# Fixing probability errors with normalization

A common error in models is when the branch probabilities do not sum to 1 (or 100%).

There is a feature in TreeAge Pro where you can automatically normalize the branch probabilities of a chance node to ensure this error does not occur.

• Probability expressions:
• branch A prob is pA
• branch B prob is pB
• branch C prob is #
• Scenario 1, no normalization needed.
• pA = 0.2
• pB = 0.3
• pC = # , and would calculate to 0.5
• Scenario 2, normalization needed.
• pA = 0.5 - normalized to 0.5/(0.5+0.6)
• pB = 0.6 - normalized to 0.6/(0.5+0.6)
• pC = #, and would calculate to 0. Without normalization, this would create an error because (0.5 + 0.6) > 1.

This is a relatively blunt tool, but it will automatically reduce the probabilities (including the # towards 0) as your probabilities (pA and pB) go up to the point where  "un-normalized" probabilities exceed 100%.

To use this function, right-click on the chance node and choose Change Coherence > Normalize Probabilities from the context menu.