Reduce Forecast Error with Cleaned Anomalies

Business Science

2019-09-20

Forecasting error can often be reduced 20% to 50% by repairing anomolous data

Example - Reducing Forecasting Error by 32%

We can often get better forecast performance by cleaning anomalous data prior to forecasting. This is the perfect use case for integrating the clean_anomalies() function into your forecast workflow.

library(tidyverse)
library(tidyquant)
library(anomalize)
library(timetk)

Here is a short example with the tidyverse_cran_downloads dataset that comes with anomalize. We’ll see how we can reduce the forecast error by 32% simply by repairing anomalies.

tidyverse_cran_downloads
#> # A time tibble: 6,375 x 3
#> # Index:  date
#> # Groups: package [15]
#>    date       count package
#>    <date>     <dbl> <chr>  
#>  1 2017-01-01   873 tidyr  
#>  2 2017-01-02  1840 tidyr  
#>  3 2017-01-03  2495 tidyr  
#>  4 2017-01-04  2906 tidyr  
#>  5 2017-01-05  2847 tidyr  
#>  6 2017-01-06  2756 tidyr  
#>  7 2017-01-07  1439 tidyr  
#>  8 2017-01-08  1556 tidyr  
#>  9 2017-01-09  3678 tidyr  
#> 10 2017-01-10  7086 tidyr  
#> # … with 6,365 more rows

Let’s take one package with some extreme events. We can hone in on lubridate, which has some outliers that we can fix.

tidyverse_cran_downloads %>%
  ggplot(aes(date, count, color = package)) +
  geom_point(alpha = 0.5) +
  facet_wrap(~ package, ncol = 3, scales = "free_y") +
  scale_color_viridis_d() +
  theme_tq() 

Forecasting Lubridate Downloads

Let’s focus on downloads of the lubridate R package.

lubridate_tbl <- tidyverse_cran_downloads %>%
  ungroup() %>%
  filter(package == "lubridate")

First, we’ll make a function, forecast_mae(), that can take the input of both cleaned and uncleaned anomalies and calculate forecast error of future uncleaned anomalies.

The modeling function uses the following criteria:

• Split the data into training and testing data that maintains the correct time-series sequence using the prop argument.
• Models the daily time series of the training data set from observed (demonstrates no cleaning) or observed and cleaned (demonstrates improvement from cleaning). Specified by the col_train argument.
• Compares the predictions to the observed values. Specified by the col_test argument.

forecast_mae <- function(data, col_train, col_test, prop = 0.8) {
  
  predict_expr <- enquo(col_train)
  actual_expr <- enquo(col_test)
  
  idx_train <- 1:(floor(prop * nrow(data)))
  
  train_tbl <- data %>% filter(row_number() %in% idx_train)
  test_tbl  <- data %>% filter(!row_number() %in% idx_train)
  
  # Model using training data (training) 
  model_formula <- as.formula(paste0(quo_name(predict_expr), " ~ index.num + year + quarter + month.lbl + day + wday.lbl"))
  
  model_glm <- train_tbl %>%
    tk_augment_timeseries_signature() %>%
    glm(model_formula, data = .)
  
  # Make Prediction
  suppressWarnings({
    # Suppress rank-deficit warning
    prediction <- predict(model_glm, newdata = test_tbl %>% tk_augment_timeseries_signature()) 
    actual     <- test_tbl %>% pull(!! actual_expr)
  })
  
  # Calculate MAE
  mae <- mean(abs(prediction - actual))
  
  return(mae)
  
}

Workflow for Cleaning Anomalies

We will use the anomalize workflow of decomposing (time_decompose()) and identifying anomalies (anomalize()). We use the function, clean_anomalies(), to add new column called “observed_cleaned” that is repaired by replacing all anomalies with the trend + seasonal components from the decompose operation. We can now experiment to see the improvment in forecasting performance by comparing a forecast made with “observed” versus “observed_cleaned”

lubridate_anomalized_tbl <- lubridate_tbl %>%
  time_decompose(count) %>%
  anomalize(remainder) %>%
  
  # Function to clean & repair anomalous data
  clean_anomalies()
#> frequency = 7 days
#> trend = 91 days

lubridate_anomalized_tbl
#> # A time tibble: 425 x 9
#> # Index: date
#>    date       observed season trend remainder remainder_l1 remainder_l2
#>    <date>        <dbl>  <dbl> <dbl>     <dbl>        <dbl>        <dbl>
#>  1 2017-01-01      643 -2078. 2474.      246.       -3323.        3310.
#>  2 2017-01-02     1350   518. 2491.    -1659.       -3323.        3310.
#>  3 2017-01-03     2940  1117. 2508.     -685.       -3323.        3310.
#>  4 2017-01-04     4269  1220. 2524.      525.       -3323.        3310.
#>  5 2017-01-05     3724   865. 2541.      318.       -3323.        3310.
#>  6 2017-01-06     2326   356. 2558.     -588.       -3323.        3310.
#>  7 2017-01-07     1107 -1998. 2574.      531.       -3323.        3310.
#>  8 2017-01-08     1058 -2078. 2591.      545.       -3323.        3310.
#>  9 2017-01-09     2494   518. 2608.     -632.       -3323.        3310.
#> 10 2017-01-10     3237  1117. 2624.     -504.       -3323.        3310.
#> # … with 415 more rows, and 2 more variables: anomaly <chr>,
#> #   observed_cleaned <dbl>

Before Cleaning with anomalize

lubridate_anomalized_tbl %>%
  forecast_mae(col_train = observed, col_test = observed, prop = 0.8)
#> [1] 4054.053

After Cleaning with anomalize

lubridate_anomalized_tbl %>%
  forecast_mae(col_train = observed_cleaned, col_test = observed, prop = 0.8)
#> [1] 2755.297

32% Reduction in Forecast Error

This is approximately a 32% reduction in forecast error as measure by Mean Absolute Error (MAE).

(2755 - 4054) / 4054 
#> [1] -0.3204243

Interested in Learning Anomaly Detection?

Business Science offers two 1-hour courses on Anomaly Detection:

• Learning Lab 18 - Time Series Anomaly Detection with anomalize

• Learning Lab 17 - Anomaly Detection with H2O Machine Learning