# Violations of the Assumptions for Linear Regression (Day 2): Independence of the Residuals

**Recap: ***Lionel Loosefit has been arrested and hauled to court for violating the assumptions of regression analysis. In the previous court session, the prosecution presented evidence to show that the errors in Mr. Loosefit’s model were not normally distributed. Today, the prosecution addresses the second alleged violation: namely, that the errors in the defendant’s regression model are not independent. Dr. Minnie Tabber, a world-renowned statistician, is on the witness stand. *

**Prosecutor**: Let me remind the members of the jury that a residual is simply the difference between the data value estimated by the regression model and the actual observed data value. Now, the law clearly states that besides being normally distributed, residuals should be independent. Why is that so important, Dr. Tabber?

**Dr. Tabber**: If the assumption of independence is violated, some model-fitting results may be questionable. For example, a positive correlation between error terms can inflate the t-values for coefficients.

**Prosecutor**: Inflated t-values. That sounds rather serious.

**Dr. Tabber**. It can be. An inflated t-value can make predictors in a model appear to be significant when they’re really not.

**Prosecutor**: So an unsuspecting victim might be led to think they’ve found a statistically significant association between variables, but really it’s just a fluke, an illusion...a shadow puppet show of p-values, as it were.

**Dr. Tabber**: Umm, something like that.

**Prosecutor**: Still, this all sounds a bit abstract to most of us. Is there any practical way an average person without statistical background can quickly check the independence of the residual errors?

**Dr. Tabber**. Yes, using a plot of the residuals vs the order of observations in Minitab. The plot provides a quick, practical means of visually examining residuals for potential correlation.

**Prosecutor**: Your honor, I’d like to present Exhibit E, a residuals vs order plot from the defendant’s regression model. But before doing so, I must warn our spectators in the courtroom that these Minitab results are extremely graphic.

[Hushed murmurs.]

**Judge**: Those with delicate constitutions may leave the courtroom.

[Stenographer gets up and exits.]

**Judge**: All right, let’s see the evidence, counselor.

**Prosecutor**: Ladies and gentlemen, I give you Exhibit E.

[Spectator stands, screams, swoons, and faints.]

**Judge**: Is there a medic in the house? Bailiff, please assist this unfortunate person.

**Prosecutor**: These kinds of things are never easy to see, are they Dr. Tabber?

**Dr Tabber** [shuddering]: No. Not even after years in the field.

**Prosecutor**: So tell us...would you call *these *residual errors random and independent?

**Dr. Tabber**: No, not at all. When errors are random and independent, you expect to see the points on the plot “bounce up and down” haphazardly at various heights on both sides of the 0 axis line. But here, there’s a clear pattern—a long sequence of negative residuals from observations 6 to 22.

**Prosecutor**: What, pray tell, does this ominous pattern portend?

**Dr. Tabber**: Well, residuals that cluster on the same sign—that is, either on the positive or the negative side of the 0-axis—often indicate a positive correlation.

**Prosecutor**: What might cause this insidious correlation, Dr. Tabber? What is shackling these residuals and preventing them from being truly independent, truly free?

**Dr. Tabber**: Not being familiar with the specifics of this experiment, I can’t really say. It could be something related to the experimental conditions...or the measuring instruments...or even data recording errors. Any number of things.

**Prosecutor**: Thank you, doctor. That’s all for now.

## The Cross-Examination: Does the State Lack Hard Evidence?

**Judge** [to defense attorney]: You may cross-examine the witness.

**Defense Attorney**: Dr. Tabber, that plot of residuals vs. order is certainly a dramatic visual, isn’t it? It really gets the attention of our spectators.

**Dr. Tabber**: Graphic analyses in Minitab often do that.

**Defense Attorney**: Yes, of course. But isn’t it true, Dr. Tabber, that the plot doesn’t provide a *formal* statistical assessment of autocorrelation? In reality, isn’t it just a rough way to eyeball whether there *may appear to be* *an association* between the residuals and the order of the observations?

**Dr. Tabber**: Technically that’s true. Still, the plot is an extremely useful tool. If there’s nothing amiss on the plot, you can usually safely assume that the assumption of independence is satisfied.

**Defense Attorney**: Yes, yes, we understand all that. But isn’t true, Dr. Tabber, that there exists a more formal, definitive way to assess autocorrelation of the residuals in a linear regression model?

**Dr. Tabber** [stiffening]: Certainly. One can formally evaluate the presence of autocorrelation of the residuals using the Durbin-Watson statistic.

**Defense Attorney**: The Durbin-Watson statistic. What a lovely name. Yet this sublime value was never calculated by the State on Mr. Loosefit’s residuals, was it?

**Dr. Tabber**: Not that I’m aware of.

**Defense Attorney**: Why do you think that is?

**Dr. Tabber**: Well, a lot of people don’t know about the test. You’d need to use **Stat > Regression > General Regression **in Minitab, the most versatile linear regression command. The statistic is not displayed by default. So you need to click **Results**, and check *Durbin-Watson statistic*.

**Defense Attorney**: As shown in this Minitab subdialog box?

**Dr. Tabber**: Yes.

**Defense Attorney**: So you just check that box and click **OK** and that’s it? That seems pretty straightforward.

**Dr. Tabber**: Well, it’s not that simple. The interpretation is bit technical. To reach a conclusion from the test, you need to compare the displayed D-W statistic with lower and upper bounds in a statistical table. The bounds depend on the number of predictors in your model and the alpha level you’re using. If D-W is greater than the upper bound, no correlation exists; if D-W is less than the lower bound, positive correlation exists. If D-W is in between the two bounds, the test is inconclusive.

**Defense Attorney**: Be that as it may, the State never even bothered to perform the test. Hmmmm. What a pity. No further questions, your honor.

**Judge**: Dr. Tabber, you may be seated.

## A Surprise Witness Takes the Stand

**Prosecutor**: Your honor, we’d like to call our next witness to the stand.

**Judge**: Proceed.

**Prosecutor**: The State calls Mr. Elmer Fudd, machine maintenance specialist at the defendant’s workplace.

**Defense Attorney**: Objection your honor! We did not expect to cross-examine a cartoon character! We request a temporary adjournment to prepare for this.

**Judge**: Denied. Mr. Fudd, please take the stand.

[Elmer Fudd enters the witness box and is sworn in.]

**Prosecutor**: Mr. Fudd, you are employed as a machine maintenance specialist at Mr. Loosefit’s workplace, is that correct?

**Mr. Fudd**: Cowwect.

**Prosecutor**: And you service the machine that Mr. Loosefit uses to measure his response data. What kind of machine is that?

**Mr. Fudd**: A wireless widget wedge wotater.

**Prosecutor**: Precisely. Every company has one. Now, you’ve just seen the residuals vs order plot and the striking pattern of correlation in observations 6 through 22. Can you tell us—about the time that this data was collected, did you get a call to service the machine?

**Mr. Fudd**: Yes.

**Prosecutor**: And what did you find?

**Mr. Fudd:** A wittle thing was stuck in the wotater. It alweady smelled wotten.

**Prosecutor**: Aha. Something was stuck in the rotater. And you removed the object, didn’t you? What exactly was it?

**Mr. Fudd**: It wooked like a wittle wabbit. But it was wed. And wubbewy.

**Prosecutor**: Let me get this straight. Are you telling the court that a wittle, wed, wotten, wubbewy wabbit was stuck in Mr. Woosefit’s wireless widget wedge wotater?

**Defense Attorney**: Objection your honor! The very idea of a rabbit getting stuck in a rotater is simply preposterous. This witness is a Looney Tune.

**Judge**: Ovewwuled!

**Prosecutor**: We agree it sounds preposterous, your honor. But only because Mr. Fudd *thought* this tiny, colorful object stuck in the machine resembled a small rabbit. Mr. Fudd, do recognize this?

**Mr. Fudd**: Thaiw he iz!!! That danged wabbit!

**Prosecutor**: Not a rabbit, Mr. Fudd, but a bear. A gummi bear, to be precise. Which, stuck in any wireless wedge rotater over time, produces a powerful alloy of citric acid, glucose syrup, and gelatinous prions, which causes the vertical feed handwheel behind the column saddle of the locking lever to stick against the gibhead key, triggering a consistency backlash on the y-axis locks that control the magnetic agitator for the hydraulic friction clutch in the hydro-extractor. And we all know what that leads to, don’t we? Progressive high-frequency belt creep on the driving face of the pneumatic prong of the grubscrew pully! Hence, the sequence of correlated residuals!

**Lionel Loosefit** [jumps up]: HE'S LYING! I DON’T EVEN LIKE GUMMI BEARS! THEY GET STUCK IN MY TEETH!

**Judge:** Order! Mr. Loosefit, another outburst like that and I’ll have you forcibly removed.

**Prosecutor**: You may or may not like gummi bears, Mr. Loosefit. That’s beside the point. Because we’ve discovered that an esteemed colleague of yours has been conducting an elaborate set of designed experiments on gummi bears for many months now, including a novel catapault experiment that is likely to have launched this red gummi bear directly into your rotater.

**Lionel Loosefit **[jumps up again]: No. NO! It’s not true. You’re using these graphs to plot against me!

**Judge:** We’ve heard enough for one day. Bailiff, take Mr. Loosefit away.

Name: Alex• Tuesday, January 22, 2013Your honour! Is there a real plain way in MINITAB to compare the displayed D-W statistic with lower and upper bounds in a statistical table or I need some additional math stuff? Thanks!

Name: Alex• Tuesday, January 22, 2013I have not found any rule of thumb in MINITAB help for interpreting the Durbin–Watson statistic. Is it clear for everybody but me?

http://en.wikipedia.org/wiki/Durbin%E2%80%93Watson_statistic

Name: Patrick Runkel• Tuesday, January 22, 2013Are you kidding? You’re probably the only one who’s daring enough to go there! I applaud your courage.

This is a fairly high-level procedure—that’s why it’s not default output in Minitab. Although the defense is using it to undermine the prosecution’s case, oftentimes you’re fine just using the residuals vs. order plot to make an informal check of this assumption of independent errors, as Dr. Tabber notes.

But I’m feeling bold today (probably the coffee) so let’s see if I can explain how to interpret the Durbin-Watson statistic in a comment. If I just confuse you, let me know—I’ll try tackle the topic later in a separate post.

After you display the D-W value in the output, you need to compare it with the values in a table of Durbin-Watson test bounds. This table is often included in textbooks on linear regression. The reference I have is Kutner, Nachtsheim, and Neter, Applied Linear Statistical Models, 5th edition. The table of Durbin-Watson test bounds (pages 1330 and 1331). You can also find the table online—here’s an example from the Six Sigma Handbook: http://my.safaribooksonline.com/book/quality-management/9780071410151/durbin-watson-test-bounds/durbin-watson_test_bounds

In many applications, such as business and economics, the most common infraction occurs when the residuals show positive correlation. Generally speaking, the smaller the Durbin-Watson statistic is, the more likely that there is statistically significant positive autocorrelation of the residuals. The actual Durbin-Watson statistic calculated for Lionel Loosefit’s model is 0.705957.

That’s pretty low--but to definitively determine whether to reject the null hypothesis that the autocorrelation parameter is 0, and accept the alternative hypothesis that the autocorrelation parameter is greater than 0, we have to compare this value of 0.705957 with the upper and lower bounds in the Durbin-Watson test bounds table. The bounds will differ depending on the number of observations in your data, the number of predictors in the model and the alpha level you’re using for the test.

If the D-W value is greater than the upper bound in the table, you conclude that there is not sufficient evidence to conclude positive autocorrelation. If the D-W value is less than the lower bound in the table, you reject the null hypothesis and conclude that there is statistically significant positive autocorrelation in the residuals.

Back to our D-W value of 0.705957. Mr. Loosefit has only 15 data values (tsk tsk) in his data set. His model has only one X variable. Using an alpha level of 0.05, the table of D-W test bounds for n=15 with one X variable (p-1=1) gives a lower bound of 1.08 and a higher bound of 1.36. Because 0.705957 is less than the lower bound, we reject the null hypothesis and conclude that the residuals show positive autocorrelation.

The test for negative autocorrelation using the D-W value is trickier—but you can see it in the Wikipedia link you included.

By the way, for the residuals vs. order plot and the Durbin-Watson calculation, Minitab assumes that the observations are in the worksheet in a meaningful order, such as time order (the order they were collected). If they aren’t, you need to sort the data into meaningful order before running Regression.

Does this make sense?

Name: Alex• Wednesday, January 23, 2013Hi, Patrick! That will do. Special thanks for your reference of Six Sigma book and D-W table. Regression analysis is a real art. Ars longa, vita brevis!