There are two ways to define fast-and-frugal trees manually when using the `FFTrees()`

function, either as a sentence using the `my.tree`

argument (the easier way), or as a dataframe using the `tree.definitions`

argument (the harder way). Both of these methods will bypass the tree construction algorithms built into `FFTrees`

.

`my.tree`

The first method is to use the `my.tree`

argument, where `my.tree`

is a sentence describing a (single) FFT. When this argument is specified in `FFTrees()`

, the function (specifically `wordstoFFT()`

will try to extract the specified FFT from the argument.

For example, let’s look at the columns sex, age and thal in the heartdisease data:

`head(heartdisease[c("sex", "age", "thal")])`

```
## # A tibble: 6 x 3
## sex age thal
## <dbl> <dbl> <chr>
## 1 1 63 fd
## 2 1 67 normal
## 3 1 67 rd
## 4 1 37 normal
## 5 0 41 normal
## 6 1 56 normal
```

Here’s how we could specify an FFT using these cues as a sentence:

```
my.tree = "If sex = 1, predict True.
If age < 45, predict False.
If thal = {fd, normal}, predict True. Otherwise, predict False"
```

Here are some notes on specifying trees manually:

- Each node must start with the word “If” and should in the form:
`If CUE DIRECTION THRESHOLD, predict EXIT`

. - Numeric thresholds shold be specified directly (without brackets).
- Factor thresholds must be specified within braces like
`sex = {male}`

. For factors with sets of values, values within a threshold should be separated by commas like`eyecolor = {blue,brown}`

- Standard logical comparisons
`=`

,`!=`

,`<`

,`>=`

(etc.) are valid. For numeric cues, only use`>`

,`>=`

,`<`

,`<=`

. For factors, only use`=`

and`!=`

. - Positive exits are indicated by
`True`

, while negative exits are specified by`False`

. The final node will be forced to have a bidirectional exit. The text`Otherwise, predict EXIT`

I’ve included in the example above is actually not necessary.

Now, let’s pass the `my.tree`

argument to `FFTrees()`

to force apply our FFT to the heartdisease data:

```
# Pass a verbally defined FFT to FFTrees with the my.tree argument
my.heart.fft <- FFTrees(diagnosis ~.,
data = heartdisease,
my.tree = "If sex = 1, predict True.
If age < 45, predict False.
If thal = {fd, normal}, predict True.
Otherwise, predict False")
```

Let’s see how well our FFT did:

```
# Plot
plot(my.heart.fft)
```

As you can see, this FFT is pretty terrible – it has a high sensitivity, but a terrible specificity.

Let’s see if we can come up with a better one using the cues `thal`

, `cp`

, and `ca`

```
# Specify an FFt verbally with the my.tree argument
my.heart.fft <- FFTrees(diagnosis ~.,
data = heartdisease,
my.tree = "If thal = {rd,fd}, predict True.
If cp != {a}, predict False.
If ca > 1, predict True.
Otherwise, predict False")
# Plot
plot(my.heart.fft)
```

This one looks much better!

Here’s one more example using the `titanic`

data. We’ll create an FFT predicting whether a person survived the Titanic using the cues class, age, and sex:

`head(titanic[c("class", "age", "sex", "survived")])`

```
## class age sex survived
## 1 first adult male 1
## 2 first adult male 1
## 3 first adult male 1
## 4 first adult male 1
## 5 first adult male 1
## 6 first adult male 1
```

```
my.titanic.tree <- "If age = {child}, predict True.
If sex = {female}, predict True.
If class = {first}, predict True.
Otherwise, predict False"
titanic.fft <- FFTrees(survived ~.,
data = titanic,
my.tree = my.titanic.tree,
comp = FALSE)
```

```
## Warning in FFTrees(survived ~ ., data = titanic, my.tree =
## my.titanic.tree, : The argument comp is depricated. Use do.comp instead.
```

`plot(titanic.fft)`

`tree.definitions`

The second way to define one (or more) fast-and-frugal trees is with the `tree.definitions`

argument. This argument should be a dataframe with the following structure:

```
## tree nodes classes cues directions thresholds exits
## 1 1 3 c;c;n thal;cp;ca =;=;> rd,fd;a;0 1;0;0.5
## 2 2 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;0;1;0.5
## 3 3 3 c;c;n thal;cp;ca =;=;> rd,fd;a;0 0;1;0.5
## 4 4 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;1;0;0.5
## 5 5 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 0;0;1;0.5
## 6 6 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;1;1;0.5
## 7 7 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 0;0;0;0.5
```

The dataframe should have 7 columns:

`tree`

: An indexing integer`nodes`

: The number of nodes in the tree.

The following 5 columns define each node in an FFT, where nodes are separated by semi-colons `;`

:

`classes`

: The class of each node in the tree.`c`

= character,`n`

= numeric,`i`

= integert.`cues`

: The names of the cues`directions`

: The direction of*positive*decisions for that cue. Even if a cue only has a negative exit branch, the direction should always be specified as if it was making a positive decision.`thresholds`

: The decision threshold for the cue. For numeric cues, thresholds are single numbers. For factor cues, they are sets of factor values (separted by commas)`exits`

: The exit direction for the cue.`0`

= negative exit,`1`

= positive exit,`.5`

= both a negative and a positive exit (only for the final node in a tree)

On can see examples of `tree.definitions`

dataframes in an `FFTrees`

object. For example, the definitions above can be obtained as follows:

```
# Create an FFTrees object
heart.fft <- FFTrees(diagnosis ~.,
data = heartdisease)
# Get the tree definitions
heart.tree.definitions <- heart.fft$tree.definitions
# Print the result
heart.tree.definitions
```

```
## tree nodes classes cues directions thresholds exits
## 1 1 3 c;c;n thal;cp;ca =;=;> rd,fd;a;0 1;0;0.5
## 2 2 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;0;1;0.5
## 3 3 3 c;c;n thal;cp;ca =;=;> rd,fd;a;0 0;1;0.5
## 4 4 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;1;0;0.5
## 5 5 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 0;0;1;0.5
## 6 6 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 1;1;1;0.5
## 7 7 4 c;c;n;n thal;cp;ca;exang =;=;>;> rd,fd;a;0;0 0;0;0;0.5
```

One can use `tree.definitions`

dataframes created from `FFTrees()`

as a template, make adjustments, and then feed the dataframe back into `FFTrees()`

to create new, customized trees. Below, I’ll create definitions of two FFTs, then pass them to `FFTrees()`

. The two FFTS can be described as follows.

- Tree #1 “If slope != {down, up}, then predict False. If ca is greater than 1, predict True. Otherwise, predict False”
- Tree #2 “If chol < 300, then predict True If oldpeak is greater than 2, predict True. If restecg is
*not*normal, then predict False. Otherwise, predict True”

```
# Define two trees
my.tree.definitions <- data.frame(tree = c(1, 2),
nodes = c(2, 3),
classes = c("c;n", "n;n;f"),
cues = c("slope;ca", "chol;oldpeak;restecg"),
directions = c("=;>", "<;>;!="),
thresholds = c("down,up;1", "300;2;normal"),
exits = c("0;.5", "1;1;.5"),
stringsAsFactors = FALSE)
```

Now, we can pass these trees to `FFTrees()`

and view their resulting performance:

```
#Pass trees to FFTrees with tree.definitions
my.heart.fft <- FFTrees(diagnosis ~ .,
data = heartdisease,
tree.definitions = my.tree.definitions)
# Show summary statistics
my.heart.fft
```

```
## FFT 1 (of 2) predicts diagnosis using 2 cues: {slope, ca}
##
## [1] If slope != {down,up}, decide False.
## [2] If ca <= 1, decide False, otherwise, decide True.
##
## train test
## cases .n 303.000 --
## hits .hi 16.000 --
## misses .mi 123.000 --
## false al .fa 8.000 --
## corr rej .cr 156.000 --
## speed .mcu 1.538 --
## frugality .pci 0.890 --
## cost .cost 0.432 --
## accuracy .acc 0.568 --
## balanced .bacc 0.533 --
## sensitivity .sens 0.115 --
## specificity .spec 0.951 --
##
## pars: algorithm = 'ifan', goal = 'wacc', goal.chase = 'wacc', sens.w = 0.5, max.levels = 4
```

```
# Plot Tree 2
plot(my.heart.fft, tree = 2)
```

Here is Tree #1: “If slope != {down, up}, then predict False. If ca is greater than 1, predict True. Otherwise, predict False”

```
# Plot Tree 1
plot(my.heart.fft, tree = 1)
```

Here is Tree #2: “If chol < 300, then predict True. If oldpeak is greater than 2, predict True. If restecg is *not* normal, then predict False. Otherwise, predict True”

```
# Plot Tree 2
plot(my.heart.fft, tree = 2)
```