What is partial autocorrelation?

This is the final post I have planned in the autocorrelation series. I’ll introduce a new concept called partial autocorrelation and show a couple examples of the concept in action. We’ll learn some important things about the stock market.

Without getting into the math, here’s a couple definitions you should commit to memory:

  1. Autocorrelation (“ACF”) measures the correlation between observations in a time series and observations a fixed unit of time away.
  2. Partial autocorrelation (“PACF”) measures the correlation between observations in a time series and observations a fixed unit of time away while controlling for indirect effects.

For example, here’s both ACF and PACF for monthly U.S. unemployment.
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This is a common pattern you’ll see in the field often: a slow decay of ACF, and a spike in PACF. Starting at a lag of two months, PACF is basically zero.

This means that January’s unemployment influences March’s unemployment, but only because January influences February and February influences March. If unemployment were high in January and low in February, we would predict March’s unemployment to also be low.

In the Box-Jenkins methodology, ACF and PACF plots like this one are used to determine the correct model. Applying this step to a preexisting model will address seasonal patterns in residuals.

A few notes: Continue reading →

Grand Theft Autocorrelation

Michael-Counting-Money-16_RGBIn my last post, I discussed the concept of correlation. In my free time since then, I’ve been playing a lot of Grand Theft Auto V. It’s time to merge these noble pursuits.

As you may know, GTA V includes an online stock market that allows players to invest their ill-gotten gains in fictitious companies. Naturally, a Reddit user has created an updating database of Stock Market prices. I play on a 360, so I’ve analyzed the Xbox prices. I’ve developed a strategy that will earn money in the long-run, but first let’s do some learning.

As we previously discussed, correlation is a measure of how well the highs of one series line up with the highs of another series. Autocorrelation is a measure of how well highs of a series line up with the highs of the previous observation of the same series. Continue reading →

What is Correlation?

Besides not being causation, many pedantically smart laymen don’t know what correlation is. I’m here to fix that with a mathematical, an intuitive explanation and a brief philosophical comment.

Intuitive Explanation

Correlation is a quantitative measure of how well the highs line up with the highs and the lows line up with the lows between two arrays. Correlation will always be between -1 and 1, inclusively. A correlation of 1 indicates a linear, positive relationship between two variables. A correlation of zero indicates no correlation. A correlation of negative one indicates a perfect, negative relationship.

Mathematical Definition

This value can be computed in Excel through the CORREL() function, but stepping into the formula helps enhance the understanding. Feel free to skim over this part to the applications and philosophy sections. Mathematically, correlation can be computed as follows: Continue reading →