How to use Exploratory Data Analysis to drive information from time series data and enhance feature engineering using Python
Time series analysis certainly represents one of the most widespread topics in the field of data science and machine learning: whether predicting financial events, energy consumption, product sales or stock market trends, this field has always been of great interest to businesses.
Obviously, the great increase in data availability, combined with the constant progress in machine learning models, has made this topic even more interesting today. Alongside traditional forecasting methods derived from statistics (e.g. regressive models, ARIMA models, exponential smoothing), techniques relating to machine learning (e.g. tree-based models) and deep learning (e.g. LSTM Networks, CNNs, Transformer-based Models) have emerged for some time now.
Despite the huge differences between these techniques, there is a preliminary step that must be done, no matter what the model is: Exploratory Data Analysis.
In statistics, Exploratory Data Analysis (EDA) is a discipline consisting in analyzing and visualizing data in order to summarize their main characteristics and gain relevant information from them. This is of considerable importance in the data science field because it allows to lay the foundations to another important step: feature engineering. That is, the practice that consists on creating, transforming and extracting features from the dataset so that the model can work to the best of its possibilities.
The objective of this article is therefore to define a clear exploratory data analysis template, focused on time series, which can summarize and highlight the most important characteristics of the dataset. To do this, we will use some common Python libraries such as Pandas, Seaborn and Statsmodel.
Let’s first define the dataset: for the purposes of this article, we will take Kaggle’s Hourly Energy Consumption data. This dataset relates to PJM Hourly Energy Consumption data, a regional transmission organization in the United States, that serves electricity to Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia, and the District of Columbia.
The hourly power consumption data comes from PJM’s website and are in megawatts (MW).
Let’s now define which are the most significant analyses to be performed when dealing with time series.
For sure, one of the most important thing is to plot the data: graphs can highlight many features, such as patterns, unusual observations, changes over time, and relationships between variables. As already said, the insight that emerge from these plots must then be taken into consideration, as much as possible, into the forecasting model. Moreover, some mathematical tools such as descriptive statistics and time series decomposition, will also be very useful.
Said that, the EDA I’m proposing in this article consists on six steps: Descriptive Statistics, Time Plot, Seasonal Plots, Box Plots, Time Series Decomposition, Lag Analysis.
1. Descriptive Statistics
Descriptive statistic is a summary statistic that quantitatively describes or summarizes features from a collection of structured data.
Some metrics that are commonly used to describe a dataset are: measures of central tendency (e.g. mean, median), measures of dispersion (e.g. range, standard deviation), and measure of position (e.g. percentiles, quartile). All of them can be summarized by the so called five number summary, which include: minimum, first quartile (Q1), median or second quartile (Q2), third quartile (Q3) and maximum of a distribution.
In Python, these information can be easily retrieved using the well know describe method from Pandas:
import pandas as pd# Loading and preprocessing steps
df = pd.read_csv('../input/hourly-energy-consumption/PJME_hourly.csv')
df = df.set_index('Datetime')
df.index = pd.to_datetime(df.index)
df.describe()
2. Time plot
The obvious graph to start with is the time plot. That is, the observations are plotted against the time they were observed, with consecutive observations joined by lines.
In Python , we can use Pandas and Matplotlib:
import matplotlib.pyplot as plt# Set pyplot style
plt.style.use("seaborn")
# Plot
df('PJME_MW').plot(title='PJME - Time Plot', figsize=(10,6))
plt.ylabel('Consumption (MW)')
plt.xlabel('Date')
This plot already provides several information:
- As we could expect, the pattern shows yearly seasonality.
- Focusing on a single year, it seems that more pattern emerges. Likely, the consumptions will have a peak in winter and one another in summer, due to the greater electricity consumption.
- The series does not exhibit a clear increasing/decreasing trend over the years, the average consumptions remains stationary.
- There is an anomalous value around 2023, probably it should be imputed when implementing the model.
3. Seasonal Plots
A seasonal plot is fundamentally a time plot where data are plotted against the individual “seasons” of the series they belong.
Regarding energy consumption, we usually have hourly data available, so there could be several seasonality: yearly, weekly, daily. Before going deep into these plots, let’s first set up some variables in our Pandas dataframe:
# Defining required fields
df('year') = (x for x in df.index.year)
df('month') = (x for x in df.index.month)
df = df.reset_index()
df('week') = df('Datetime').apply(lambda x:x.week)
df = df.set_index('Datetime')
df('hour') = (x for x in df.index.hour)
df('day') = (x for x in df.index.day_of_week)
df('day_str') = (x.strftime('%a') for x in df.index)
df('year_month') = (str(x.year) + '_' + str(x.month) for x in df.index)
3.1 Seasonal plot — Yearly consumption
A very interesting plot is the one referring to the energy consumption grouped by year over months, this highlights yearly seasonality and can inform us about ascending/descending trends over the years.
Here is the Python code:
import numpy as np# Defining colors palette
np.random.seed(42)
df_plot = df(('month', 'year', 'PJME_MW')).dropna().groupby(('month', 'year')).mean()(('PJME_MW')).reset_index()
years = df_plot('year').unique()
colors = np.random.choice(list(mpl.colors.XKCD_COLORS.keys()), len(years), replace=False)
# Plot
plt.figure(figsize=(16,12))
for i, y in enumerate(years):
if i > 0:
plt.plot('month', 'PJME_MW', data=df_plot(df_plot('year') == y), color=colors(i), label=y)
if y == 2018:
plt.text(df_plot.loc(df_plot.year==y, :).shape(0)+0.3, df_plot.loc(df_plot.year==y, 'PJME_MW')(-1:).values(0), y, fontsize=12, color=colors(i))
else:
plt.text(df_plot.loc(df_plot.year==y, :).shape(0)+0.1, df_plot.loc(df_plot.year==y, 'PJME_MW')(-1:).values(0), y, fontsize=12, color=colors(i))
# Setting labels
plt.gca().set(ylabel= 'PJME_MW', xlabel = 'Month')
plt.yticks(fontsize=12, alpha=.7)
plt.title("Seasonal Plot - Monthly Consumption", fontsize=20)
plt.ylabel('Consumption (MW)')
plt.xlabel('Month')
plt.show()
This plot shows every year has actually a very predefined pattern: the consumption increases significantly during winter and has a peak in summer (due to heating/cooling systems), while has a minima in spring and in autumn when no heating or cooling is usually required.
Furthermore, this plot tells us that’s not a clear increasing/decreasing pattern in the overall consumptions across years.
3.2 Seasonal plot — Weekly consumption
Another useful plot is the weekly plot, it depicts the consumptions during the week over months and can also suggest if and how weekly consumptions are changing over a single year.
Let’s see how to figure it out with Python:
# Defining colors palette
np.random.seed(42)
df_plot = df(('month', 'day_str', 'PJME_MW', 'day')).dropna().groupby(('day_str', 'month', 'day')).mean()(('PJME_MW')).reset_index()
df_plot = df_plot.sort_values(by='day', ascending=True)months = df_plot('month').unique()
colors = np.random.choice(list(mpl.colors.XKCD_COLORS.keys()), len(months), replace=False)
# Plot
plt.figure(figsize=(16,12))
for i, y in enumerate(months):
if i > 0:
plt.plot('day_str', 'PJME_MW', data=df_plot(df_plot('month') == y), color=colors(i), label=y)
if y == 2018:
plt.text(df_plot.loc(df_plot.month==y, :).shape(0)-.9, df_plot.loc(df_plot.month==y, 'PJME_MW')(-1:).values(0), y, fontsize=12, color=colors(i))
else:
plt.text(df_plot.loc(df_plot.month==y, :).shape(0)-.9, df_plot.loc(df_plot.month==y, 'PJME_MW')(-1:).values(0), y, fontsize=12, color=colors(i))
# Setting Labels
plt.gca().set(ylabel= 'PJME_MW', xlabel = 'Month')
plt.yticks(fontsize=12, alpha=.7)
plt.title("Seasonal Plot - Weekly Consumption", fontsize=20)
plt.ylabel('Consumption (MW)')
plt.xlabel('Month')
plt.show()
3.3 Seasonal plot — Daily consumption
Finally, the last seasonal plot I want to show is the daily consumption plot. As you can guess, it represents how consumption change over the day. In this case, data are first grouped by day of week and then aggregated taking the mean.
Here’s the code:
import seaborn as sns# Defining the dataframe
df_plot = df(('hour', 'day_str', 'PJME_MW')).dropna().groupby(('hour', 'day_str')).mean()(('PJME_MW')).reset_index()
# Plot using Seaborn
plt.figure(figsize=(10,8))
sns.lineplot(data = df_plot, x='hour', y='PJME_MW', hue='day_str', legend=True)
plt.locator_params(axis='x', nbins=24)
plt.title("Seasonal Plot - Daily Consumption", fontsize=20)
plt.ylabel('Consumption (MW)')
plt.xlabel('Hour')
plt.legend()
Often, this plot show a very typical pattern, someone calls it “M profile” since consumptions seems to depict an “M” during the day. Sometimes this pattern is clear, others not (like in this case).
However, this plots usually shows a relative peak in the middle of the day (from 10 am to 2 pm), then a relative minima (from 2 pm to 6 pm) and another peak (from 6 pm to 8 pm). Finally, it also shows the difference in consumptions from weekends and other days.
3.4 Seasonal plot — Feature Engineering
Let’s now see how to use this information for feature engineering. Let’s suppose we are using some ML model that requires good quality features (e.g. ARIMA models or tree-based models).
These are the main evidences coming from seasonal plots:
- Yearly consumptions do not change a lot over years: this suggests the possibility to use, when available, yearly seasonality features coming from lag or exogenous variables.
- Weekly consumptions follow the same pattern across months: this suggests to use weekly features coming from lag or exogenous variables.
- Daily consumption differs from normal days and weekends: this suggest to use categorical features able to identify when a day is a normal day and when it is not.
4. Box Plots
Boxplot are a useful way to identify how data are distributed. Briefly, boxplots depict percentiles, which represent 1st (Q1), 2nd (Q2/median) and 3rd (Q3) quartile of a distribution and whiskers, which represent the range of the data. Every value beyond the whiskers can be thought as an outlier, more in depth, whiskers are often computed as:
4.1 Box Plots — Total consumption
Let’s first compute the box plot regarding the total consumption, this can be easily done with Seaborn:
plt.figure(figsize=(8,5))
sns.boxplot(data=df, x='PJME_MW')
plt.xlabel('Consumption (MW)')
plt.title(f'Boxplot - Consumption Distribution');
Even if this plot seems not to be much informative, it tells us we are dealing with a Gaussian-like distribution, with a tail more accentuated towards the right.
4.2 Box Plots — Day month distribution
A very interesting plot is the day/month box plot. It is obtained creating a “day month” variable and grouping consumptions by it. Here is the code, referring only from year 2017:
df('year') = (x for x in df.index.year)
df('month') = (x for x in df.index.month)
df('year_month') = (str(x.year) + '_' + str(x.month) for x in df.index)df_plot = df(df('year') >= 2017).reset_index().sort_values(by='Datetime').set_index('Datetime')
plt.title(f'Boxplot Year Month Distribution');
plt.xticks(rotation=90)
plt.xlabel('Year Month')
plt.ylabel('MW')
sns.boxplot(x='year_month', y='PJME_MW', data=df_plot)
plt.ylabel('Consumption (MW)')
plt.xlabel('Year Month')
It can be seen that consumption are less uncertain in summer/winter months (i.e. when we have peaks) while are more dispersed in spring/autumn (i.e. when temperatures are more variable). Finally, consumption in summer 2018 are higher than 2017, maybe due to a warmer summer. When feature engineering, remember to include (if available) the temperature curve, probably it can be used as an exogenous variable.
4.3 Box Plots — Day distribution
Another useful plot is the one referring consumption distribution over the week, this is similar to the weekly consumption seasonal plot.
df_plot = df(('day_str', 'day', 'PJME_MW')).sort_values(by='day')
plt.title(f'Boxplot Day Distribution')
plt.xlabel('Day of week')
plt.ylabel('MW')
sns.boxplot(x='day_str', y='PJME_MW', data=df_plot)
plt.ylabel('Consumption (MW)')
plt.xlabel('Day of week')
As seen before, consumptions are noticeably lower on weekends. Anyway, there are several outliers pointing out that calendar features like “day of week” for sure are useful but could not fully explain the series.
4.4 Box Plots — Hour distribution
Let’s finally see hour distribution box plot. It is similar to the daily consumption seasonal plot since it provides how consumptions are distributed over the day. Following, the code:
plt.title(f'Boxplot Hour Distribution');
plt.xlabel('Hour')
plt.ylabel('MW')
sns.boxplot(x='hour', y='PJME_MW', data=df)
plt.ylabel('Consumption (MW)')
plt.xlabel('Hour')
Note that the “M” shape seen before is now much more crushed. Furthermore there are a lot of outliers, this tells us data not only relies on daily seasonality (e.g. the consumption on today’s 12 am is similar to the consumption of yesterday 12 am) but also on something else, probably some exogenous climatic feature like temperature or humidity.
5. Time Series Decomposition
As already said, time series data can exhibit a variety of patterns. Often, it is helpful to split a time series into several components, each representing an underlying pattern category.
We can think of a time series as comprising three components: a trend component, a seasonal component and a remainder component (containing anything else in the time series). For some time series (e.g., energy consumption series), there can be more than one seasonal component, corresponding to different seasonal periods (daily, weekly, monthly, yearly).
There are two main type of decomposition: additive and multiplicative.
For the additive decomposition, we represent a series (𝑦) as the sum of a seasonal component (𝑆), a trend (𝑇) and a remainder (𝑅):
Similarly, a multiplicative decomposition can be written as:
Generally speaking, additive decomposition best represent series with constant variance while multiplicative decomposition best suits time series with non-stationary variances.
In Python, time series decomposition can be easily fulfilled with Statsmodel library:
df_plot = df(df('year') == 2017).reset_index()
df_plot = df_plot.drop_duplicates(subset=('Datetime')).sort_values(by='Datetime')
df_plot = df_plot.set_index('Datetime')
df_plot('PJME_MW - Multiplicative Decompose') = df_plot('PJME_MW')
df_plot('PJME_MW - Additive Decompose') = df_plot('PJME_MW')# Additive Decomposition
result_add = seasonal_decompose(df_plot('PJME_MW - Additive Decompose'), model='additive', period=24*7)
# Multiplicative Decomposition
result_mul = seasonal_decompose(df_plot('PJME_MW - Multiplicative Decompose'), model='multiplicative', period=24*7)
# Plot
result_add.plot().suptitle('', fontsize=22)
plt.xticks(rotation=45)
result_mul.plot().suptitle('', fontsize=22)
plt.xticks(rotation=45)
plt.show()
The above plots refers to 2017. In both cases, we see the trend has several local peaks, with higher values in summer. From the seasonal component, we can see the series actually has several periodicities, this plot highlights more the weekly one, but if we focus on a particular month (January) of the same year, daily seasonality emerges too:
df_plot = df((df('year') == 2017)).reset_index()
df_plot = df_plot(df_plot('month') == 1)
df_plot('PJME_MW - Multiplicative Decompose') = df_plot('PJME_MW')
df_plot('PJME_MW - Additive Decompose') = df_plot('PJME_MW')df_plot = df_plot.drop_duplicates(subset=('Datetime')).sort_values(by='Datetime')
df_plot = df_plot.set_index('Datetime')
# Additive Decomposition
result_add = seasonal_decompose(df_plot('PJME_MW - Additive Decompose'), model='additive', period=24*7)
# Multiplicative Decomposition
result_mul = seasonal_decompose(df_plot('PJME_MW - Multiplicative Decompose'), model='multiplicative', period=24*7)
# Plot
result_add.plot().suptitle('', fontsize=22)
plt.xticks(rotation=45)
result_mul.plot().suptitle('', fontsize=22)
plt.xticks(rotation=45)
plt.show()
6. Lag Analysis
In time series forecasting, a lag is simply a past value of the series. For example, for daily series, the first lag refers to the value the series had the previous day, the second to the value of the day before and so on.
Lag analysis is based on computing correlations between the series and a lagged version of the series itself, this is also called autocorrelation. For a k-lagged version of a series, we define the autocorrelation coefficient as:
Where y bar represent the mean value of the series and k the lag.
The autocorrelation coefficients make up the autocorrelation function (ACF) for the series, this is simply a plot depicting the auto-correlation coefficient versus the number of lags taken into consideration.
When data has a trend, the autocorrelations for small lags are usually large and positive because observations close in time are also nearby in value. When data show seasonality, autocorrelation values will be larger in correspondence of seasonal lags (and multiples of the seasonal period) than for other lags. Data with both trend and seasonality will show a combination of these effects.
In practice, a more useful function is the partial autocorrelation function (PACF). It is similar to the ACF, except that it shows only the direct autocorrelation between two lags. For example, the partial autocorrelation for lag 3 refers to the only correlation lag 1 and 2 do not explain. In other words, the partial correlation refers to the direct effect a certain lag has on the current time value.
Before moving to the Python code, it is important to highlight that autocorrelation coefficient emerges more clearly if the series is stationary, so often is better to first differentiate the series to stabilize the signal.
Said that, here is the code to plot PACF for different hours of the day:
from statsmodels.graphics.tsaplots import plot_pacfactual = df('PJME_MW')
hours = range(0, 24, 4)
for hour in hours:
plot_pacf(actual(actual.index.hour == hour).diff().dropna(), lags=30, alpha=0.01)
plt.title(f'PACF - h = {hour}')
plt.ylabel('Correlation')
plt.xlabel('Lags')
plt.show()
As you can see, the PACF simply consists on plotting Pearson partial auto-correlation coefficients for different lags. Of course, the non-lagged series shows a perfect auto-correlation with itself, so lag 0 will always be 1. The blue band represent the confidence interval: if a lag exceed that band, then it is statistically significant and we can assert it is has great importance.
6.1 Lag analysis — Feature Engineering
Lag analysis is one of the most impactful study on time series feature engineering. As already said, a lag with high correlation is an important lag for the series, then it should be taken into consideration.
A widely used feature engineering technique consists on making an hourly division of the dataset. That is, splitting data in 24 subset, each one referring to an hour of the day. This has the effect to regularize and smooth the signal, making it more simple to forecast.
Each subset should then be feature engineered, trained and fine-tuned. The final forecast will be achieved combining the results of these 24 models. Said that, every hourly model will have its peculiarities, most of them will regard important lags.
Before moving on, let’s define two types of lag we can deal with when doing lag analysis:
- Auto-regressive lags: lags close to lag 0, for which we expect high values (recent lags are more likely to predict the present value). They are a representation on how much trend the series shows.
- Seasonal lags: lags referring to seasonal periods. When hourly splitting the data, they usually represent weekly seasonality.
Note that auto-regressive lag 1 can also be taught as a daily seasonal lag for the series.
Let’s now discuss about the PACF plots printed above.
Night Hours
Consumption on night hours (0, 4) relies more on auto-regressive than on weekly lags, since the most important are all localized on the first five. Seasonal periods such as 7, 14, 21, 28 seems not to be too much important, this advises us to pay particular attention on lag 1 to 5 when feature engineering.
Day Hours
Consumption on day hours (8, 12, 16, 20) exhibit both auto-regressive and seasonal lags. This particularly true for hours 8 and 12 – when consumption is particularly high — while seasonal lags become less important approaching the night. For these subsets we should also include seasonal lag as well as auto-regressive ones.
Finally, here are some tips when feature engineering lags:
- Do not to take into consideration too many lags since this will probably lead to over fitting. Generally, auto-regressive lags goes from 1 to 7, while weekly lags should be 7, 14, 21 and 28. But it’s not mandatory to take each of them as features.
- Taking into consideration lags that are not auto-regressive or seasonal is usually a bad idea since they could bring to overfitting as well. Rather, try to understand while a certain lag is important.
- Transforming lags can often lead to more powerful features. For example, seasonal lags can be aggregated using a weighted mean to create a single feature representing the seasonality of the series.
Finally, I would like to mention a very useful (and free) book explaining time series, which I have personally used a lot: Forecasting: Principles and Practice.
Even though it is meant to use R instead of Python, this textbook provides a great introduction to forecasting methods, covering the most important aspects of time series analysis.
The aim of this article was to present a comprehensive Exploratory Data Analysis template for time series forecasting.
EDA is a fundamental step in any type of data science study since it allows to understand the nature and the peculiarities of the data and lays the foundation to feature engineering, which in turn can dramatically improve model performance.
We have then described some of the most used analysis for time series EDA, these can be both statistical/mathematical and graphical. Obviously, the intention of this work was only to give a practical framework to start with, subsequent investigations need to be carried out based on the type of historical series being examined and the business context.
Thanks for having followed me until the end.
Unless otherwise noted, all images are by the author.
