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https://github.com/sibylhe/mmm_stan
Python/STAN Implementation of Multiplicative Marketing Mix Model, with deep dive into Adstock (carry-over effect), ROAS, and mROAS
https://github.com/sibylhe/mmm_stan
bayesian-regression constrained-regression marketing-mix-modeling media-mix-modeling pystan roas stan
Last synced: 3 months ago
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Python/STAN Implementation of Multiplicative Marketing Mix Model, with deep dive into Adstock (carry-over effect), ROAS, and mROAS
- Host: GitHub
- URL: https://github.com/sibylhe/mmm_stan
- Owner: sibylhe
- License: mit
- Created: 2020-11-30T09:46:54.000Z (almost 4 years ago)
- Default Branch: main
- Last Pushed: 2022-05-28T05:45:02.000Z (over 2 years ago)
- Last Synced: 2024-05-14T00:14:55.473Z (6 months ago)
- Topics: bayesian-regression, constrained-regression, marketing-mix-modeling, media-mix-modeling, pystan, roas, stan
- Language: Jupyter Notebook
- Homepage:
- Size: 2.46 MB
- Stars: 326
- Watchers: 12
- Forks: 159
- Open Issues: 5
-
Metadata Files:
- Readme: README.md
- License: LICENSE
Awesome Lists containing this project
- awesome-marketing-machine-learning - mmm-stan
README
# Python/STAN Implementation of Multiplicative Marketing Mix Model
The methodology of this project is based on [this paper](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/46001.pdf) by Google, but is applied to a more complicated, real-world setting, where 1) there are 13 media channels and 46 control variables; 2) models are built in a stacked way.
# 1. Introduction
Marketing Mix Model, or Media Mix Model (MMM) is used by advertisers to measure how their media spending contributes to sales, so as to optimize future budget allocation. **ROAS** (return on ad spend) and **mROAS** (marginal ROAS) are the key metrics to look at. High ROAS indicates the channel is efficient, high mROAS means increasing spend in the channel will yield a high return based on current spending level.
**Procedures**
1. Fit a regression model with priors on coefficients, using media channels' impressions (or spending) and control variables to predict sales;
2. Decompose sales to each media channel's contribution. Channel contribution is calculated by comparing original sales and predicted sales upon removal of the channel;
3. Compute ROAS and mROAS using channel contribution and spending.
**Intuition of MMM**
- Offline channel's influence is hard to track. E.g., a customer saw a TV ad, and made a purchase at store.
- Media channels' influences are intertwined.
**Actual Customer Journey: Multiple Touchpoints**
A customer saw a product on TV > clicked on a display ad > clicked on a paid seach ad > made a purchase of $30. In this case, 3 touchpoints contributed to the conversion, and they should all get credits for this conversion.
**What's trackable: Last Digital Touchpoint**
Usually, only the last digital touchpoint can be tracked. In this case, SEM, and it will get all credits for this conversion.
So, a good attribution model should take into account all the relevant variables leading to conversion.
## 1.1 Multiplicative MMM
Since media channels work interactively, a multiplicative model structure is adopted:
Take log of both sides, we get the linear form (log-log model):
**Constraints on Coefficients**
1. Media coefficients are positive.
2. Control variables like discount, macro economy, event/retail holiday are expected to have positive impact on sales, their coefficients should also be positive.
## 1.2 Adstock
Media effect on sales may lag behind the original exposure and extend several weeks. The carry-over effect is modeled by Adstock:
L: length of the media effect
P: peak/delay of the media effect, how many weeks it's lagging behind first exposure
D: decay/retention rate of the media channel, concentration of the effect
The media effect of current weeks is a weighted average of current week and previous (L− 1) weeks.
**Adstock Example**
**Adstock with Varying Decay**
The larger the decay, the more scattered the effect.
**Adstock with Varying Length**
The impact of length is relatively minor. In model training, length could be fixed to 8 weeks or a period long enough for the media effect to finish.
```python
import numpy as np
import pandas as pd
def apply_adstock(x, L, P, D):
'''
params:
x: original media variable, array
L: length
P: peak, delay in effect
D: decay, retain rate
returns:
array, adstocked media variable
'''
x = np.append(np.zeros(L-1), x)
weights = np.zeros(L)
for l in range(L):
weight = D**((l-P)**2)
weights[L-1-l] = weight
adstocked_x = []
for i in range(L-1, len(x)):
x_array = x[i-L+1:i+1]
xi = sum(x_array * weights)/sum(weights)
adstocked_x.append(xi)
adstocked_x = np.array(adstocked_x)
return adstocked_x
```
## 1.3 Diminishing Return
After a certain saturation point, increasing spend will yield diminishing marginal return, the channel will be losing efficiency as you keep overspending on it. The diminishing return is modeled by Hill function:
K: half saturation point
S: slope
**Hill function with varying K and S**
```python
def hill_transform(x, ec, slope):
return 1 / (1 + (x / ec)**(-slope))
```
# 2. Model Specification & Implementation
## Data
Four years' (209 weeks) records of sales, media impression and media spending at weekly level.
**1. Media Variables**
- Media Impression (prefix='mdip_'): impressions of 13 media channels: direct mail, insert, newspaper, digital audio, radio, TV, digital video, social media, online display, email, SMS, affiliates, SEM.
- Media Spending (prefix='mdsp_'): spending of media channels.
**2. Control Variables**
- Macro Economy (prefix='me_'): CPI, gas price.
- Markdown (prefix='mrkdn_'): markdown/discount.
- Store Count ('st_ct')
- Retail Holidays (prefix='hldy_'): one-hot encoded.
- Seasonality (prefix='seas_'): month, with Nov and Dec further broken into to weeks. One-hot encoded.
**3. Sales Variable** ('sales')
```python
df = pd.read_csv('data.csv')
# 1. media variables
# media impression
mdip_cols=[col for col in df.columns if 'mdip_' in col]
# media spending
mdsp_cols=[col for col in df.columns if 'mdsp_' in col]
# 2. control variables
# macro economics variables
me_cols = [col for col in df.columns if 'me_' in col]
# store count variables
st_cols = ['st_ct']
# markdown/discount variables
mrkdn_cols = [col for col in df.columns if 'mrkdn_' in col]
# holiday variables
hldy_cols = [col for col in df.columns if 'hldy_' in col]
# seasonality variables
seas_cols = [col for col in df.columns if 'seas_' in col]
base_vars = me_cols+st_cols+mrkdn_cols+va_cols+hldy_cols+seas_cols
# 3. sales variables
sales_cols =['sales']
```
## Model Architecture
The model is built in a stacked way. Three models are trained:
- Control Model
- Marketing Mix Model
- Diminishing Return Model
## 2.1 Control Model / Base Sales Model
**Goal**: predict base sales (X_ctrl) as an input variable to MMM, this represents the baseline sales trend without any marketing activities.
X1: control variables positively related with sales, including macro economy, store count, markdown, holiday.
X2: control variables that may have either positive or negtive impact on sales: seasonality.
Target variable: ln(sales).
The variables are centralized by mean.
**Priors**
```python
import pystan
import os
os.environ['CC'] = 'gcc-10'
os.environ['CXX'] = 'g++-10'
# mean-centralize: sales, numeric base_vars
df_ctrl, sc_ctrl = mean_center_trandform(df, ['sales']+me_cols+st_cols+mrkdn_cols)
df_ctrl = pd.concat([df_ctrl, df[hldy_cols+seas_cols]], axis=1)
# variables positively related to sales: macro economy, store count, markdown, holiday
pos_vars = [col for col in base_vars if col not in seas_cols]
X1 = df_ctrl[pos_vars].values
# variables may have either positive or negtive impact on sales: seasonality
pn_vars = seas_cols
X2 = df_ctrl[pn_vars].values
ctrl_data = {
'N': len(df_ctrl),
'K1': len(pos_vars),
'K2': len(pn_vars),
'X1': X1,
'X2': X2,
'y': df_ctrl['sales'].values,
'max_intercept': min(df_ctrl['sales'])
}
ctrl_code1 = '''
data {
int N; // number of observations
int K1; // number of positive predictors
int K2; // number of positive/negative predictors
real max_intercept; // restrict the intercept to be less than the minimum y
matrix[N, K1] X1;
matrix[N, K2] X2;
vector[N] y;
}
parameters {
vector[K1] beta1; // regression coefficients for X1 (positive)
vector[K2] beta2; // regression coefficients for X2
real alpha; // intercept
real noise_var; // residual variance
}
model {
// Define the priors
beta1 ~ normal(0, 1);
beta2 ~ normal(0, 1);
noise_var ~ inv_gamma(0.05, 0.05 * 0.01);
// The likelihood
y ~ normal(X1*beta1 + X2*beta2 + alpha, sqrt(noise_var));
}
'''
sm1 = pystan.StanModel(model_code=ctrl_code1, verbose=True)
fit1 = sm1.sampling(data=ctrl_data, iter=2000, chains=4)
fit1_result = fit1.extract()
```
MAPE of control model: 8.63%
Extract control model parameters from the fit object and predict base sales -> df['base_sales']
## 2.2 Marketing Mix Model
**Goal**:
- Find appropriate adstock parameters for media channels;
- Decompose sales to media channels' contribution (and non-marketing contribution).
L: length of media impact
P: peak of media impact
D: decay of media impact
X: adstocked media impression variables and base sales
Target variable: ln(sales)
Variables are centralized by mean.
**Priors**
```python
df_mmm, sc_mmm = mean_log1p_trandform(df, ['sales', 'base_sales'])
mu_mdip = df[mdip_cols].apply(np.mean, axis=0).values
max_lag = 8
num_media = len(mdip_cols)
# padding zero * (max_lag-1) rows
X_media = np.concatenate((np.zeros((max_lag-1, num_media)), df[mdip_cols].values), axis=0)
X_ctrl = df_mmm['base_sales'].values.reshape(len(df),1)
model_data2 = {
'N': len(df),
'max_lag': max_lag,
'num_media': num_media,
'X_media': X_media,
'mu_mdip': mu_mdip,
'num_ctrl': X_ctrl.shape[1],
'X_ctrl': X_ctrl,
'y': df_mmm['sales'].values
}
model_code2 = '''
functions {
// the adstock transformation with a vector of weights
real Adstock(vector t, row_vector weights) {
return dot_product(t, weights) / sum(weights);
}
}
data {
// the total number of observations
int N;
// the vector of sales
real y[N];
// the maximum duration of lag effect, in weeks
int max_lag;
// the number of media channels
int num_media;
// matrix of media variables
matrix[N+max_lag-1, num_media] X_media;
// vector of media variables' mean
real mu_mdip[num_media];
// the number of other control variables
int num_ctrl;
// a matrix of control variables
matrix[N, num_ctrl] X_ctrl;
}
parameters {
// residual variance
real noise_var;
// the intercept
real tau;
// the coefficients for media variables and base sales
vector[num_media+num_ctrl] beta;
// the decay and peak parameter for the adstock transformation of
// each media
vector[num_media] decay;
vector[num_media] peak;
}
transformed parameters {
// the cumulative media effect after adstock
real cum_effect;
// matrix of media variables after adstock
matrix[N, num_media] X_media_adstocked;
// matrix of all predictors
matrix[N, num_media+num_ctrl] X;
// adstock, mean-center, log1p transformation
row_vector[max_lag] lag_weights;
for (nn in 1:N) {
for (media in 1 : num_media) {
for (lag in 1 : max_lag) {
lag_weights[max_lag-lag+1] <- pow(decay[media], (lag - 1 - peak[media]) ^ 2);
}
cum_effect <- Adstock(sub_col(X_media, nn, media, max_lag), lag_weights);
X_media_adstocked[nn, media] <- log1p(cum_effect/mu_mdip[media]);
}
X <- append_col(X_media_adstocked, X_ctrl);
}
}
model {
decay ~ beta(3,3);
peak ~ uniform(0, ceil(max_lag/2));
tau ~ normal(0, 5);
for (i in 1 : num_media+num_ctrl) {
beta[i] ~ normal(0, 1);
}
noise_var ~ inv_gamma(0.05, 0.05 * 0.01);
y ~ normal(tau + X * beta, sqrt(noise_var));
}
'''
sm2 = pystan.StanModel(model_code=model_code2, verbose=True)
fit2 = sm2.sampling(data=model_data2, iter=1000, chains=3)
fit2_result = fit2.extract()
```
**Distribution of Media Coefficients**
red line: mean, green line: median
### Decompose sales to media channels' contribution
Each media channel's contribution = total sales - sales upon removal of the channel
In the previous model fitting step, parameters of the log-log model have been found:
Plug them into the multiplicative model:
```python
# decompose sales to media contribution
mc_df = mmm_decompose_media_contrib(mmm, df, y_true=df['sales_ln'])
adstock_params = mmm['adstock_params']
mc_pct, mc_pct2 = calc_media_contrib_pct(mc_df, period=52)
```
RMSE (log-log model): 0.04977
MAPE (multiplicative model): 15.71%
**Adstock Parameters**
```python
{'dm': {'L': 8, 'P': 0.8147057071636012, 'D': 0.5048365638721349},
'inst': {'L': 8, 'P': 0.6339321363933637, 'D': 0.40532404247040194},
'nsp': {'L': 8, 'P': 1.1076944292039324, 'D': 0.4612905130128658},
'auddig': {'L': 8, 'P': 1.8834110997525702, 'D': 0.5117823761413419},
'audtr': {'L': 8, 'P': 1.9892680621155827, 'D': 0.5046141055524362},
'vidtr': {'L': 8, 'P': 0.05520253973872224, 'D': 0.0846136627657064},
'viddig': {'L': 8, 'P': 1.862571613911107, 'D': 0.5074553132446618},
'so': {'L': 8, 'P': 1.7027472358912694, 'D': 0.5046386226501091},
'on': {'L': 8, 'P': 1.4169662215350334, 'D': 0.4907407637366824},
'em': {'L': 8, 'P': 1.0590065753144235, 'D': 0.44420264450045377},
'sms': {'L': 8, 'P': 1.8487648735160152, 'D': 0.5090970201714644},
'aff': {'L': 8, 'P': 0.6018657109295106, 'D': 0.39889023002777724},
'sem': {'L': 8, 'P': 1.34945185610011, 'D': 0.47875793676213835}}
```
**Notes**:
- For SEM, P=1.3, D=0.48 does not make a lot of sense to me, because SEM is expected to have immediate and concentrated impact (P=0, low decay). Same with online display.
- Try more specific priors in future model.
## 2.3 Diminishing Return Model
**Goal**: for each channel, find the relationship (fit a Hill function) between spending and contribution, so that ROAS and marginal ROAS can be calculated.
x: adstocked media channel spending
K: half saturation
S: shape
Target variable: the media channel's contribution
Variables are centralized by mean.
**Priors**
```python
def create_hill_model_data(df, mc_df, adstock_params, media):
y = mc_df['mdip_'+media].values
L, P, D = adstock_params[media]['L'], adstock_params[media]['P'], adstock_params[media]['D']
x = df['mdsp_'+media].values
x_adstocked = apply_adstock(x, L, P, D)
# centralize
mu_x, mu_y = x_adstocked.mean(), y.mean()
sc = {'x': mu_x, 'y': mu_y}
x = x_adstocked/mu_x
y = y/mu_y
model_data = {
'N': len(y),
'y': y,
'X': x
}
return model_data, sc
model_code3 = '''
functions {
// the Hill function
real Hill(real t, real ec, real slope) {
return 1 / (1 + (t / ec)^(-slope));
}
}
data {
// the total number of observations
int N;
// y: vector of media contribution
vector[N] y;
// X: vector of adstocked media spending
vector[N] X;
}
parameters {
// residual variance
real noise_var;
// regression coefficient
real beta_hill;
// ec50 and slope for Hill function of the media
real ec;
real slope;
}
transformed parameters {
// a vector of the mean response
vector[N] mu;
for (i in 1:N) {
mu[i] <- beta_hill * Hill(X[i], ec, slope);
}
}
model {
slope ~ gamma(3, 1);
ec ~ beta(2, 2);
beta_hill ~ normal(0, 1);
noise_var ~ inv_gamma(0.05, 0.05 * 0.01);
y ~ normal(mu, sqrt(noise_var));
}
'''
# train hill models for all media channels
sm3 = pystan.StanModel(model_code=model_code3, verbose=True)
hill_models = {}
to_train = ['dm', 'inst', 'nsp', 'auddig', 'audtr', 'vidtr', 'viddig', 'so', 'on', 'sem']
for media in to_train:
print('training for media: ', media)
hill_model = train_hill_model(df, mc_df, adstock_params, media, sm3)
hill_models[media] = hill_model
```
**Diminishing Return Model (Fitted Hill Curve)**
### Calculate overall ROAS and weekly ROAS
- Overall ROAS = total media contribution / total media spending
- Weekly ROAS = weekly media contribution / weekly media spending
**Distribution of Weekly ROAS** (Recent 1 Year)
red line: mean, green line: median
### Calculate mROAS
Marginal ROAS represents the return of incremental spending based on current spending. For example, I've spent $100 on SEM, how much will the next $1 bring.
mROAS is calculated by increasing the current spending level by 1%, the incremental channel contribution over incremental channel spending.
1. Current spending level ```cur_sp``` is an array of weekly spending in a given period.
Next spending level ```next_sp``` is increasing ```cur_sp``` by 1%.
2. Plug ```cur_sp``` and ```next_sp``` into the Hill function:
Current media contribution ```cur_mc``` = beta * Hill(```cur_sp```)
Next-level media contribution ```next_mc``` = beta * Hill(```next_sp```)
3. **mROAS** = (sum(```next_mc```) - sum(```cur_mc```)) / sum(0.01 * ```cur_sp```)
# 3. Results & Marketing Budget Optimization
**Media Channel Contribution**
80% sales are contributed by non-marketing factors, marketing channels contributed 20% sales.
Top contributors: TV, affiliates, SEM
**ROAS**
High ROAS: TV, insert, online display
**mROAS**
High mROAS: TV, insert, radio, online display
Note: trivial channels: newspaper, digital audio, digital video, social (spending/impression too small to be qualified, so that their results are not trustworthy).
## Q&A
Please check this running list of [FAQ](https://github.com/sibylhe/mmm_stan/discussions/7). If you have questions, comments, suggestions, and practical problems (when applying this script to your datasets) that are unaddressed in this list, feel free to [open a discussion](https://github.com/sibylhe/mmm_stan/discussions). You may also comment on my [Medium article](https://towardsdatascience.com/python-stan-implementation-of-multiplicative-marketing-mix-model-with-deep-dive-into-adstock-a7320865b334).
For bugs/errors in code, please open an issue. An issue is expected to be addressed in the following weekend.
## References
[1] Bayesian Methods for Media Mix Modeling with Carryover and Shape Effects. https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/46001.pdf
[2] STAN tutorials:
Prior Choice Recommendations. https://github.com/stan-dev/stan/wiki/Prior-Choice-Recommendations
Pystan Documentation. https://www.cnpython.com/pypi/pystan
Pystan Workflow. https://mc-stan.org/users/documentation/case-studies/pystan_workflow.html
A quick-start introduction to Stan for economists. https://nbviewer.jupyter.org/github/QuantEcon/QuantEcon.notebooks/blob/master/IntroToStan_basics_workflow.ipynb
HMC sampling. https://education.illinois.edu/docs/default-source/carolyn-anderson/edpsy590ca/lectures/9-hmc-and-stan/hmc_n_stan_post.pdf
**If you like this project, please leave a :star2: for motivation:)**