption flow using just defined auxiliares. The two sources of adoption are assumed to be independent. Thus, the total adoption rate is the sum of adoptions resulting from word of mouth driven by the population of adopters and adoptions resulting from advertising.

**Define the formula for the adoption rate**

- Click the just created flow variable on the diagram.
- Go to the
**General**page of the**Properties**view. - Specify the formula expression in the
**AdoptionRate**= field:`AdoptionFromAd+AdoptionFromWOM`

Now we have completely defined our model. The stock and flow diagram of the model should look like as on the figure below.

You may examine the causal dependencies between stocks, flows and auxiliaries in your model. They are denoted with arrows as in standard SD notation.

- A thick arrow going from flow to stock means that this flow acts as inflow for this stock.
- A thick arrow going from stock to flow means that this flow acts as outflow.
- A thin arrow going from A variable to B means that A causes to change B.

You can see that our model has one balancing and one reinforcing feedback loop.

- A balancing feedback loop affects the adoption rate due to advertising. The adoption rate reduces the pool of the potential adopters, which in turn decreases the adoption rate.
- A reinforcing loop affects the adoption rate due to word of mouth. The adoption rate increases the adopter population, resulting in an increase of word of mouth, and thus the increase of the adoption rate.