5.5 Entering probabilities and payoffs

The model now is now configured, it has a structure and we have created the numeric values. The final part of building the model is referencing the numeric values in the correct places.

When the tree is calculated it will use the likelihood of the patient pathways and the values of each scenario (cost and effectiveness) to estimate the Expected Values for each strategy. Separate values are entered for cost and effectiveness. If each scenario has several path probabilities then these are multiplied together to give the likelihood of the scenario.

Probabilities

We can enter the probability for Radiation: Treatment eradicates tumor, which is the top branch. To enter the probability:

  • Select the Enter prob text below the Treatment eradicates tumor branch.

  • In the dialogue type pEradicateRad, and select enter. (If you do not type the already defined variable exactly, TreeAge will create a new variable for you. See below for other fail safe methods to enter variables).

At each chance node, the probabilities for the branches need to sum to 1 (or 100%). To assist with this we can enter the compliment for the other probability (Radiation: Treatment fails to eradicate tumor) in the model. Select the textEnter prob below the second branch and then enter '#'. When the model is calculated this will evaluate '#' as (1 - pEradicateRad). You can use the '#' for one branch of any chance node to calculate the compliment.

Repeat the same process for the remaining probabilities, as per the figure below.

Payoffs

The final values which need to be entered into the model before the model can be analyzed are the payoffs. In this model, we have two active payoffs: cost and effectiveness.

To edit the payoffs for the top scenario:

  • Double click on the top most Terminal node and the Edit Payoff editor will open.

  • For the Cost payoff, click in the value at node column and enter the formula: cRadiation + cFollowupAnnual*lyEradicated

  • For the Effectiveness payoff, select in the value at node column and enter lyEradicated.

  • The payoff editor should look like the image below. Select OK, and the payoffs will now be visible in the tree.

Repeat this process for all the other payoffs with the appropriate values of cost and effectiveness.

The completed tree should look like the image below.

..

We can now analyze the model.

OTHER INFO

Entering variables in the model above, we typed the variable names we had defined directly into the fields as required. But this can lead to mistakes. Luckily there are some fail safe methods to enter variables (or any other defined numeric value) into your model. You can use one of the following two options:

  1. Use Ctrl + Spacebar: To enter a variable into the model, start typing the first letters of the variable you want to reference. For example for pRadiation, type: pr. Then hold down Ctrl and the Spacebar and a list of all the references in the model starting with pr will be listed. Just select the one you want from the drop down menu and it will fill it out.

  2. Use the function editor: When you start to enter the values in the variable field, you will see a =fx editor which you can select. Once you open the =fx, you are presented options of different groups (such as Variables) which show you elements on the right (your already defined variables) which you can select and enter into the Build Expression definition at the top. This is really useful for defining expressions or formulas.