Cohort Analysis是将某一个时期内的用户划分为一个cohort，并将多个cohort进行时间上的某个属性的比较的一种分析方法。Cohort Analysis在有些场景下非常有用。比如一个网站或App，在某个连续的4周里陆续更新或新增了一个功能或设计，想要知道这些功能和设计上的改动对用户的影响，就可以将每周的新注册作为一个cohort，观察这4个cohort在接下来的一段实际里的行为数据，就可以很清楚地观察到4个改动的影响。

最近要做Cohort Analysis，数据都在数据库里，就直接想用Python接数据库直接分析了。Google到一篇讲得很清楚的文章，学着用Python实现一遍。示例数据从这里下载。

1. 读取数据

据说 from ... import ... 的方式在性能上有弊端，一般都推荐直接 import。示例数据是典型的购物数据，按客户的第一次消费时间将客户分为不同的cohort。

import pandas as pd

import numpy as np

import matplotlib.pyplot as plt

%matplotlib inline

df = pd.read_excel('relay-foods.xlsx', sheet='Purchase Data')

df.head()

	OrderId	OrderDate	UserId	TotalCharges	CommonId	PupId	PickupDate
0	262	2009-01-11	47	50.67	TRQKD	2	2009-01-12
1	278	2009-01-20	47	26.60	4HH2S	3	2009-01-20
2	294	2009-02-03	47	38.71	3TRDC	2	2009-02-04
3	301	2009-02-06	47	53.38	NGAZJ	2	2009-02-09
4	302	2009-02-06	47	14.28	FFYHD	2	2009-02-09

2. 确定 OrderDate 的月份，根据 OrderDate 分群

分别创建两个字段 OrderPeriod 和 CohortGroup。OrderPeriod是购买日期的月份，CohortGroup是根据购买日期对 UserId 的分群。

df['OrderPeriod'] = df.OrderDate.map(lambda x: x.strftime('%Y-%m'))

df = df.assign(OrderPeriod = df.OrderDate.map(lambda x: x.strftime('%Y-%m'))) \

       .set_index('UserId')

# goupby(level=0): level 是index的level， 对于multiIndex， 可用level=0或1指定根据那个index来group

df = df.assign(CohortGroup = df.groupby(level=0).OrderDate.min().apply(lambda x: x.strftime('%Y-%m'))) \

       .reset_index()

df.head()

	UserId	OrderId	OrderDate	TotalCharges	CommonId	PupId	PickupDate	OrderPeriod	CohortGroup
0	47	262	2009-01-11	50.67	TRQKD	2	2009-01-12	2009-01	2009-01
1	47	278	2009-01-20	26.60	4HH2S	3	2009-01-20	2009-01	2009-01
2	47	294	2009-02-03	38.71	3TRDC	2	2009-02-04	2009-02	2009-01
3	47	301	2009-02-06	53.38	NGAZJ	2	2009-02-09	2009-02	2009-01
4	47	302	2009-02-06	14.28	FFYHD	2	2009-02-09	2009-02	2009-01

3. 计算每个CohortGroup在各个OrderPeriod的用户量

# pd.Series.nunique --> Return number of unique elements in the object.

cohorts = df.groupby(['CohortGroup', 'OrderPeriod']) \

            .agg({'UserId': pd.Series.nunique,

                 'OrderId': pd.Series.nunique,

                 'TotalCharges': 'sum'})

cohorts.rename(columns={'UserId': 'TotalUsers', 'OrderId': 'TotalOrders'}, inplace=True)

cohorts.head()

		TotalCharges	TotalOrders	TotalUsers
CohortGroup	OrderPeriod
2009-01	2009-01	1850.255	30	22
	2009-02	1351.065	25	8
	2009-03	1357.360	26	10
	2009-04	1604.500	28	9
	2009-05	1575.625	26	10

4. 标记每个CohortGroup的Cohort时期

比如，对于2009-01 cohort，其第一个时期是 2009-01，第二到第五个时期为 2009-02，...，2009-05。这里需要将每个CohortGroup的OrderPeriod对应到其第1，2，...个时期。

def cohort_period(df):

    df['CohortPeriod'] = np.arange(len(df)) + 1

    return df

cohorts = cohorts.groupby(level=0).apply(cohort_period)

cohorts.head()

		TotalCharges	TotalOrders	TotalUsers	CohortPeriod
CohortGroup	OrderPeriod
2009-01	2009-01	1850.255	30	22	1
	2009-02	1351.065	25	8	2
	2009-03	1357.360	26	10	3
	2009-04	1604.500	28	9	4
	2009-05	1575.625	26	10	5

上面 level=0 实际上就是对 group by CohortGroup，然后对每个group结果 apply cohort_period。groupby 后的结果是这样的：

[(k, v) for k, v in cohorts.head(5).groupby(level=0)]

[('2009-01',

                           TotalCharges  TotalOrders  TotalUsers  CohortPeriod

  CohortGroup OrderPeriod

  2009-01     2009-01          1850.255           30          22             1

              2009-02          1351.065           25           8             2

              2009-03          1357.360           26          10             3

              2009-04          1604.500           28           9             4

              2009-05          1575.625           26          10             5)]

5. 确定分群后的结果正确

x = df[(df.CohortGroup=='2009-01') & (df.OrderPeriod=='2009-01')]

y = cohorts.ix[('2009-01', '2009-01')]

assert(x.UserId.nunique()==y.TotalUsers)

assert(x.OrderId.nunique()==y.TotalOrders)

assert(x.TotalCharges.sum()==y.TotalCharges)

x = df[(df.CohortGroup=='2009-03') & (df.OrderPeriod=='2009-05')]

y = cohorts.ix[('2009-03', '2009-05')]

assert(x.UserId.nunique()==y.TotalUsers)

assert(x.OrderId.nunique()==y.TotalOrders)

assert(x.TotalCharges.sum()==y.TotalCharges)

每个CohortGroup的留存

6. 计算每个CohortGroup在第一个CohortPeriod的用户数量

cohorts = cohorts.reset_index() \

                 .set_index(['CohortGroup', 'CohortPeriod'])

cohort_group_size = cohorts.TotalUsers.groupby(level=0).first()

cohort_group_size

CohortGroup

2009-01     22

2009-02     15

2009-03     13

2009-04     39

2009-05     50

2009-06     32

2009-07     50

2009-08     31

2009-09     37

2009-10     54

2009-11    130

2009-12     65

2010-01     95

2010-02    100

2010-03     24

Name: TotalUsers, dtype: int64

7. 计算每个CohortPeriod的留存率

user_retention = cohorts.TotalUsers.unstack(0).divide(cohort_group_size, axis=1)

user_retention.head()

CohortGroup	2009-01	2009-02	2009-03	2009-04	2009-05	2009-06	2009-07	2009-08	2009-09	2009-10	2009-11	2009-12	2010-01	2010-02	2010-03
CohortPeriod
1	1.000000	1.000000	1.000000	1.000000	1.00	1.00000	1.00	1.000000	1.000000	1.000000	1.000000	1.000000	1.000000	1.00	1.0
2	0.363636	0.200000	0.307692	0.333333	0.26	0.46875	0.46	0.354839	0.405405	0.314815	0.246154	0.261538	0.526316	0.19	NaN
3	0.454545	0.333333	0.384615	0.256410	0.24	0.28125	0.26	0.290323	0.378378	0.222222	0.200000	0.276923	0.273684	NaN	NaN
4	0.409091	0.066667	0.307692	0.333333	0.10	0.18750	0.20	0.225806	0.216216	0.240741	0.223077	0.107692	NaN	NaN	NaN
5	0.454545	0.266667	0.076923	0.153846	0.08	0.21875	0.22	0.193548	0.351351	0.240741	0.100000	NaN	NaN	NaN	NaN

留存率曲线

user_retention[['2009-01', '2009-05', '2009-08']] \

  .plot(figsize=(11, 6), color=['#4285f4', '#EA4335', '#A60628'])

plt.title("Cohorts: User Retention")

plt.xticks(np.arange(1, len(user_retention)+1, 1))

plt.xlim(1, len(user_retention))

plt.ylabel('% of Cohort Purchasing', fontsize=16)

留存率热力图

import seaborn as sns

sns.set(style='white')

plt.figure(figsize=(16, 8))

plt.title('Cohorts: User Retetion', fontsize=14)

sns.heatmap(user_retention.T,

            mask=user_retention.T.isnull(),

            annot=True, fmt='.0%')

Cohort Analysis Using Python的相关教程结束。