Exploring project data

With Data Science & AI Workbench, you can explore project data using visualization libraries such as Bokeh and Matplotlib, and numeric libraries such as NumPy, SciPy, and Pandas.

Use these tools to discover patterns and relationships in your datasets, and develop approaches for your analysis and deployment pipelines.

The following examples use the Iris flower data set, and this mini customer data set (customers.csv):

customer_id,title,industry
1,data scientist,retail
2,data scientist,academia
3,compiler optimizer,academia
4,data scientist,finance
5,compiler optimizer,academia
6,data scientist,academia
7,compiler optimizer,academia
8,data scientist,retail
9,compiler optimizer,finance

1. Begin by importing libraries, and reading data into a Pandas DataFrame:

   import pandas as pd
   import numpy as np
   import matplotlib.pyplot as plt
   irisdf = pd.read_csv('iris.csv')
   customerdf = pd.read_csv('customers.csv')
   
   %matplotlib inline
   

2. Then list column / variable names:

   print(irisdf.columns)
   

   Index(['sepal_length', 'sepal_width', 'petal_length', 'petal_width', 'class'], dtype='object')
   

3. Summary statistics include minimum, maximum, mean, median, percentiles, and more:

   print('length:', len(irisdf)) # length of data set
   print('shape:', irisdf.shape) # length and width of data set
   print('size:', irisdf.size) # length * width
   print('min:', irisdf['sepal_width'].min())
   print('max:', irisdf['sepal_width'].max())
   print('mean:', irisdf['sepal_width'].mean())
   print('median:', irisdf['sepal_width'].median())
   print('50th percentile:', irisdf['sepal_width'].quantile(0.5)) # 50th percentile, also known as median
   print('5th percentile:', irisdf['sepal_width'].quantile(0.05))
   print('10th percentile:', irisdf['sepal_width'].quantile(0.1))
   print('95th percentile:', irisdf['sepal_width'].quantile(0.95))
   

   length: 150
   shape: (150, 5)
   size: 750
   min: 2.0
   max: 4.4
   mean: 3.0573333333333337
   median: 3.0
   50th percentile: 3.0
   5th percentile: 2.3449999999999998
   10th percentile: 2.5
   95th percentile: 3.8
   

4. Use the value_counts function to show the number of items in each category, sorted from largest to smallest. You can also set the ascending argument to True to display the list from smallest to largest.

   print(customerdf['industry'].value_counts())
   print()
   print(customerdf['industry'].value_counts(ascending=True))
   

   academia    5
   finance     2
   retail      2
   Name: industry, dtype: int64
   
   retail      2
   finance     2
   academia    5
   Name: industry, dtype: int64
   

Categorical variables

In statistics, a categorical variable may take on a limited number of possible values. Examples could include blood type, nation of origin, or ratings on a Likert scale.

Like numbers, the possible values may have an order, such as from disagree to neutral to agree. The values cannot, however, be used for numerical operations such as addition or division.

Categorical variables tell other Python libraries how to handle the data, so those libraries can default to suitable statistical methods or plot types.

The following example converts the class variable of the Iris dataset from object to category.

print(irisdf.dtypes)
print()
irisdf['class'] = irisdf['class'].astype('category')
print(irisdf.dtypes)

sepal_length    float64
sepal_width     float64
petal_length    float64
petal_width     float64
class            object
dtype: object

sepal_length     float64
sepal_width      float64
petal_length     float64
petal_width      float64
class           category
dtype: object

Within Pandas, this creates an array of the possible values, where each value appears only once, and replaces the strings in the DataFrame with indexes into the array. In some cases, this saves significant memory.

A categorical variable may have a logical order different than the lexical order. For example, for ratings on a Likert scale, the lexical order could alphabetize the strings and produce agree, disagree, neither agree nor disagree, strongly agree, strongly disagree. The logical order could range from most negative to most positive as strongly disagree, disagree, neither agree nor disagree, agree, strongly agree.

Time series data visualization

The following code sample creates four series of random numbers over time, calculates the cumulative sums for each series over time, and plots them.

timedf = pd.DataFrame(np.random.randn(1000, 4), index=pd.date_range('1/1/2015', periods=1000), columns=list('ABCD'))
timedf = timedf.cumsum()
timedf.plot()

This example was adapted from https://pandas.pydata.org/docs/user_guide/visualization.html.

Histograms

This code sample plots a histogram of the sepal length values in the Iris data set:

plt.hist(irisdf['sepal_length'])
plt.show()

Bar charts

The following sample code produces a bar chart of the industries of customers in the customer data set.

industries = customerdf['industry'].value_counts()

fig, ax = plt.subplots()

ax.bar(np.arange(len(industries)), industries)

ax.set_xlabel('Industry')
ax.set_ylabel('Customers')
ax.set_title('Customer industries')
ax.set_xticks(np.arange(len(industries)))
ax.set_xticklabels(industries.index)

plt.show()

This example was adapted from https://matplotlib.org/gallery/statistics/barchart_demo.

Scatter plots

This code sample makes a scatter plot of the sepal lengths and widths in the Iris data set:

fig, ax = plt.subplots()
ax.scatter(irisdf['sepal_length'], irisdf['sepal_width'], color='green')
ax.set(
    xlabel="length",
    ylabel="width",
    title="Iris sepal sizes",
)
plt.show()

Sorting

To show the customer data set:

customerdf

row	customer_id	title	industry
0	1	data scientist	retail
1	2	data scientist	academia
2	3	compiler optimizer	academia
3	4	data scientist	finance
4	5	compiler optimizer	academia
5	6	data scientist	academia
6	7	compiler optimizer	academia
7	8	data scientist	retail
8	9	compiler optimizer	finance

To sort by industry and show the results:

customerdf.sort_values(by=['industry'])

row	customer_id	title	industry
1	2	data scientist	academia
2	3	compiler optimizer	academia
4	5	compiler optimizer	academia
5	6	data scientist	academia
6	7	compiler optimizer	academia
3	4	data scientist	finance
8	9	compiler optimizer	finance
0	1	data scientist	retail
7	8	data scientist	retail

To sort by industry and then title:

customerdf.sort_values(by=['industry', 'title'])

row	customer_id	title	industry
2	3	compiler optimizer	academia
4	5	compiler optimizer	academia
6	7	compiler optimizer	academia
1	2	data scientist	academia
5	6	data scientist	academia
8	9	compiler optimizer	finance
3	4	data scientist	finance
0	1	data scientist	retail
7	8	data scientist	retail

The sort_values function can also use the following arguments:

• axis to sort either rows or columns
• ascending to sort in either ascending or descending order
• inplace to perform the sorting operation in-place, without copying the data, which can save space
• kind to use the quicksort, merge sort, or heapsort algorithms
• na_position to sort not a number (NaN) entries at the end or beginning

Grouping

customerdf.groupby('title')['customer_id'].count() counts the items in each group, excluding missing values such as not-a-number values (NaN). Because there are no missing customer IDs, this is equivalent to customerdf.groupby('title').size().

print(customerdf.groupby('title')['customer_id'].count())
print()
print(customerdf.groupby('title').size())
print()
print(customerdf.groupby(['title', 'industry']).size())
print()
print(customerdf.groupby(['industry', 'title']).size())

title
compiler optimizer    4
data scientist        5
Name: customer_id, dtype: int64

title
compiler optimizer    4
data scientist        5
dtype: int64

title               industry
compiler optimizer  academia    3
                    finance     1
data scientist      academia    2
                    finance     1
                    retail      2
dtype: int64

industry  title
academia  compiler optimizer    3
          data scientist        2
finance   compiler optimizer    1
          data scientist        1
retail    data scientist        2
dtype: int64

By default groupby sorts the group keys. You can use the sort=False option to prevent this, which can make the grouping operation faster.

Binning

Binning or bucketing moves continuous data into discrete chunks, which can be used as ordinal categorical variables.

You can divide the range of the sepal length measurements into four equal bins:

pd.cut(irisdf['sepal_length'], 4).head()

0    (4.296, 5.2]
1    (4.296, 5.2]
2    (4.296, 5.2]
3    (4.296, 5.2]
4    (4.296, 5.2]
Name: sepal_length, dtype: category
Categories (4, interval[float64]): [(4.296, 5.2] < (5.2, 6.1] < (6.1, 7.0] < (7.0, 7.9]]

Or make a custom bin array to divide the sepal length measurements into integer-sized bins from 4 through 8:

custom_bin_array = np.linspace(4, 8, 5)
custom_bin_array

array([4., 5., 6., 7., 8.])

Copy the Iris data set, and apply the binning to it:

iris2=irisdf.copy()
iris2['sepal_length'] = pd.cut(iris2['sepal_length'], custom_bin_array)
iris2['sepal_length'].head()

0    (5.0, 6.0]
1    (4.0, 5.0]
2    (4.0, 5.0]
3    (4.0, 5.0]
4    (4.0, 5.0]
Name: sepal_length, dtype: category
Categories (4, interval[float64]): [(4.0, 5.0] < (5.0, 6.0] < (6.0, 7.0] < (7.0, 8.0]]

Then plot the binned data:

plt.style.use('ggplot')
categories = iris2['sepal_length'].cat.categories
ind = np.array([x for x, _ in enumerate(categories)])
plt.bar(ind, iris2.groupby('sepal_length').size(), width=0.5, label='Sepal length')
plt.xticks(ind, categories)
plt.show()

This example was adapted from http://benalexkeen.com/bucketing-continuous-variables-in-pandas/.