Introduction

An important goal of our Win Vector LLC teaching offerings is to instill in engineers some familiarity with, and empathy for, how data is likely to be used for analytics and business. Having such engineers in your organization greatly increases the quality of the data later available to your analysts and data scientists. This in turn expands what is possible in prediction and forecasting, which can lead to significant revenue opportunities.

In the following, I’d like to illustrate a data issue that can squander such opportunities.

An Example Problem

Suppose you are purchasing data on movie attendance in your region; data for both past attendance, and future projected attendance. In particular, you are concerned about planning for popcorn sales at the The Roxie movie house.

(NOTE: While the Roxie is an actual movie theater, please note that we are using synthetic attendance numbers, for the purposes of this example.)

Photo by Simon Durkin – originally posted to Flickr as Roxie Theatre – Mission SF, CC BY-SA 2.0, Link

# attach our packages
library(ggplot2)
library(dplyr)

# read our data
d <- read.csv(
  'Roxie_schedule_as_known_after_August.csv', 
  strip.white = TRUE, 
  stringsAsFactors = FALSE)
d$Date <- as.Date(d$Date)

d |>
  head() |>
  knitr::kable(row.names = NA)

Our business goal is to build a model relating attendance to popcorn sales, which we will apply to future data in order to predict future popcorn sales. This allows us to plan staffing and purchasing, and also to predict snack bar revenue.

In the above example data, all dates in August of 2024 are “in the past” (available as training and test/validation data) and all dates in September of 2024 are “in the future” (dates we want to make predictions for). The movie attendance service we are subscribing to supplies

• past schedules
• past (recorded) attendance
• future schedules, and
• (estimated) future attendance.

The fly in the ointment

The above already has the flaw we are warning about: we have mixed past attendance and (estimated) future attendance. In machine learning modeling we want our explanatory variables (in this case attendance) to be produced the same way when training a model as when applying the model. Here, we are using recorded attendance for the past, and some sort of estimated future attendance for the future. Without proper care, these are not necessarily the same thing.

Continuing the example

Our intermediate goal is to build a model relating past popcorn (unit) purchases to past attendance.

To do this we join in our own past popcorn sales data (in units sold) and build a predictive model.

# join in popcorn sales records
popcorn_sales <- read.csv(
  'popcorn_sales.csv', 
  strip.white = TRUE, 
  stringsAsFactors = FALSE)
popcorn_sales$Date <- as.Date(popcorn_sales$Date)

popcorn_sales |>
  head() |>
  knitr::kable(row.names = NA)

d_train <- d |>
  filter(is.na(Attendance) == FALSE) |>
  group_by(Date) |>
  summarize(Attendance = sum(Attendance)) |>
  inner_join(popcorn_sales, by='Date')

d_train |>
  head() |>
  knitr::kable(row.names = NA)

# model popcorn sales as a function of attendance
model <- lm(PopcornSales ~ Attendance, data=d_train)
d$PredictedPopcorn <- round(pmax(0, 
                                 predict(model, newdata=d)), 
                            digits=1)
train_R2 <- summary(model)$adj.r.squared
summary(model)

## 
## Call:
## lm(formula = PopcornSales ~ Attendance, data = d_train)
## 
## Residuals:
##      Min       1Q   Median       3Q      Max 
## -11.1726  -2.2676   0.5702   3.2467   7.3703 
## 
## Coefficients:
##              Estimate Std. Error t value Pr(>|t|)    
## (Intercept) -2.684809   1.897652  -1.415    0.171    
## Attendance   0.164917   0.006236  26.445   <2e-16 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 5.047 on 22 degrees of freedom
## Multiple R-squared:  0.9695, Adjusted R-squared:  0.9681 
## F-statistic: 699.4 on 1 and 22 DF,  p-value: < 2.2e-16

We get what appears to be a good result: a highly predictive model that shows about a 15% attachment rate from attendance to popcorn purchase.

Let’s plot our predictions in the past and future, and actuals in the past.

subtitle = paste("Training R-Squared:", sprintf('%.2f', train_R2))

d_daily <- d |> 
  group_by(Date) |>
  summarize(PredictedPopcorn = sum(PredictedPopcorn)) |>
  ungroup() |>
  full_join(popcorn_sales, by='Date') |>
  mutate(Month = format(Date, '%B')) |>
  group_by(Month) |>
  mutate(
    MeanPredictedPopcorn = mean(PredictedPopcorn), 
    MeanPopcornSales = mean(PopcornSales)) |>
  ungroup()
ggplot(
  data=d_daily,
  mapping=aes(x=Date)) +
  geom_point(mapping=aes(y=PopcornSales)) +
  geom_line(
    mapping=aes(y=PredictedPopcorn), 
    color='Blue') +
  geom_step(
    mapping=aes(y=MeanPredictedPopcorn), 
    direction='mid', 
    color='Blue', 
    alpha=0.5, 
    linetype=2) +
  ggtitle('Misusing corrected data\npopcorn sales: actual as points, predicted as lines, monthly mean as dashed',
      subtitle=subtitle)

Now we really see the problem. Our model predicts popcorn sales in the presumed future month of September are going to be double what was seen in the past training month of August. As we don’t have the future data yet, we don’t immediately know this is wrong. But without a presumed cause, it is suspicious.

Diagnosing

Let’s plot how our explanatory variable changes form the past month to the future month.

d_plot = d
d_plot$Month = format(d_plot$Date, '%B')

ggplot(
  data=d_plot,
  mapping=aes(
    x=Attendance, 
    color=Month, 
    fill=Month, 
    linetype=Month)) +
  geom_density(adjust = 0.2, alpha=0.5) +
  scale_color_brewer(type="qual", palette="Dark2") + 
  scale_fill_brewer(type="qual", palette="Dark2") +
  ggtitle("distribution of attendance by month")

The months look nothing alike. The estimated future attendances (which we purchased from our data supplier) look nothing like what the (same) data supplier said past attendances were.

Let’s look at a few rows of future application data.

d |>
  tail() |>
  knitr::kable(row.names = NA)

This looks like only a few different attendance values are reported. Let’s dig deeper into that.

table(
  Attendance = d[format(d$Date, '%B') == 'September',
                 'Attendance']) |>
  knitr::kable(row.names = NA)

We are seeing only two values for estimated future attendance: 47 and 233. It turns out that these are the reported sizes of the two theaters comprising the Roxie (ref).

A guess

Here’s what we guess is happening: For future events, the data supplier is using the venue size as the size estimate. For past events they edit the event record to reflect actual ticketed attendance. This correction seems like an improvement, until one attempts a project spanning both past (used for training) and future (used for application) data. The individual record may seem better, but its relation to other records is made worse. This is a severe form of undesirable concept-drift or data non-exchangeability. We need the imposed practice or rehearsal conditions to simulate the required performance conditions.

No amount of single-time-index back-testing on past data would show the effect. Only by tracking what was the recorded attendance for a given date as a function of when we ask will we see what is going on.

The fix

To fix this issue, we need “versioned”, “as of”, or “bitemporal” data. For the August data we don’t want the actual known attendance (as nice as that is), but in fact what the estimated attendance for August looked like way back in July. That way the Attendance variable we use in training is an estimate, just like it will be in future applications of the model.

If our vendor supplies versioned data we can then use that. Even though it is “inferior” it is better suited to our application.

Let’s see that in action. To do this we need older projections for attendance that have not been corrected. If we have such we can proceed, if not we are stuck. Let’s suppose we have the older records.

# read our data
d_est <- read.csv(
  'Roxie_schedule_as_known_before_August.csv', 
  strip.white = TRUE, 
  stringsAsFactors = FALSE)
d_est$Date <- as.Date(d$Date, format='%Y-%B-%d')

d_est |>
  head() |>
  knitr::kable(row.names = NA)

Let’s repeat our modeling effort with the uncorrected (not retouched) data.

# predict popcorn sales as a function of attendance
d_est_train <- d_est |>
  filter(is.na(EstimatedAttendance) == FALSE) |>
  group_by(Date) |>
  summarize(EstimatedAttendance = sum(EstimatedAttendance)) |>
  inner_join(popcorn_sales, by='Date')

model_est <- lm(PopcornSales ~ EstimatedAttendance, data=d_est_train)
d_est$PredictedPopcorn <- round(pmax(0, 
                                     predict(model_est, newdata=d_est)), 
                                digits=1)
train_est_R2 <- summary(model_est)$adj.r.squared

subtitle = paste("Training R-Squared:", sprintf('%.2f', train_est_R2))

d_est_daily <- d_est |> 
  group_by(Date) |>
  summarize(PredictedPopcorn = sum(PredictedPopcorn)) |>
  ungroup() |>
  full_join(popcorn_sales, by='Date') |>
  mutate(Month = format(Date, '%B')) |>
  group_by(Month) |>
  mutate(
    MeanPredictedPopcorn = mean(PredictedPopcorn), 
    MeanPopcornSales = mean(PopcornSales)) |>
  ungroup()
ggplot(
  data=d_est_daily,
  mapping=aes(x=Date)) +
  geom_point(mapping=aes(y=PopcornSales)) +
  geom_line(mapping=aes(
    y=PredictedPopcorn), 
    color='Blue') +
  geom_step(mapping=aes(
    y=MeanPredictedPopcorn), 
    directon='mid', 
    color='Blue', 
    alpha=0.5, 
    linetype=2) +
  ggtitle("Properly Using Non-corrected data\npopcorn sales: actual as points, predicted as lines, monthly mean as dashed",
          subtitle=subtitle)

Using the estimated attendance to train (instead of actual) gives a vastly inferior R-squared as measured on training data. However, using the estimated attendance (without corrections) gives us a model that performs much better in the future (which is the actual project goal)! The idea is that we expect our model to be applied to rough, estimated future inputs, so we need to train it on such estimates, and not on cleaned up values that will not be available during application. A production model must be trained in the same rough seas that it will sail in.

Conclusion

The performance of a model on held-out data is only a proxy measure for future model performance. In our example we see that the desired connection breaks down when there is a data concept-change between the training and application periods. The fix is to use “as of” data or bitemporal modeling.

A common way to achieve a full bitemporal data model is to have reversible time stamped audit logging on any field edits. One keeps additional records of the form “at this time this value was changed from A to B in this record.” An engineer unfamiliar with how forecasts are applied may not accept the cost of the audit or roll-back logging. So one needs to convert these engineers into modeling peers and allies.

Data users should insist on bitemporal data for forecasting applications. When date or time enter the picture- it is rare that there is only one key. Most date/time questions unfortunately can not be simplified down to “what is the prediction for date x?” Instead one needs to respect structures such as “what is the best prediction for date x, using a model trained up through what was known at date y, and taking inputs known up through date z?” To even back test such models one needs a bitemporal database, to control what data looked like at different times.

Appendix

All the code and data to reproduce this example can be found here.