To calculate the retention index for a phylogenetic tree, we use the following formula:

$$\frac{\text{maximum number of steps on tree - number of steps on the tree}} {\text{maximum number of steps on tree - minimum number of steps in the data}}$$

To calculate the maximum number of steps, we look at the number of observed character states for a character and take the lowest number. For each character, we take the lowest number of observed traits and then sum across all characters.

Take the following dataset:

          123456
Outgroup  000000
Taxa A    100001
Taxa B    110001
Taxa C    111001
Taxa D    111101
Taxa E    111111
Taxa F    111110

The maximum number of steps for the first character is 1, since the character state 0 is attested once and character state 1 occurs six times.

What I don't understand is why we do not use 6 for the maximum number of changes for the first character.

To calculate the retention index for a phylogenetic tree, we use the following formula:

$$\frac{\text{maximum number of steps on tree - number of steps on the tree}} {\text{maximum number of steps on tree - minimum number of steps in the data}}$$

To calculate the maximum number of steps, we look at the number of observed character states for a character and take the lowest number. For each character, we take the lowest number of observed traits and then sum across all characters.

Take the following dataset (columns are characters, taxa are rows):

          123456
Outgroup  000000
Taxa A    100001
Taxa B    110001
Taxa C    111001
Taxa D    111101
Taxa E    111111
Taxa F    111110

The maximum number of steps for the first character is 1, since the character state 0 is attested once and character state 1 occurs six times.

What I don't understand is why we do not use 6 for the maximum number of changes for the first character.