# Statistical Models from formulas#

This week, I taught a course on statistical modeling in statsmodels. For those of you who have never used or heard of this Python package, it began as a subpackage in `scipy`

called `scipy.models`

. However, as it grew in size and complexity, it was removed from `scipy`

, and then it became its own package, `statsmodels`

.

As a package, it is a great way to carry out statistical modeling as it
provides a great deal of model introspection right out of the box, enabling users to fine-tune their model specification. In this regard, it is similar to the very popular scikit-learn package, but I have found the main difference between the two is that `statsmodels`

is more for introspecting single models, while `scikit-learn`

provides a powerful, object-oriented interface for creating predictive pipelines.

While each of these statistical packages has their place, `statsmodels`

has
a home primarily in academic research AND with users transitioning from R to Python.

## Patsy: formulaic model specification in Python#

The primary reason, `statsmodels`

sees a large influx of R users is due to its
explicit support for formula-specified models. The formula mini-language originated
in S and was subsequentally implemented in R. The whole idea behind the formula language
was to provide a simple interface for representing complex statistical models.

The most common use of fomulas is to allow users to abstract away the construction of design
matrices, used very commonly in linear models. While `statsmodels`

does not
implement the formula language in Python, it is the only large statistics package
that uses them. The actual parsing and construction of the formulas is performed
by patsy.

But, before you can really appreciate working with *formulas*, let’s do some linear
model fitting (multiple regression) with design matrices we craft by hand.

## Creating dummy data#

To examine a slightly interesting problem, I wanted to ensure that we had at least two predictive variables to work with: one discrete and one continuous. The continuous values are slightly dependent on the factor creating a slight correlation between them (which we’ll ignore).

Importantly, our y values, `ys`

, are highly dependent on predictive variables, which
is what we’ll try to model.

```
from pandas import DataFrame
from numpy import array, choose
from numpy.random import default_rng
rng = default_rng(0)
df = DataFrame({
'factor': rng.choice(['a', 'b', 'c', 'd'], size=100),
}).astype({'factor': 'category'})
df['xs'] = choose(
df['factor'].cat.codes,
rng.normal(
[[60], [50], [40], [35]], scale=5, size=(df['factor'].nunique(), len(df))
)
)
df['ys'] = (
df['xs'] - array([50, 40, 30, 20]).take(df['factor'].cat.codes)
+ 20 + rng.normal(0, scale=3, size=len(df))
)
df.head()
```

factor | xs | ys | |
---|---|---|---|

0 | d | 27.354536 | 28.259441 |

1 | c | 39.805825 | 31.084139 |

2 | c | 45.331623 | 37.864355 |

3 | b | 47.446888 | 27.142061 |

4 | b | 49.942335 | 28.892937 |

```
from matplotlib.pyplot import subplots
fig, ax = subplots()
for factor, group in df.groupby('factor'):
p = ax.scatter('xs', 'ys', label=factor, data=group, ec='white')
ax.legend(loc='upper left', bbox_to_anchor=(1, .9), title='factor')
ax.set(xlabel='xs', ylabel='ys')
fig.tight_layout()
```

## Design Matrices in NumPy#

Let’s first take the simple route: predict `ys`

directly from `xs`

without worrying
about factor.

```
from numpy.linalg import lstsq
from numpy import column_stack, ones_like
from pandas import get_dummies
# create our design matrix. `xs` is passed through as is
# ones_like represents the intercept of our model
design = column_stack([df['xs'], ones_like(df['xs'])])
design[:5]
```

```
array([[27.35453563, 1. ],
[39.80582481, 1. ],
[45.33162277, 1. ],
[47.44688766, 1. ],
[49.94233469, 1. ]])
```

```
coefs, *_ = lstsq(design, df['ys'], rcond=None)
pred_df = df.assign(pred_ys=design @ coefs)
pred_df.head()
fig, ax = subplots()
ax.scatter('xs', 'ys', data=pred_df, alpha=.8)
ax.plot('xs', 'pred_ys', data=pred_df.sort_values('xs'))
```

```
[<matplotlib.lines.Line2D at 0x7f08e76976d0>]
```

As you can see in the code above, the actual setup of our design was fairly simple—no preprocessing needed to be done, and all we had to do was add a column of `1`

s
to represent our intercept in our least squares operation.

But, what about when we add in a discrete variable such as our `factor`

column?
For these values to be appropriately modeled in our design matrix,
we will need to sparsely encode them. The most common coding of which is
referred to as “dummy coding,” and, thankfully, `pandas`

has a function for that.

```
dummies = get_dummies(df['factor'])
dummies.head()
```

a | b | c | d | |
---|---|---|---|---|

0 | 0 | 0 | 0 | 1 |

1 | 0 | 0 | 1 | 0 |

2 | 0 | 0 | 1 | 0 |

3 | 0 | 1 | 0 | 0 |

4 | 0 | 1 | 0 | 0 |

This transforms each level in our factor into its own column. We will need to
remove one of these columns (because we are including an intercept in our model,
we need to ensure that our columns cannot be linear combination of other columns—there is a lot more to this, but I’ll save those details for a future post on
linear regression) and combine with our `xs`

column to complete our
design matrix.

```
from numpy import nan
from pandas import get_dummies
dummies = get_dummies(df['factor'])
design = column_stack([df['xs'], dummies.drop(columns=['a']), ones_like(df['xs'])])
design[:5]
```

```
array([[27.35453563, 0. , 0. , 1. , 1. ],
[39.80582481, 0. , 1. , 0. , 1. ],
[45.33162277, 0. , 1. , 0. , 1. ],
[47.44688766, 1. , 0. , 0. , 1. ],
[49.94233469, 1. , 0. , 0. , 1. ]])
```

```
coefs, *_ = lstsq(design, df['ys'], rcond=None)
fig, ax = subplots()
for factor, group in df.groupby('factor'):
p = ax.scatter('xs', 'ys', label=factor, data=group, ec='white')
ax.legend(loc='upper left', bbox_to_anchor=(1, .9), title='factor')
ax.set(xlabel='xs', ylabel='ys')
fig.tight_layout()
pred_df = df.assign(pred_ys=design @ coefs)
# slight matplotlib trick here- using nan to break apart contiguous lines
pred_df = pred_df.sort_values(['factor', 'xs'], ascending=[False, True])
pred_df.loc[~pred_df.duplicated(['factor'], keep='last')] = nan
ax.plot('xs', 'pred_ys', data=pred_df)
```

```
[<matplotlib.lines.Line2D at 0x7f08e7541780>]
```

Creating the design matrix by hand isn’t too bad, but we lose some *at a glance*
information about what the model is actually comprised of. We would need to be familiar
with `numpy.column_stack`

and with `pandas.get_dummies`

in order to make
sense of the code above.

This is where `patsy`

and `statsmodels`

come into play.

## Linear Regression in Statsmodels#

```
from matplotlib.pyplot import subplots, show
from numpy import column_stack, ones_like, array, choose
from pandas import read_pickle
from statsmodels.formula.api import ols
# model specification: ys predicted by a combination of xs and categorical factors
# the dummy coding is taken care of for us (Treatment coding by default)
model = ols('ys ~ xs + C(factor)', data=df)
model.exog[:5, :] # design matrix created for us!
```

```
array([[ 1. , 0. , 0. , 1. , 27.35453563],
[ 1. , 0. , 1. , 0. , 39.80582481],
[ 1. , 0. , 1. , 0. , 45.33162277],
[ 1. , 1. , 0. , 0. , 47.44688766],
[ 1. , 1. , 0. , 0. , 49.94233469]])
```

```
fit = model.fit()
# Why not show yet ANOTHER way to plot these data
fig, ax = subplots()
for label, g in df.assign(fitted=fit.fittedvalues).groupby('factor'):
ax.scatter('xs', 'ys', label=label, data=g, ec='white')
ax.plot(g['xs'], g['fitted'], color='tab:blue')
ax.legend(loc='upper left', bbox_to_anchor=(1, .9), title='factor')
ax.set(xlabel='xs', ylabel='ys')
fig.tight_layout()
```

The formula `'ys ~ xs + C(factor)'`

provides a concise manner of specifying our
design matrix, abstracting the need to drop into NumPy ourselves. Furthermore, the
`ols`

(ordinary least squares) abstraction enables us to easily model our data and
inspect its results. This is a utility that `numpy.linalg.lstsq`

does not provide.

## Wrap up#

That brings us to a close! This is such a vast topic that I am planning to revisit soon.
Hopefully this post left you with some questions about constructing design matrices
and how we can use `patsy`

without `statsmodels`

to create them for
other programs to use. See you all again next time in a narrower dive
into this topic!