Basic Statistics and Data Analysis

Lecture notes, MCQS of Statistics

Tag: Time Series Analysis

Objectives of Time Series Analysis

There are many objectives related to time series analysis, objectives of time series analysis may be classified as

  1. Description
  2. Explanation
  3. Prediction
  4. Control

The description of the objectives of time series analysis are as follows:

Description

The first step in the analysis is to plot the data and obtain simple descriptive measures (such as plotting data, looking for trends,  seasonal fluctuations and so on) of the main properties of the series. Seasonal EffectIn above figure , there is a regular seasonal pattern of price change although this price pattern is not consistent. Graph enables to look for “wild” observations or outlier (not appear to be consistent with the rest of the data). Graphing the time series make possible the presence of turning points where upward trend suddenly changed to a downward trend. If there is turning point, different models may have to be fitted to the two parts of the series.

Explanation

Observations taken on two or more variables, making possible to use the variation in one time series to explain the variation in another series. This may lead to deeper understanding. Multiple regression model may be helpful in this case.

Prediction

Given an observed time series, one may want to predict the future values of the series. It is an important task in sales of forecasting and is the analysis of economic and industrial time series. Prediction and forecasting used interchangeably.

Control

When time series generated to measure the quality of a manufacturing process (the aim may be) to control the process. Control procedures are of several different kinds. In quality control, the observations are plotted on control chart and the controller takes action as a result of studying the charts. A stochastic model is fitted to the series. Future values of the series are predicted and then the input process variables are adjusted so as to keep the process on target.

Time Series Analysis and Forecasting

A time series $\{Y_t\}$ or $\{y_1,y_2,\cdots,y_T\}$ is a discrete time, continuous state process where time $t=1,2,\cdots,=T$ are certain discrete time points spaced at uniform time intervals.

Usually time is taken at more or less equally spaced intervals such as hour, day, month, quarter or year. More specifically, it is set of data in which observations are arranged in a chronological order (A set of repeated observations of the same variable).

Time series are used in different fields of sciences such as statistics, signal processing, pattern recognition, econometrics, mathematical finance, weather forecasting, earthquake prediction, electroencephalography, control engineering, astronomy, and communications engineering among many other fields.

Time series analysis is the analysis of a series of data-points over time, allowing one to answer question such as what is the causal effect on a variable $Y$ of a change in variable $X$ over time? An important difference between time series and cross section data is that the ordering of cases does matter in time series.

Definition: A sequence of random variables indexed by time is called a stochastic process (stochastic means random) or time series for mere mortals. A data set is one possible outcome (realization) of the stochastic process. If history had been different, we would observe a different outcome, thus we can think of time series as the outcome of a random variable.

Rather than dealing with individuals as units, the unit of interest is time: the value of Y at time $t$ is $Y_t$. The unit of time can be anything from days to election years. The value of $Y_t$ in the previous period is called the first lag value: $Y_{t-1}$. The jth lag is denoted: $Y_{t-j}$. Similarly $Y_{t+1}$ is the value of $Y_t$ in the next period. So a simple bivariate regression equation for time series data looks like: \[Y_t = \beta_0 + \beta X_t + u_t\]

Continuous Time Series

A time series is said to be continuous when observation are made continuously in time. The term continuous is used for series of this type even when the measured variable can only take a discrete set of values.

Discrete Time Series

A time series is said to be discrete when observations are taken at specific time, usually equally spaced. The term discrete is used for series of this type even when the measured variable is continuous variable.

Most of Macroeconomic and finance data comes in form of time series. GNP or Stock Return is example fo time series data.

We can write a series as $\{x_1,x_2,x_3,\cdots,x_T\}$ or $\{x_t\}$, where $t=1,2,3,\cdots,T$. $x_t$ is treated as random variable.

Time series analysis refers to the branch of statistics where observations are collected sequentially in time, usually but not necessarily at equal spaced time points. The arcane difference between time series and other variable is use of subscript.

Time series analysis comprises methods for analyzing time series data in order to extract some useful (meaningful) statistics and other characteristics of the data, while Time series forecasting is the use of a model to predict future values based on previously observed values.

Given an observed time series, the first step in analyzing a time series is to plot the given series on a graph taking time intervals (t) along X-axis (as independent variable) and the observed value ($Y_t$) on Y-axis (as dependent variable). Such a graph will show various types of fluctuations and other point of interest.

Note

  • $Y_t$ is treated as random variable. If $Y_t$ is generated by some model (Regression model for time series i.e. $Y_t=x_t\beta +\varepsilon_t$, $E(\varepsilon_t|x_t)=0$, then ordinary least square (OLS) provides a consistent estimates of $\beta$.
  • Time series interchangeably used for sample $\{x_t\}$ and probability model. A possible probability model for the joint distribution of a time series $\{x_t\}$ is $x_t=\varepsilon_t$, $\varepsilon_t\sim iid  N(0,\sigma_\varepsilon^2)$
  • Time series are typically not iid (Independent Identically Distributed) e.g. If GNP today is unusually high, GNP tomorrow will also likely to be unusually high.

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