StoTree software for modeling medical interventions

Gordon Hazen, Ph.D.

StoTree is software tool for the formulation and solution of continuous-time Markov models of medical interventions. Markov chain models provide a convenient means to account for medical treatment options and risks that occur not only in the present but also in the near and distant future. See my introductory video for an overview.

StoTree is implemented in the Excel spreadsheet environment in Windows (no Macintosh version is available). It is a Microsoft Excel addin for Microsoft Office 2007, 2010, 2013, 2016 and Office 365 that occupies its own tab on the Office ribbon. It represents Markov models using the graphic technique of stochastic trees.

Obtaining a copy of StoTree

StoTree works only on a Windows computer. I do not believe it will fully function on a MacIntosh computer. However, there is software available that lets a MacIntosh emulate a PC. It is possible that StoTree will function properly on such a system.

To obtain a free version of StoTree and test it out, complete the following steps, which apply to Office 365 systems:

1. Download the StoTree file.

2. Find the file in your Downloads folder and unblock it.

3. Move the StoTree file to the default location for Excel add-ins.

4. Open Excel and activate StoTree.

In more detail,

1. Click here to download the StoTree file, which is an Excel add-in named StoTree7v.xlam. (The letter “v” may be replaced by another letter for later editions of StoTree.)

2. Unless you select otherwise, the StoTree add-in will be saved to your Downloads folder. Find it there or wherever you saved it. Then right click on it and select Properties. In the resulting window, near the bottom of the General tab, check the box next to Unblock.

3. Use File Explorer to move the StoTree add-in to the default location for Excel add-ins, which on a computer running Office is the path

c:\Users\[your name]\Appdata\Roaming\Microsoft\Addins\.

Here, [your name] is some designation of your account under Windows, and will vary with your computer. Alternatively, you can leave it where you saved it and find it in the next step.

4. Open a workbook in Excel. Then

a. Make sure the Developer tab is visible. If it is not, then in Excel, follow the path File/Options/Customize Ribbon, then check the textbox next to Developer, and click OK.

b. To activate StoTree, click on Developer/Excel Add-ins, and click the checkbox next to the StoTree add-in. If it is not present, then you will have to Browse to the location you saved it to.

The StoTree tab should now appear on your Excel ribbon:

Playing with StoTree

You can click around the StoTree menu items (Hint: Start at the left). You may find my introductory video useful to get you started. But at some point, you should read the available documentation for StoTree, which will guide you through constructing a simple stochastic tree. You can also download an Excel workbook containing a completed stochastic tree model, and examine its features using the StoTree tab in Excel.

Opening workbooks containing stochastic tree models

When you open a workbook that contains a stochastic tree that you or others have created using StoTree, you should be aware of the following:

1. StoTree does insert macros into the workbooks it creates. This means Excel believes they could be dangerous workbooks. While macros can be used maliciously, the macros in these workbooks are not malicious. In order to inspect these workbooks properly, you need to

a. Make sure the workbook is unblocked, as described for the StoTree file in step 2 above. You need to unblock not only StoTree but also workbooks others have created and transmitted to you.

b. Make sure all macros are enabled if they are not already (Developer tab/ Macro Security/…).

2. Even with all macros enabled, Excel may issue various warnings when you open a workbook created by StoTree. You can ignore all such warnings. In detail:

a. A PROTECTED VIEW warning about files from the Internet that can contain viruses. These workbooks do contain macros, which is what Excel is worried about. You can stay in protected view if you wish, but you will not be able to use the StoTree ribbon to explore the downloaded workbook. To be able to do so, you should Enable Editing.

b. A warning about circular references. StoTree sets up circular references in workbooks it creates. Simply click OK to dismiss this warning.

c. A security warning that automated updating of links has been disabled, and an option to enable content. The links Excel refers to are from the graphics in this workbook to the computer (probably my computer) on which this workbook was created. You do not want to update these links, so simply close the warning banner.

d. When you click on a graphic, Excel may produce a warning that it cannot find the needed link. This is because the link may be on another computer, which means the link is orphaned. Click OK to dismiss the warning, but see the next comment.

3. You need to convert the orphaned links in a workbook you open in order to point to the version of StoTree that now resides on your computer. To do so, click on the Convert button in the About section of the StoTree tab. After a short pause in which conversion occurs, you should see the following acknowledgement:

Interesting workbooks constructed with StoTree

The Excel workbooks below contain completed stochastic tree models that you may like to inspect. To download a workbook, click on the link provided.

- Beck and Sonnenberg (1993). Modeling whether to cease post-transplant immunosuppression when melanoma occurs (the model illustrated in my introductory StoTree video). In FA Sonnenberg and JR Beck. Markov Models in Medical Decision Making: A Practical Guide. Med Decis Making 1993 (13): 322-338.

- Test(7f).xlsm. A simple model of testing for a generic disease.

- Exer 2_5 HIV-AIDS (StoTree 7f).xlsm. A stochastic tree cost-effectiveness model of the decision to use add lamivudine to zidovudine therapy for HIV-positive individuals. Based on Chapter 2 from Briggs, Sculpher, Claxton (2006), Decision Modeling for Health Economic Evaluation.

- Exer 3_5 (StoTree7f).xlsm. A stochastic tree cost-effectiveness model of the decision to recommend a new prosthesis for hip replacement. Based on Chapter 3 from Briggs, Sculpher, Claxton (2006), Decision Modeling for Health Economic Evaluation.

- Matchar and Pauker 1986.xlsm. A stochastic tree cost-effectiveness model of the decision to undertake carotid endarterectomy. Based on Matchar DB, Pauker SG. Transient ischemic attacks in a man with coronary artery disease: two strategies neck and neck. Med Decis Making 1986 (6): 239-49.

- Tsevat et al (1989).xlsm. A multi-factor stochastic tree cost-effectiveness model of the decision to administer the anticoagulant warfarin. Based on Tsevat J, Eckman MH, McNutt RA, Pauker SG. Warfarin for dilated cardiomyopathy: a bloody tough pill to swallow? Med Decis Making 1989 (9): 162-9.

- Chang Pelliessier Hazen 1996.xlsm. A multi-factor stochastic tree cost-effectiveness model of the hip replacement decision for elderly adults. Based on Chang RW, Pellissier JM, Hazen GB. A cost-effectiveness analysis of total hip arthroplasty for osteoarthritis of the hip. JAMA March 20, 1966-275(11) 858-865.

These workbooks contain named cells that hold parameter settings used in the QALY and cost calculations. You might try changing values in these cells and recalculating the cohort sheet to do simple what-if analyses.